TRAFFIC MANAGEMENT AND SPEED - TRIPPtripp.iitd.ernet.in/assets/newsimage/KRR-Speed-safety.pdf ·...
Transcript of TRAFFIC MANAGEMENT AND SPEED - TRIPPtripp.iitd.ernet.in/assets/newsimage/KRR-Speed-safety.pdf ·...
TRAFFIC MANAGEMENT
AND SPEED
WORKSHOP-CUM-TRAINING PROGRAMME ON ROAD SAFETY
17th ndash 21st September 2018
Dr K Ramachandra RaoProfessor
Department of Civil Engineering and Transportation Research and Injury Prevention Programmme (TRIPP)
Indian Institute of Technology Delhi
Outline
bull Introduction
bull Evidence of speed impacts on injury
bull Reaction time
bull Braking distance
bull Speed ndash injury severity relationship
bull Speed limits
bull Road structure - speed
bull ITS and Safety ndash Michelin Report
bull Way forward
Speed - safety 2
Speed - safety 3
Introductionbull Travelling at higher speeds ndash seen as a benefit of vehicle
use speed WAS NOT CONSIDERED to be a MAJOR
RISK
bull Many studies in 1960rsquos attempted to understand various
factors associated with incidence of road traffic crashes
including variables
bull Traffic volume age sex of drivers width and curvature of roads
weather driver error but did not include SPEED
bull A study in 1964 reports that ldquoOne of the important
findings of this study is that the greater the differential in
speed of a driver and his vehicle from the average speed
of all traffic the greater the chance of that driver being
involved in an accidentrdquo
Speed - safety 4
Introductionbull Further ldquoThe accident-involvement injury and property-damage rates
were highest at very low speeds lowest at about the average speed
of all traffic and increased at the very high speeds particularly at
nightrdquo
bull Thus the greater the variation in speed of any vehicle from the
average speed of all traffic the greater its chance of being involved in
an accidentrdquo
bull ldquoThe fatality rate was highest at very high speeds and lowest at about
the average speedrdquo (Solomon 1964)
bull ldquoAs is well known the primary factor affecting the likelihood of driver
injury is speedrdquo (White and Clayton 1972)
bull Another study concluded that there was a reduction in speeds
brought about by the 55 mph law and the interstate rural system had
a 30 reduction in fatality rates which prevented 4700 fatalities and
81000 injuries in 1974
Speed - safety 5
Evidencebull Over whelming evidence ndash
bull increase in speed is always accompanied by increase in the number of crashes and in the average severity of road traffic injuries (Peden et al 2004 Elvik and Vaa 2004)
bull There are still some skeptics who suggest that increasing speed limits does not result in a greater loss of lives ldquoAll of the evidence thus far indicates that Americans have not responded to higher speed limits by converting the highways into stretches of the Indianapolis 500
bull Any loss of life has been very minimal and at most a tiny fraction of what had been predicted by the safety lobby
bull All carefully done studies and meta analyses show conclusively that there is a very strong statistical relationship between speed and road safety ndash when speeds go up the number of crashes and severity of injuries increase and when speeds go down the numbers and severities reduce
Speed - safety 6
Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden
United Kingdom and the Netherlands and concluded that
bull Fatalities tend to change proportionally with change of average speed in a double quadratic way
bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)
bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)
bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities
Speed - safety 7
Speeding - highways
Speed - safety 8
Speeding
Speed - safety 9
Person struck by car gt 5 death rate
Field of Vision at 30 kmh
Livable StreetsMore than just traffic volume ndash lower speeds
IIT Delhi 17 September 2018
Speed - safety 10
Field of Vision at 50 kmh
Person struck by car gt 45 death rate
IIT Delhi 17 September 2018
Speed - safety 11
Field of Vision at 65 kmh
IIT Delhi 17 September 2018
Speed - safety 12
Probability of pedestrian fatality
Speed - safety 13
Probability of
pedestrian fatality
increases 4-fold for
a 10 kmh (40 to 50
kmh) increase in
impacting speed
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Outline
bull Introduction
bull Evidence of speed impacts on injury
bull Reaction time
bull Braking distance
bull Speed ndash injury severity relationship
bull Speed limits
bull Road structure - speed
bull ITS and Safety ndash Michelin Report
bull Way forward
Speed - safety 2
Speed - safety 3
Introductionbull Travelling at higher speeds ndash seen as a benefit of vehicle
use speed WAS NOT CONSIDERED to be a MAJOR
RISK
bull Many studies in 1960rsquos attempted to understand various
factors associated with incidence of road traffic crashes
including variables
bull Traffic volume age sex of drivers width and curvature of roads
weather driver error but did not include SPEED
bull A study in 1964 reports that ldquoOne of the important
findings of this study is that the greater the differential in
speed of a driver and his vehicle from the average speed
of all traffic the greater the chance of that driver being
involved in an accidentrdquo
Speed - safety 4
Introductionbull Further ldquoThe accident-involvement injury and property-damage rates
were highest at very low speeds lowest at about the average speed
of all traffic and increased at the very high speeds particularly at
nightrdquo
bull Thus the greater the variation in speed of any vehicle from the
average speed of all traffic the greater its chance of being involved in
an accidentrdquo
bull ldquoThe fatality rate was highest at very high speeds and lowest at about
the average speedrdquo (Solomon 1964)
bull ldquoAs is well known the primary factor affecting the likelihood of driver
injury is speedrdquo (White and Clayton 1972)
bull Another study concluded that there was a reduction in speeds
brought about by the 55 mph law and the interstate rural system had
a 30 reduction in fatality rates which prevented 4700 fatalities and
81000 injuries in 1974
Speed - safety 5
Evidencebull Over whelming evidence ndash
bull increase in speed is always accompanied by increase in the number of crashes and in the average severity of road traffic injuries (Peden et al 2004 Elvik and Vaa 2004)
bull There are still some skeptics who suggest that increasing speed limits does not result in a greater loss of lives ldquoAll of the evidence thus far indicates that Americans have not responded to higher speed limits by converting the highways into stretches of the Indianapolis 500
bull Any loss of life has been very minimal and at most a tiny fraction of what had been predicted by the safety lobby
bull All carefully done studies and meta analyses show conclusively that there is a very strong statistical relationship between speed and road safety ndash when speeds go up the number of crashes and severity of injuries increase and when speeds go down the numbers and severities reduce
Speed - safety 6
Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden
United Kingdom and the Netherlands and concluded that
bull Fatalities tend to change proportionally with change of average speed in a double quadratic way
bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)
bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)
bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities
Speed - safety 7
Speeding - highways
Speed - safety 8
Speeding
Speed - safety 9
Person struck by car gt 5 death rate
Field of Vision at 30 kmh
Livable StreetsMore than just traffic volume ndash lower speeds
IIT Delhi 17 September 2018
Speed - safety 10
Field of Vision at 50 kmh
Person struck by car gt 45 death rate
IIT Delhi 17 September 2018
Speed - safety 11
Field of Vision at 65 kmh
IIT Delhi 17 September 2018
Speed - safety 12
Probability of pedestrian fatality
Speed - safety 13
Probability of
pedestrian fatality
increases 4-fold for
a 10 kmh (40 to 50
kmh) increase in
impacting speed
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed - safety 3
Introductionbull Travelling at higher speeds ndash seen as a benefit of vehicle
use speed WAS NOT CONSIDERED to be a MAJOR
RISK
bull Many studies in 1960rsquos attempted to understand various
factors associated with incidence of road traffic crashes
including variables
bull Traffic volume age sex of drivers width and curvature of roads
weather driver error but did not include SPEED
bull A study in 1964 reports that ldquoOne of the important
findings of this study is that the greater the differential in
speed of a driver and his vehicle from the average speed
of all traffic the greater the chance of that driver being
involved in an accidentrdquo
Speed - safety 4
Introductionbull Further ldquoThe accident-involvement injury and property-damage rates
were highest at very low speeds lowest at about the average speed
of all traffic and increased at the very high speeds particularly at
nightrdquo
bull Thus the greater the variation in speed of any vehicle from the
average speed of all traffic the greater its chance of being involved in
an accidentrdquo
bull ldquoThe fatality rate was highest at very high speeds and lowest at about
the average speedrdquo (Solomon 1964)
bull ldquoAs is well known the primary factor affecting the likelihood of driver
injury is speedrdquo (White and Clayton 1972)
bull Another study concluded that there was a reduction in speeds
brought about by the 55 mph law and the interstate rural system had
a 30 reduction in fatality rates which prevented 4700 fatalities and
81000 injuries in 1974
Speed - safety 5
Evidencebull Over whelming evidence ndash
bull increase in speed is always accompanied by increase in the number of crashes and in the average severity of road traffic injuries (Peden et al 2004 Elvik and Vaa 2004)
bull There are still some skeptics who suggest that increasing speed limits does not result in a greater loss of lives ldquoAll of the evidence thus far indicates that Americans have not responded to higher speed limits by converting the highways into stretches of the Indianapolis 500
bull Any loss of life has been very minimal and at most a tiny fraction of what had been predicted by the safety lobby
bull All carefully done studies and meta analyses show conclusively that there is a very strong statistical relationship between speed and road safety ndash when speeds go up the number of crashes and severity of injuries increase and when speeds go down the numbers and severities reduce
Speed - safety 6
Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden
United Kingdom and the Netherlands and concluded that
bull Fatalities tend to change proportionally with change of average speed in a double quadratic way
bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)
bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)
bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities
Speed - safety 7
Speeding - highways
Speed - safety 8
Speeding
Speed - safety 9
Person struck by car gt 5 death rate
Field of Vision at 30 kmh
Livable StreetsMore than just traffic volume ndash lower speeds
IIT Delhi 17 September 2018
Speed - safety 10
Field of Vision at 50 kmh
Person struck by car gt 45 death rate
IIT Delhi 17 September 2018
Speed - safety 11
Field of Vision at 65 kmh
IIT Delhi 17 September 2018
Speed - safety 12
Probability of pedestrian fatality
Speed - safety 13
Probability of
pedestrian fatality
increases 4-fold for
a 10 kmh (40 to 50
kmh) increase in
impacting speed
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Introductionbull Travelling at higher speeds ndash seen as a benefit of vehicle
use speed WAS NOT CONSIDERED to be a MAJOR
RISK
bull Many studies in 1960rsquos attempted to understand various
factors associated with incidence of road traffic crashes
including variables
bull Traffic volume age sex of drivers width and curvature of roads
weather driver error but did not include SPEED
bull A study in 1964 reports that ldquoOne of the important
findings of this study is that the greater the differential in
speed of a driver and his vehicle from the average speed
of all traffic the greater the chance of that driver being
involved in an accidentrdquo
Speed - safety 4
Introductionbull Further ldquoThe accident-involvement injury and property-damage rates
were highest at very low speeds lowest at about the average speed
of all traffic and increased at the very high speeds particularly at
nightrdquo
bull Thus the greater the variation in speed of any vehicle from the
average speed of all traffic the greater its chance of being involved in
an accidentrdquo
bull ldquoThe fatality rate was highest at very high speeds and lowest at about
the average speedrdquo (Solomon 1964)
bull ldquoAs is well known the primary factor affecting the likelihood of driver
injury is speedrdquo (White and Clayton 1972)
bull Another study concluded that there was a reduction in speeds
brought about by the 55 mph law and the interstate rural system had
a 30 reduction in fatality rates which prevented 4700 fatalities and
81000 injuries in 1974
Speed - safety 5
Evidencebull Over whelming evidence ndash
bull increase in speed is always accompanied by increase in the number of crashes and in the average severity of road traffic injuries (Peden et al 2004 Elvik and Vaa 2004)
bull There are still some skeptics who suggest that increasing speed limits does not result in a greater loss of lives ldquoAll of the evidence thus far indicates that Americans have not responded to higher speed limits by converting the highways into stretches of the Indianapolis 500
bull Any loss of life has been very minimal and at most a tiny fraction of what had been predicted by the safety lobby
bull All carefully done studies and meta analyses show conclusively that there is a very strong statistical relationship between speed and road safety ndash when speeds go up the number of crashes and severity of injuries increase and when speeds go down the numbers and severities reduce
Speed - safety 6
Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden
United Kingdom and the Netherlands and concluded that
bull Fatalities tend to change proportionally with change of average speed in a double quadratic way
bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)
bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)
bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities
Speed - safety 7
Speeding - highways
Speed - safety 8
Speeding
Speed - safety 9
Person struck by car gt 5 death rate
Field of Vision at 30 kmh
Livable StreetsMore than just traffic volume ndash lower speeds
IIT Delhi 17 September 2018
Speed - safety 10
Field of Vision at 50 kmh
Person struck by car gt 45 death rate
IIT Delhi 17 September 2018
Speed - safety 11
Field of Vision at 65 kmh
IIT Delhi 17 September 2018
Speed - safety 12
Probability of pedestrian fatality
Speed - safety 13
Probability of
pedestrian fatality
increases 4-fold for
a 10 kmh (40 to 50
kmh) increase in
impacting speed
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Introductionbull Further ldquoThe accident-involvement injury and property-damage rates
were highest at very low speeds lowest at about the average speed
of all traffic and increased at the very high speeds particularly at
nightrdquo
bull Thus the greater the variation in speed of any vehicle from the
average speed of all traffic the greater its chance of being involved in
an accidentrdquo
bull ldquoThe fatality rate was highest at very high speeds and lowest at about
the average speedrdquo (Solomon 1964)
bull ldquoAs is well known the primary factor affecting the likelihood of driver
injury is speedrdquo (White and Clayton 1972)
bull Another study concluded that there was a reduction in speeds
brought about by the 55 mph law and the interstate rural system had
a 30 reduction in fatality rates which prevented 4700 fatalities and
81000 injuries in 1974
Speed - safety 5
Evidencebull Over whelming evidence ndash
bull increase in speed is always accompanied by increase in the number of crashes and in the average severity of road traffic injuries (Peden et al 2004 Elvik and Vaa 2004)
bull There are still some skeptics who suggest that increasing speed limits does not result in a greater loss of lives ldquoAll of the evidence thus far indicates that Americans have not responded to higher speed limits by converting the highways into stretches of the Indianapolis 500
bull Any loss of life has been very minimal and at most a tiny fraction of what had been predicted by the safety lobby
bull All carefully done studies and meta analyses show conclusively that there is a very strong statistical relationship between speed and road safety ndash when speeds go up the number of crashes and severity of injuries increase and when speeds go down the numbers and severities reduce
Speed - safety 6
Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden
United Kingdom and the Netherlands and concluded that
bull Fatalities tend to change proportionally with change of average speed in a double quadratic way
bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)
bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)
bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities
Speed - safety 7
Speeding - highways
Speed - safety 8
Speeding
Speed - safety 9
Person struck by car gt 5 death rate
Field of Vision at 30 kmh
Livable StreetsMore than just traffic volume ndash lower speeds
IIT Delhi 17 September 2018
Speed - safety 10
Field of Vision at 50 kmh
Person struck by car gt 45 death rate
IIT Delhi 17 September 2018
Speed - safety 11
Field of Vision at 65 kmh
IIT Delhi 17 September 2018
Speed - safety 12
Probability of pedestrian fatality
Speed - safety 13
Probability of
pedestrian fatality
increases 4-fold for
a 10 kmh (40 to 50
kmh) increase in
impacting speed
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Evidencebull Over whelming evidence ndash
bull increase in speed is always accompanied by increase in the number of crashes and in the average severity of road traffic injuries (Peden et al 2004 Elvik and Vaa 2004)
bull There are still some skeptics who suggest that increasing speed limits does not result in a greater loss of lives ldquoAll of the evidence thus far indicates that Americans have not responded to higher speed limits by converting the highways into stretches of the Indianapolis 500
bull Any loss of life has been very minimal and at most a tiny fraction of what had been predicted by the safety lobby
bull All carefully done studies and meta analyses show conclusively that there is a very strong statistical relationship between speed and road safety ndash when speeds go up the number of crashes and severity of injuries increase and when speeds go down the numbers and severities reduce
Speed - safety 6
Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden
United Kingdom and the Netherlands and concluded that
bull Fatalities tend to change proportionally with change of average speed in a double quadratic way
bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)
bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)
bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities
Speed - safety 7
Speeding - highways
Speed - safety 8
Speeding
Speed - safety 9
Person struck by car gt 5 death rate
Field of Vision at 30 kmh
Livable StreetsMore than just traffic volume ndash lower speeds
IIT Delhi 17 September 2018
Speed - safety 10
Field of Vision at 50 kmh
Person struck by car gt 45 death rate
IIT Delhi 17 September 2018
Speed - safety 11
Field of Vision at 65 kmh
IIT Delhi 17 September 2018
Speed - safety 12
Probability of pedestrian fatality
Speed - safety 13
Probability of
pedestrian fatality
increases 4-fold for
a 10 kmh (40 to 50
kmh) increase in
impacting speed
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Evidence ndash double quadratic lawbull Koornstra et al (2002) compared traffic fatality rates in Sweden
United Kingdom and the Netherlands and concluded that
bull Fatalities tend to change proportionally with change of average speed in a double quadratic way
bull For example an average speed reduction by a factor 95 gives an expected reduction of fatalities by a factor (0952)2=0815 or 5 reduction of average speed gives 185 reduction in expected fatalities This relationship has been verified by Nilsson (2004) and (Elvik Christensen and Amundsen 2004)
bull 1113088There is enough evidence to show that lowering of speed limits on expressways and urban roads results in fewer fatalities and injuries (ETSC 1995)
bull Reduction of speed limits by 10ndash20 kmh on motorways and rural roads in Switzerland and Sweden resulted in 6ndash21 fewer fatalities
Speed - safety 7
Speeding - highways
Speed - safety 8
Speeding
Speed - safety 9
Person struck by car gt 5 death rate
Field of Vision at 30 kmh
Livable StreetsMore than just traffic volume ndash lower speeds
IIT Delhi 17 September 2018
Speed - safety 10
Field of Vision at 50 kmh
Person struck by car gt 45 death rate
IIT Delhi 17 September 2018
Speed - safety 11
Field of Vision at 65 kmh
IIT Delhi 17 September 2018
Speed - safety 12
Probability of pedestrian fatality
Speed - safety 13
Probability of
pedestrian fatality
increases 4-fold for
a 10 kmh (40 to 50
kmh) increase in
impacting speed
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speeding - highways
Speed - safety 8
Speeding
Speed - safety 9
Person struck by car gt 5 death rate
Field of Vision at 30 kmh
Livable StreetsMore than just traffic volume ndash lower speeds
IIT Delhi 17 September 2018
Speed - safety 10
Field of Vision at 50 kmh
Person struck by car gt 45 death rate
IIT Delhi 17 September 2018
Speed - safety 11
Field of Vision at 65 kmh
IIT Delhi 17 September 2018
Speed - safety 12
Probability of pedestrian fatality
Speed - safety 13
Probability of
pedestrian fatality
increases 4-fold for
a 10 kmh (40 to 50
kmh) increase in
impacting speed
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speeding
Speed - safety 9
Person struck by car gt 5 death rate
Field of Vision at 30 kmh
Livable StreetsMore than just traffic volume ndash lower speeds
IIT Delhi 17 September 2018
Speed - safety 10
Field of Vision at 50 kmh
Person struck by car gt 45 death rate
IIT Delhi 17 September 2018
Speed - safety 11
Field of Vision at 65 kmh
IIT Delhi 17 September 2018
Speed - safety 12
Probability of pedestrian fatality
Speed - safety 13
Probability of
pedestrian fatality
increases 4-fold for
a 10 kmh (40 to 50
kmh) increase in
impacting speed
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Person struck by car gt 5 death rate
Field of Vision at 30 kmh
Livable StreetsMore than just traffic volume ndash lower speeds
IIT Delhi 17 September 2018
Speed - safety 10
Field of Vision at 50 kmh
Person struck by car gt 45 death rate
IIT Delhi 17 September 2018
Speed - safety 11
Field of Vision at 65 kmh
IIT Delhi 17 September 2018
Speed - safety 12
Probability of pedestrian fatality
Speed - safety 13
Probability of
pedestrian fatality
increases 4-fold for
a 10 kmh (40 to 50
kmh) increase in
impacting speed
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Field of Vision at 50 kmh
Person struck by car gt 45 death rate
IIT Delhi 17 September 2018
Speed - safety 11
Field of Vision at 65 kmh
IIT Delhi 17 September 2018
Speed - safety 12
Probability of pedestrian fatality
Speed - safety 13
Probability of
pedestrian fatality
increases 4-fold for
a 10 kmh (40 to 50
kmh) increase in
impacting speed
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Field of Vision at 65 kmh
IIT Delhi 17 September 2018
Speed - safety 12
Probability of pedestrian fatality
Speed - safety 13
Probability of
pedestrian fatality
increases 4-fold for
a 10 kmh (40 to 50
kmh) increase in
impacting speed
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Probability of pedestrian fatality
Speed - safety 13
Probability of
pedestrian fatality
increases 4-fold for
a 10 kmh (40 to 50
kmh) increase in
impacting speed
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Evidence ndash Reaction timebull Lower initial speeds mean that the driver has better
control of the vehicle and the vehicle can stop much
earlier and reduce the probability of a crash
bull Say a vehicle travelling at 80 kmh (22 ms) than a
50 kmh (139 ms) to avoid crash
bull First uses steering 17 s (14 m extra)
bull Then leaves accelerator pedal and hit brakes 22 ndash 26 s (19 m
extra) before brakes are applied
Speed - safety 14
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Evidence ndash braking distancebull The stopping distance of a vehicle under braking depends
on the square of the original velocity
bull When this is combined with the extra reaction time we
see that distances covered at higher speeds are much
higher than those at lower speeds
bull Total stopping distance = reaction distance + braking
distance
d = dr + db
dr = vit
db = (v12-v2
2)2g(f001G)
bull Vehicles traveling at higher speeds continue to travel at
high speeds and the effect of braking manifests itself later
and later the higher the speed
Speed - safety 15
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Relationship of speed ndash braking distance
Speed - safety 16
Mohan (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Evidence ndash braking distancebull An increase of 10 20 and 30 kmh in speed over 40 kmh
can have the effect of an increase in impact speeds of 32
53 and 70 kmh respectively
bull This shows how increases in speed affect the outcome in
proportions that are disproportionately higher
Speed - safety 17
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Evidence ndash speed to injury severitybull In the event of a crash the injuries are less severe at
lower impact velocities
bull This is because the severity of injuries depends to large
extent on the energy transferred to the human body
during an impact
bull The relationship between speed and energy is given
below
bull E = (frac12 ) M V 2 where E ndash energy M ndash mass of the object
V ndash velocity of the object
bull Thus small increases in velocity effect much larger
increases in energy
Speed - safety 18
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed ndash impacts on car occupants and
pedestrians
Impact
velocity
(kmh)
Estimated injury
(Belted car)
Risk of fatality
(Belted car driver)
Road user fatality
(pedestrian Car impact)
30 004 001 008
50 010 003 087
80 042 021 100
100 080 061 100
120 100 100 100
Speed - safety 19
Mohan (2017)
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Probability of fatality ndash pedestrian and car
occupant
Speed - safety 20
Mohan (2016)
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed limits
bull The speed at which a driver travels depends on many factors such as the vehiclersquos engine power and stability road and traffic conditions perception of safety speed limits and the level of enforcement travel motivations personal characteristics and behaviour of other drivers (Noguchi 1990)
bull Among the factors above a significant influence by policy only can happen over the setting of speed limits and enforcement road design and vehicle design
bull Speed limits ndash setting and compliance
bull Mannering (2009) observes that in observed speed data show that the majority of drivers routinely exceed posted speed limits
Speed - safety 21
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed limits
bull A key motivating factor in driversrsquo tendency to exceed the
speed limit is that they believe that the excess speed
does not threaten safety
bull The findings show that driversrsquo perception of the speed
above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash suggesting that enforcement
plays an important role in safety perceptions
Speed - safety 22
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed limits - perceptions
bull Manneringrsquos study show that driversrsquo perception of the
speed above the speed limit at which they will receive a
speeding ticket is a critical determinant of what they
believe is a safe speed ndash
bull Enforcement plays an important role in safety perceptions
bull Other significant factors in determining the speed above
the speed limit at which safety is first threatened include
bull age gender being previously stopped for speeding and driversrsquo
ethnicity
Speed - safety 23
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed management
bull Speed limits
bull Good information about the local speed limit
bull Infrastructural measures
bull Police surveillance and enforcement
bull Education and information
bull Intelligent Speed Assistance (ISA)
Speed - safety 24
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed management - measures
bull Speed management consists of a combination of
measures in a logical order (Wegman amp Aarts 2006 Van
Schagen amp Feypell 2011)
bull Step 1 Determine the safe speed limit
bull When motorized traffic mixes with pedestrians and cyclists the
speed must be low
bull the possibility of certain conflicts such as a lateral or a head-on
conflict influences the safe speed
Speed - safety 25
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed management - measures
bull Step 2 Make sure the limit is credible
bull Credible means that the limit is consistent with the
expectations
bull drivers are more inclined to keep to the limit (Van
Schagen Wegman amp Roszbach 2004)
Speed - safety 26
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed management - measures
bull Step 3 Give good information about the local limit
bull In practice it is not always clear which limit applies at a certain
location
bull Information on the local limit is usually given by road signs
Speed - safety 27
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed management - measures
bull Step 4 Support the limit with speed inhibitors
bull Where necessary (eg near schools pedestrian and bicycle
crossings single-level intersections) physical speed inhibitors help
drivers to choose a lower speed
bull speed humps road narrowings plateaus or roundabouts
Speed - safety 28
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed management - measures
bull Step 5 Police enforcement
bull With the above measures we may assume that many of
the speeding offences are prevented
bull But because drivers can ultimately choose their own
speed offences will always be committed
bull Therefore focused police enforcement remains necessary
for the time being aimed at both general deterrence and
prevention
Speed - safety 29
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed management - measures
bull Step 5 Education and information
bull Education and information offer support for each of the
above measures
They can be used to explain speed measures such as
police enforcement and speed humps and make people
aware of the risks of (too) fast driving
bull It appears very difficult to influence speed behaviour with
education and information (Van Schagen et al 2016)
Speed - safety 30
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Road features ndash enhancereduce speeds
Speed - safety 31
SWOV Fact sheet 3 copy SWOV Leidschendam the Netherlands November 2012
Reproduction is only permitted with due acknowledgement
lanes and the road markings Variations were chosen in such a way that the final layout was in line with the current Dutch CROW guidelines In the first intuitive drive the credibility deviated from the intended speed limit in two directions the limit was either not or less credible because it was too high (in that case the intuitive speed was lower than the intended speed limit) or because the limit was too low (in that case the intuitive speed was higher than the intended speed limit) In the second experimental drive the speed limits were indicated and the degree of credibility was indeed found to influence the driving speed More credible limits resulted in an average driving speed that was closer to the limit and on average less time was spent speeding In addition the results indicated smaller differences between drivers When the limit was experienced as being too low the average speed was considerably higher than the limit for limits that were experienced as being too high the average speed was lower than the limit There are indications that drivers older than 50 are more influenced by the credibility of limits than young drivers gender and sensation seeking were found to have no influence here What to do if a speed limit is not credible
If a speed limit is not credible there basically are two possibilities to do something about it either change the limit or change the layout of the road or the environment In case of the first option
adapting the speed limit this must not be done at the expense of road safety A safe limit remains the starting point no matter what The roads function traffic composition potential conflict types traffic volume etc will always need to be considered (see also SWOV Fact sheet Measures for speed management) Finally the possibility remains that for certain reasons neither the speed limit nor the
road image can be changed when the limit is not credible An example is the 80 kmh speed limit on motorways for environmental reasons In these cases it is advisable to explicitly communicate the reason for the low limit to the road user as is done in eg Germany (Umweltschutz) and France (Pollution) It goes without saying that the reason given should also be credible What practical use is there for credible speed limits
All things considered the concept of credible speed limits has sufficient potential to be translated into actual practical applications and a first step has been taken towards drawing up a credibility checklist (Van Nes et al 2007a) In a simplified way the checklist has divided credibility into a limited number of separate elements that can be assessed easily However it is important to be aware that credibility involves an overall picture The starting point for drawing up the checklist was that the road layout was already in accordance with the Dutch guidelines for different road categories Furthermore five accelerators or decelerators were identified (see Table 1) Accelerators are elements of the road or environment that intuitively independent of the limit elicit
a higher speed Decelerators are elements of the road or environment that intuitively independent of the limit elicit a lower speed
Next a distinction was made between primary and secondary accelerators and decelerators Short road sections and physical speed limiters literally force drivers to drive at a lower speed On long tangents and when physical speed limiters are absent the physical obstacles to prevent high speeds are missing These two elements have been called primary accelerators and decelerators
Accelerators Decelerators
1 Tangents Long tangents Short tangents (many bends or intersections)
2 Physical speed limiters Physical speed limiters not present Physical speed limiters present
3 Openness of the situation Open clear road environment Closed inconveniently arranged road environment
4 Road width Wide road Narrow road
5 Road surface Smooth road surface Rough road surface
Table 1 Five features of road and road environment that can function as accelerators or decelerators The other three elements are secondary accelerators and decelerators an openclosed environment road width and road surface An open road environment offers a driver an unobstructed view both left
and right and thus will generally elicit acceleration In a closed environment the view is obstructed for example by buildings or vegetation which generally will elicit deceleration ndash as long as there is not too
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Road design standards - safety
The truth is that roads designed to standards are not safe
not unsafe nor are they appropriately safe roads designed
to standards have an unpremeditated level of safety
- Ezra Hauer
Speed - safety 32
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Swiss cheese model
Speed - safety 33
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed - safety 34
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed - safety 35
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Emergency vehicle pre-emption
Speed - safety 36
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Way forward
bull Speed management is mandatory in high activity
precincts
bull Credible speed limits
bull Identification of road features ndash reducing speed
bull ITS implementations for safety ndash needs further exploration
Speed - safety 37
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Referencesbull Mohan D (2016) lsquoSpeed and its effect on road traffic crashesrsquo in Transport
Planning and Traffic Safety Making cities roads and vehicles safer Taylor
and Francis
bull Mohan D (2017) Class handouts ndash Traffic Safety and environment
(CVL747) IIT Delhi
bull SWOV (2017) What measures for speed management are there
bull (httpswwwswovnlenfacts-figuresfactspeed-what-measures-speed-
management-are-there)
bull Mannering FL (2009) An empirical analysis of driver perceptions of the
relationship between speed limits and safety Transportation Research Part F
12 99ndash106
bull Elvik R (2017) Can evolutionary theory explain the slow development of
knowledge about the level of safety built into roads Accident Analysis and
Prevention 106 166ndash172
bull Michelin Report (2014) Road safety and connected mobility International
Safety amp Connected Mobility Task Force - Michelin Challenge Bibendum
Speed - safety 38
Speed - safety 39
Thank you
Speed - safety 39
Thank you