Geometric Design Guide for Canadian Roads

TO: Holders of the Geometric Design Guide for Canadian Roads (1999)

FROM: Transportation Association of Canada

SUBJECT: Updates to the Geometric Design Guide for Canadian Roads

Enclosed please find 54 new and/or revised pages for insertion into your copy of the GeometricDesign Guide for Canadian Roads. Revisions in this package will affect the following items:

Title Page – Part 1 and 2Chapter 1.2 – Design ControlsChapter 1.4 – Design ConsistencyChapter 4.3 – Index

To update your Guide simply follow these instructions:

Remove the Title Page from Part 1 and Part 2 and insert the new Title Pages.

For Chapter 1.2:

♦ Remove pages 1.2.i – 1.2.ii, 1.2.3.1 – 1.2.3.6, 1.2.R.1 – 1.2.R.2 from the Guide and insert newpages 1.2.i–1.2.ii, 1.2.3.1 – 1.2.3.9, 1.2.R.1 – 1.2.R.2 from new Chapter 1.2

♦ Leave pages 1.2.4.1 – 1.2.5.8 in the existing Chapter 1.2 of the Guide, since these are still validand remain unchanged.

For Chapter 1.4:

♦ Remove pages 1.4.i – 1.4.R.2 from the Guide and insert new pages 1.4.i – 1.4.R.2 that form thenew Chapter 1.4. You should now have a completely new Chapter 1.4 in your Guide.

For Chapter 4.3:

♦ Remove pages 4.3.1 – 4.3.8 from the existing Index and insert new pages 4.3.1 – 4.3.8.

Transportation Association of Canada2323 St. Laurent Blvd., Ottawa, Canada K1G 4J8Tel. (613) 736-1350 ~ Fax (613) 736-1395

www.tac-atc.ca

GeometricDesignGuide forCanadianRoadsPart 1

TransportationAssociation ofCanada

September 1999Updated December 2007

December 2007

© Transportation Association of Canada, 19992323 St. Laurent BoulevardOttawa, Canada K1G 4J8Tel. (613) 736-1350Fax: (613) 736-1395Web site: www.tac-atc.caISBN 1-55187-131-9

The Transportation Association of Canada is a national association with a mission to promote theprovision of safe, secure, efficient, effective and environmentally and financially sustainabletransportation services in support of Canada’s social and economic goals. The association is aneutral forum for gathering or exchanging ideas, information and knowledge on technical guidelinesand best practices. In Canada as a whole, TAC has a primary focus on roadways and their strategiclinkages and inter-relationships with other components of the transportation system. In urbanareas, TAC’s primary focus is on the movement of people, goods and services and its relationshipwith land use patterns.

L’ATC est une association d’envergure nationale dont la mission est de promouvoir la sécurité, lasûreté, l’efficience, l’efficacité et le respect de l’environnement dans le cadre de la prestation deservices financièrement durables de transport, le tout à l’appui des objectifs sociaux et économiquesdu Canada. L’ATC est une tribune neutre de collecte et d’échange d’idées, d’informations et deconnaissances à l’appui de l’élaboration de lignes directrices techniques et de bonnes pratiques.À l’échelle du pays, l’Association s’intéresse principalement au secteur routier et à ses liens etinterrelations stratégiques avec les autres composantes du réseau de transport. En milieu urbain,l’Association s’intéresse non seulement au transport des personnes et des marchandises, maisencore à la prestation de services à la collectivité et aux incidences de toutes ces activités sur lesmodèles d’aménagement du territoire.

DesignControls

Chapter 1.2

Design Controls

December 2007

Geometric Design Guide for Canadian Roads

Page 1.2.iDecember 2007

1.2 DESIGN CONTROLS

1.2.1 INTRODUCTION ........................................................................................ 1.2.1.1

1.2.2 HUMAN FACTORS ..................................................................................... 1.2.2.11.2.2.1 Expectancy .................................................................................... 1.2.2.11.2.2.2 Reaction ........................................................................................ 1.2.2.11.2.2.3 Design Response .......................................................................... 1.2.2.2

1.2.3 SPEED ........................................................................................................ 1.2.3.11.2.3.1 Introduction .................................................................................... 1.2.3.11.2.3.2 Desired Speed ............................................................................... 1.2.3.11.2.3.3 Design Speed ................................................................................ 1.2.3.21.2.3.4 Operating Speed ............................................................................ 1.2.3.31.2.3.5 Running Speed .............................................................................. 1.2.3.31.2.3.6 Posted Speed ................................................................................ 1.2.3.41.2.3.7 Limitations of Design Speed Approach .......................................... 1.2.3.51.2.3.8 Design Domain: Design Speed Selection ...................................... 1.2.3.6

1.2.4 DESIGN VEHICLES..................................................................................... 1.2.4.11.2.4.1 Introduction .................................................................................... 1.2.4.11.2.4.2 Vehicle Classifications ................................................................... 1.2.4.11.2.4.3 Vehicle Characteristics .................................................................. 1.2.4.21.2.4.4 Vehicle Turning Paths .................................................................. 1.2.4.101.2.4.5 Selecting a Design Vehicle .......................................................... 1.2.4.10

1.2.5 SIGHT DISTANCE ....................................................................................... 1.2.5.11.2.5.1 Criteria Used in Calculating Sight Distance ................................... 1.2.5.11.2.5.2 Stopping Sight Distance ................................................................ 1.2.5.21.2.5.3 Passing Sight Distance.................................................................. 1.2.5.51.2.5.4 Decision Sight Distance ................................................................. 1.2.5.7

REFERENCES........................................................................................................ 1.2.R.1

Design Controls

Page 1.2.ii December 2007

FiguresFigure 1.2.3.1 Operating Speed Approach for Design of Two-lane,

Two-way Roadways ...................................................................... 1.2.3.9Figure 1.2.4.1 Passenger Car (P) Dimensions .................................................... 1.2.4.4Figure 1.2.4.2 Light Single-Unit (LSU) Truck Dimensions .................................... 1.2.4.5Figure 1.2.4.3 Medium Single-Unit (MSU) Truck Dimensions ............................... 1.2.4.5Figure 1.2.4.4 Heavy Single-Unit (HSU) Truck Dimensions .................................. 1.2.4.6Figure 1.2.4.5 WB-19 Tractor-Semitrailer Dimensions ........................................ 1.2.4.6Figure 1.2.4.6 WB-20 Tractor-Semitrailer Dimensions ........................................ 1.2.4.7Figure 1.2.4.7 A-Train Double (ATD) Dimensions................................................. 1.2.4.7Figure 1.2.4.8 B-Train Double (BTD) Dimensions ................................................ 1.2.4.8Figure 1.2.4.9 Standard Single-Unit (B-12) Bus Dimensions................................ 1.2.4.8Figure 1.2.4.10 Articulated Bus (A-BUS) Dimensions ............................................ 1.2.4.9Figure 1.2.4.11 Intercity Bus (I-BUS) Dimensions .................................................. 1.2.4.9Figure 1.2.5.1 Elements of Passing Sight Distance ............................................. 1.2.5.6Figure 1.2.5.2 Required Passing Sight Distance for Passenger

Cars & Trucks in Comparison with AASHTO Criteria .................... 1.2.5.7

TablesTable 1.2.2.1 Perception and Reaction Time Design Domain ............................. 1.2.2.2Table 1.2.4.1 Design Dimensions for Passenger Cars ....................................... 1.2.4.2Table 1.2.4.2 Design Dimensions for Commercial Vehicles ................................ 1.2.4.2Table 1.2.4.3 Design Dimensions for Buses ....................................................... 1.2.4.3Table 1.2.4.4 Minimum Design Turning Radii for Representative Trucks,

for 180º Turns ................................................................................ 1.2.4.3Table 1.2.5.1 Object Height Design Domain ....................................................... 1.2.5.2Table 1.2.5.2 Coefficient of Friction for Wet Pavements ..................................... 1.2.5.3Table 1.2.5.3 Stopping Sight Distance for Automobiles

and Trucks with Antilock Braking Systems .................................... 1.2.5.4Table 1.2.5.4 Stopping Sight Distances for Trucks

with Conventional Braking Systems .............................................. 1.2.5.5Table 1.2.5.5 Minimum Passing Sight Distance .................................................. 1.2.5.6Table 1.2.5.6 Decision Sight Distance................................................................. 1.2.5.8


Page 1.2.3.1December 2007

1.2.3 SPEED

1.2.3.1 Introduction

Minimizing travel time is usually one of the mostimportant concerns to a driver in selectingalternate routes. The value of a road in carryingpeople and goods is judged by its convenienceand economy, which are directly related to itstravel speed.

The speed of vehicles on a roadway depends, ingeneral, on several conditions:

• vehicle operating capabilities

• driver capability, behaviour and comfort

• physical characteristics of the road and itssurroundings

• weather

• roadway conditions

• presence of other vehicles

• posted speed limits

The effects of these conditions are usuallycombined although, under varyingcircumstances, one or other of the factors maygovern.

The speed of vehicles on a roadway has asignificant bearing on safety, particularly in termsof the severity of collisions. However, therelationship of speed to the probability of acollision is not as evident, since collisions arecomplex events that can seldom be attributedto a single factor. It is now widely believed thatcollision rate is more directly affected by speedvariations than by speed per se, given thatintuitively, the probability of conflicts would belower if all vehicles were travelling at the samespeed (this has lead to the introduction ofminimum speed limits for some applications).

A recent study15 showed that both faster andslower drivers are more likely to be involved incollisions. In an analysis of speed variance, thestudy found that the major contributing factorwas the difference between design speed and

posted speed limit. For Virginia highways, theminimum speed variance was obtained when thedesign speed was 10-20 km/h higher than theposted speed. However, this finding isconfounded by their other finding that mean speedvariance and design speed do not actindependently on collision rates since they arecorrelated.

The designer needs to be cognizant of thegeneral effects of speed on safety. Collisionfrequency will be reduced on roads that do notrequire drivers to make large speed adjustmentsand that promote uniformity of speeds. Collisionseverity will increase with speed. More severecollisions not only increase the cost toindividuals and society of each collision, butresult in more collisions being reported, showingan apparent increase in collision rate. A designchoice that promotes speed is not necessarilybad since road users value the time savings.However, anticipation of the increase in speedmay call for other design improvements to bemade that compensate for the expected increasein collision severity.

In designing a road, the features and dimensionschosen must be appropriate for the travel speed.While this is a simple concept, its interpretationis complex, because the relationship betweenroad design and speed is a circular one. Whilethe designer shapes elements of the road bythe anticipated speed at which they are intendedto be used, the speed at which they will be useddepends to some extent on their chosen design.In this, the design of roads differs from the designof most other engineered systems.

1.2.3.2 Desired Speed

Desired speed is the speed at which a driverwishes to travel, determined by a combinationof motivation and comfort. Motivation is heavilyinfluenced by the length and urgency of the trip:as these factors increase, desired speed alsoincreases.

Desired speed is influenced, as part of thecomfort factor, by the driver’s expectations ofthe characteristics and quality of the road ahead.Expectations are, in turn, based on the driver’sperception of the prevailing topographic,environmental, traffic and climatic conditions.

Design Controls

Page 1.2.3.2 December 2007

Where severe topographic conditions areencountered, the driver expects to travel at lowerspeeds and is more likely to adjust to a geometricdesign consistent with a lower design speedthan where there is no apparent reason for it.Drivers do not necessarily adjust their speedsto an arbitrarily chosen design speed for aroadway, but to the physical limitations and theprevailing traffic conditions.

1.2.3.3 Design Speed

Design speed is a speed selected as a basis toestablish appropriate geometric design elementsfor a particular section of road. These designelements include horizontal and verticalalignment, superelevation and sight distance.Other elements such as lane width, shoulderwidth, sideslope ratio and clearance fromobstacles are related indirectly to design speed.

The selected design speed should be a logicalone with respect to the character of terrain,anticipated operating speed, adjacent land use(ie. urban or rural in character), and the roadclassification system as described in theChapter 1.3 of this guide. As noted elsewhere inthis section, urban and rural freeways andprimary rural arterials should be designed withthe highest practical design speed to promotea desired degree of traffic mobility, safety, andefficiency within the constraints of environmentalquality, economics and aesthetics. However,local roads are often subject to speed controlsthrough measures such as traffic calming andhence the design speed is likely to be dictatedby a speed management policy.

When planning new facilities, appropriate designspeeds should be chosen that are consistentwith the road function as perceived by the driver24.Redesigning existing roads to achieve greatercongruity between driver perceptions ofappropriate operating speeds and cues providedby the road itself (e.g. narrowing lanes) haspromise. Because of the size of the network,however, road redesign is a long term strategyand more understanding of the overall safetybenefits of alternative designs is needed.27

When designing a substantial length of road, itis desirable, although it may not be feasible, toassume a constant design speed. Changes interrain and other physical controls may dictate

a change in design speed on certain sections.Each section, however, should be of relativelylong length, compatible with the general terrainor development through which the road passes.The justification for introducing a reduced designspeed should be obvious to the driver. Moreover,the introduction of a lower or higher design speedshould not be effected abruptly but over sufficientdistance to encourage drivers to change speedgradually.

Differences in design speed from one segmentto another should not be more than 20 km/h.Drivers should be warned well in advance. Atransition section allowing for speed reductions,as from 100 to 90 to 80 km/h, should beprovided. Thus, the changing condition shouldcomprise extra long (anticipatory) sightdistances, speed-zone signs, curve speed signs,and so on.

Roads intended to provide high mobility, suchas freeways and expressways, should bedesigned with the highest practical design speedto promote traffic mobility, efficiency and safety.Pedestrians and cyclists are either discouragedor prohibited from utilizing these types of roads,so provision should be made for a speed thatsatisfies nearly all drivers (the 85th percentiledesired speed is typically used). Only a smallpercentage of drivers travel at extremely highspeed, and it is not economically feasible todesign for them.

Ideally, then, design speed should be chosen toreflect the 85th percentile desired speed, andthis is often achievable for roads for which theprimary function is mobility and where severephysical constraints do not exist. Studies16 haveshown that the 85th percentile desired speeddoes not generally exceed 120 km/h forunhindered vehicles on a four-lane dividedroadway. Use of a design speed of 120 km/hshould therefore satisfy driver demands in mostareas, although design speeds of 130 km/h areused in some jurisdictions.

The selected design speed should be a logicalone with respect to the topography and thefunctional classification of a road, but carefulconsideration should also be given to itsrelationship with other definitions of speed. Whileno hard relationships have been established,



choice of design speed can simultaneouslyaccommodate and influence desired speed,operating speed, running speed and postedspeed.

1.2.3.4 Operating Speed

Operating speed is the speed at which a driveris observed operating a vehicle (a “spot” speedat a particular location). For an individual driver,operating speed will generally be lower thandesired speed, since operating conditions arenot usually ideal. The operating speed of allvehicles at a particular location is reported aseither a mean or 85th percentile operating speed.

Where constraints exist, such that the designspeed will be less than the desired speed formany drivers, it is important that drivers are givenclear advance warning that they should modifytheir speed desires, as studies15 have shownthat collision rates increase as operating speedof a particular vehicle deviates from the meanoperating speed of the other vehicles on theroadway.

The typical driver can recognise or sense alogical operating speed for a given roadwaybased on knowledge of the system, posted speedlimits, appraisal of the ruggedness of the terrain,traffic volumes and the extent, density and sizeof development. Studies26 have shown thatcharacteristics such as the number of accesspoints, nearby commercial development, roadwidth, and number of lanes have a significantinfluence on vehicle speeds. Based on thesefactors, the driver will adjust speed to beconsistent with the conditions expected to beencountered. The driver’s initial response is toreact to the anticipated situation rather than tothe actual situation. In most instances, the twoare similar enough that no problems are created.When the initial response is incorrect operationand safety may be severely affected.

Some agencies conduct speed surveys todetermine operating speeds at various pointsalong a section of roadway. The results can becompared with the design speed used, and maylead to a policy change in the selection of designspeeds.

1.2.3.5 Running Speed

While operating speed is tied to specificlocations, running speed is defined19 as theaverage or 85th percentile speed of all vehiclesalong a specific roadway as determined by thedistance and the running times (travel timesminus involuntary delay stops) between twoselected points, usually major intersections. Fordesign purposes, the running speed during off-peak traffic periods and under favourable roadwayand weather conditions is an importantconsideration, since it is helpful to know actualvehicle speeds for traffic en masse to beexpected on roadways of different design speedsand various volume conditions. Running speedis one measure of the service that a roadwayrenders, and it affords a means of evaluating usercosts and benefits.

Running speed on a given roadway variessomewhat during the day, depending primarilyon the volume of traffic. Therefore, when referenceis made to average or 85th percentile runningspeed it should be clear whether this speed isfor peak hours or off-peak hours or whether it isan average for the day. The first two are ofconcern in design and operation; the latter is ofimportance in economic analyses.

Running speed is likely to govern the choice ofdesign speed on roads with high traffic volumes,particularly in an urban setting. Local roads, inparticular, are designed with consideration forthe safety of all users, including pedestrians andcyclists. They are subject to highly conflictinguses and movements, provide a high degree ofaccess and often accommodate parking. Thus,low design speeds are appropriate for local roadswith low mobility requirements, frequent access,and significant pedestrian and cyclist activity.

Urban arterial roads should be designated andcontrol devices regulated, where feasible, topermit running speeds of 30 to 70 km/h. Lowerspeeds in this range are applicable for local andcollector roads through residential areas and forarterial roads through the more crowdedbusiness areas, while the higher speeds applyto arterials in the outlying suburban areas. Forarterial roads through crowded business areas,coordinated signal control through successive

Design Controls


intersections generally is necessary to achieveeven the lower speeds. Many cities havesubstantial lengths of roads controlled so as tooperate at running speeds of 25 to 40 km/h. Atthe other extreme, in suburban areas, it iscommon experience on preferred roads to adoptsome form of speed zoning or speed control toprevent high operating speeds. In these areas,the infrequent pedestrian or occasional vehicleson a cross street may be exposed to potentialcollisions with through drivers, who gradually gainspeed as the frequency of urban restrictions areleft behind or retain the speed of the open roadas they enter the city.

Since much of urban road design is retrofit, theselection of design speed of roads in the urbanenvironment is often an iterative process in whichthe selection is influenced by the attainablegeometric features. An understanding ofexpected traffic running speeds is important sothat the designer can analyse a retrofit projectto ascertain the safety and efficiency of thedesign.

1.2.3.6 Posted Speed

Speed control, aimed at encouraging drivers totravel at an appropriate speed for prevailingconditons, encompasses enforcement,education and engineering techniques. Whilepolice enforcement has been the traditionalapproach to controlling speeds, research24 hasshown that significant increases in enforcementlevels are required to influence driver behaviour,and those effects are relatively short lived. Theposted speed is a speed limitation, consciouslyintroduced for reasons of safety and economy,traffic control and government regulatory policies.Consequently, the selection of posted speed isa traffic operations consideration rather than ageometric design element. Conversely, however,geometric design elements must be consideredin determining posted speed.

Design speed would appear to be the mostnatural criterion from the road agency’s viewpointin selecting a posted speed limit since bydefinition it is supposed to be the safe speed atwhich the road can be negotiated. However, thedesign speed criterion can be unrealistic from adriver’s point of view since speed affects thedesign of relatively few elements even though it

is used to classify an entire road segment. Asa result, the speed limit might appear to beunreasonable to the driver and thus lead tosubstantial speed limit violations.

Further demonstrating the effects of designelements, experience has shown that thecollision rates on freeways, which have thehighest legislated speed limits, are two to threetimes lower than two-lane, two-way highways.This safety benefit is in large part due to thedesign features of the roadway: separation ofopposing traffic; dual lanes eliminate passingconflicts; limited access to the roadway; trafficflows are separated at the interchanges; andwide, relatively flat and object free roadsidesreduce the severity of collisions when vehiclesrun off the roadway.

Design speed should not be deduced fromanticipated posted speed limits, since these canbe changed arbitrarily by policy makers. On thecontrary, a posted speed limit should be appliedat or below design speed. Where, for reasons ofconstraints, a design speed is used belowanticipated 85th percentile levels of desired orrunning speeds, continuing enforcement ofappropriate speed limits is important in reducingthe collision rate on a road.

It should be recognized that, while posted speedcan readily be changed after the road isconstructed, design speed is reflected in thephysical features of the road and cannot bealtered without reconstruction. Simply changingthe legislated speed limits has little effect ondriver behaviour. A recent study25 investigatedthe effects of raising and lowering the speed limitat 100 experimental sites on non-limited accesshighways in 22 U.S. states. A summary of theresults indicated that, overall, raising or loweringthe speed limits had little effect on the driver’sspeed choice, and did not lead to anystatistically significant changes in either totalor severe collisions. Arbitrarily posted reducedspeed zones are therefore not likely to operateeffectively. To be effective, the posted speedshould be consistent with prevailingtopographical and development conditions andsubject to reasonable enforcement. If a lowposted speed is anticipated at the outset of adesign, the selected design speed should notbe artificially lowered since a well-chosen design



speed will produce a safer road and thepossibility of a higher posted speed applied afterconstruction should be recognized.

Where a design speed can be selected to matcha high percentile desired speed, it appearsreasonable to set the posted speed at the designspeed, i.e. at the 85th percentile desired orrunning speed, rounded to the nearest 10 km/h.Speed limits set higher make very few additionaldrivers “legal” for each 10 km/h increment ofspeed increased. Conversely, speed limits setlower make a significant number of reasonabledrivers “illegal” for each 10 km/h increment ofspeed decreased, place unnecessary burdenson law enforcement personnel, lead to a lack ofcredibility of speed limits and lead to increasedtolerance by enforcement agencies14. Based onstudies16 which showed that the 85th percentiledesired speed does not generally exceed120 km/h for unhindered vehicles on a four-lanedivided roadway, it is logical to set the maximumposted speed at 120 km/h, where interchangesare provided and access is controlled.

Situations can arise where the posted speedexceeds the design speed. This is not surprisingand is not necessarily unsafe, since traditionallydesigned geometric components of a road mayincorporate considerable margins of safety,particularly for good vehicle and weatherconditions. Nevertheless, in order to avoid liabilityconcerns, this situation should give rise to anevaluation of the design of the road and theposting of advisory speed limits at criticallocations such as horizontal or vertical curves13.

Posted speeds may also be established on thebasis of specific traffic operation studies. In thesestudies other elements are often consideredincluding factors such as collision experience,the nature and intensity of adjacent land use,parking practices, access frequency andpedestrian activity.

For all types of roads in an area, it is desirableto provide a reasonable degree of uniformity inthe design speeds, operating speeds, andsubsequently the posted speeds selected withineach classification subgroup or group. Forexample, the posted speed for all minor arterialroads within a municipality should be identical

or near identical. Driver expectations are met inthis manner.

1.2.3.7 Limitations of DesignSpeed Approach

As noted in Subsection 1.2.2.3, consistency ofdesign is fundamental to good driverperformance, based on satisfying the driver’sexpectations. Design consistency exists whenthe geometric features of a continuous sectionof road are consistent with the operationalcharacteristics as perceived by the driver. Thetraditional approach to achieving designconsistency has been through the applicationof the design speed process. Once selected,the design speed is used to determine valuesfor the geometric design elements fromappropriate design domains.

However, application of this procedure alone doesnot guarantee design consistency. There areseveral limitations of the design speed conceptthat should be considered during design:

1. Selection of dimensions to accommodatespecified design speed does not necessarilyensure a consistent alignment design.Design speed is significant only whenphysical road characteristics limit the speedof travel. Thus, a road can be designed witha constant design speed, yet haveconsiderable variation in speeds achievableand therefore to a driver appear to have awide variation in design basis. For example,to maintain consistency, the design speedof curves within a road section should beuniform, not merely greater than someminimum design speed.

2. For horizontal alignments, design speedapplies only to curves, not to the tangentsthat connect those curves. Design speedhas no practical meaning on tangents. As aresult, the maximum operating speed on atangent, especially a long one, can oftensignificantly exceed the design speed of thehorizontal curves at either end of thetangent.

3. The design speed concept does not ensuresufficient coordination among individualgeometric features to ensure consistency.

Design Controls


It controls only the minimum value of themaximum speeds for the individual featuresalong an alignment. For example, a roadwith an 80 km/h design speed could haveonly one curve with a design speed of 80 km/h and all other features with design speedsof 110 km/h or greater. As a result, operatingspeeds approaching the critical curve arelikely to exceed the 80 km/h design speed.Such an alignment would comply with an80 km/h design speed, but it might violatea driver’s expectancy and result inundesirable operating speeds.

4. As discussed previously, vehicle operatingspeed is not necessarily synonymous withdesign speed. Drivers normally adjust speedaccording to their desired speed, postedspeed, traffic volumes and the perceivedalignment hazards. The perception of hazardpresented by the alignment may vary alonga road designed with a constant designspeed. The speed adopted by a driver tendsto vary accordingly and may exceed thedesign speeds. A report20 on studies inAustralia and the US concluded that85th percentile operating speedsconsistently exceed design speeds, wherethose design speeds are less than 100 km/h, at horizontal curves on rural two-lanehighways.

5. In addition, different alignment elementsmay have quite different levels of perceivedhazard. Entering a horizontal curve too fastwill almost certainly result in loss of control,so drivers adjust their speed accordingly.However, the possibility of a curtailed sightdistance concealing a hazard is consideredas a remote occurrence. Drivers do notgenerally adjust their speed to compensatefor sight distance restrictions.

6. The design speed concept could preventinconsistent operating-speeds if driverscould be presumed to know the designspeed of the road and choose an operatingspeed less than or equal to that designspeed, even though they may be comfortableat higher speeds along most of thealignment. That presumption isunreasonable, even if posted speed is set

at design speed. Therefore, the designspeed concept does not provide asystematic mechanism for preventinggeometric inconsistencies.

In order to help overcome these weaknesses inthe use of design speed to design individualgeometric elements, speed profiles are used. Aspeed profile is a graphical depiction (which canbe modelled) showing how the 85th percentileoperating speed varies along a length of road.This profile facilitates an examination of thedesign to identify undesirably large differentialsin the 85th percentile operating speed betweensuccessive geometric elements, e.g. a curvefollowing a tangent.

For an existing road being considered forimprovement, actual operating speeds can bemeasured to create a speed profile, butinterpretation of the profile can be difficult,depending on the complexity of geometric andother features which may cause drivers to changespeed. For a new road, some prediction ofoperating speeds is needed to create a speedprofile model, and a methodology for doing so isoutlined in Chapter 1.4.

1.2.3.8 Design Domain: DesignSpeed Selection

General Application Heuristics

The following factors influence the choice ofdesign speed and are presented in the form ofguidelines only.

• Design speed should be greater than orequal to the legal posted speed. A designspeed equal to the posted speed may bewarranted by such factors as low trafficvolumes, mountainous terrain, or economicconsiderations. This practice is appropriatefor minor collectors, local roads, municipalroads, and some secondary highways

• Design speed choice should reflect the 85th

percentile desired speed.

• The overall range in design speeds is 20km/h to 130 km/h and the design speedincrements are 10 km/h.



• A lower design speed should not beautomatically assumed for a secondary ruralhighway or low volume primary ruralhighway, where the terrain and speedenvironment are such that drivers are likelyto travel at higher speeds.

Rural Highways

Two-lane Locals, Collectors and Arterials

The following operating-speed based approachis recommended for use by designers seekingto establish appropriate design speeds on ruraltwo-lane, two-way highways. This approach isillustrated in Figure 1.2.3.1and consists of thefollowing steps:

• Select a nominal (trial) design speed.

• Select the design parameters for vertical andhorizontal alignment and other highwaygeometric elements.

• Develop a trial alignment.

• Estimate the 85th percentile speeds on thetrial alignment. (See Chapter 1.4)

• Check consistency; does the estimatedspeed match the design speed?

• If it does finalize the design.

• If the estimated speeds do not match thedesign speed can the alignment bemodified? If so, develop a new trialalignment.

• If the alignment cannot be modified, selectanother nominal (trial) design speed andrepeat the process.

This approach is consistent with basic designconsistency principles

Commonly used design speeds for primary ruralarterial two-lane highways in Canada varybetween 100 and 120 km/h in rolling and levelterrain. In mountainous terrain commonly useddesign speeds are 80 – 100 km/h. A designspeed equal to the legal posted speed of 90 –100 km/h is the normal practice for secondary

highways in some jurisdictions. Similarly, adesign speed of 80 km/h and a posted speed of80 km/h is the normal practice for rural municipalroads in some jurisdictions.

Divided Arterials, Expressways,and Freeways

A design speed of 110, 120 or 130 km/h shouldbe used for rural divided arterials, expressways,and freeways.

Urban Roadways

General Application Heuristics for UrbanRoadways

• In the urban environment, all users ratherthan just the motorist should be taken intoconsideration when selecting the designelements that affect speed along a street.Users other than the motorist includepedestrians, cyclists, and public transitriders24.

• Speed considerations that may influencedesign includes design speed, desiredspeed, and operating speed. A directrelationship between posted speed anddesign speed is particularly valid for newstreet facilities in the upper range of thestreet classification system such asfreeways, expressways, and major arterials.

• For design purposes, the average runningspeed during off-peak traffic periods andunder favourable street and weatherconditions is an important consideration.

• Where the urban street design is retrofit,the design speed of streets in the urbanenvironment is often an iterative process inwhich the selection is influenced by theattainable geometric features.

• Unlike rural design providing as high adesign speed as practical is not the primaryobjective.

• Selection of the most appropriate designspeed should be made on the basis of theintended service function and needs of theexpected users.

Design Controls


• The choice of design speed in the urbanenvironment will also be influenced by theconstraints of economics, social impacts,environmental controls, and aesthetics.

• The design speed selected should beconsistent with driver expectations alongthat particular urban roadway. Prevailingconditions that should be considered in thisregard include adjacent land-use,intersection spacing, access conditions,and vulnerable road user activity.

Locals and Collectors

• Choosing too high a design speed for anurban local or collector street can inducedrivers to travel beyond the safe speed fortheir surroundings and pose significant risksto vulnerable road users such as pedestriansand cyclists.

• Low design speeds are appropriate for localstreets with low mobility requirements,frequent access, and significant pedestrianand cyclist activity. Local streets must bedesigned with the vulnerable road user inmind, including cyclists and pedestrians.

• The designer’s choice of design speed forcollectors and local roadways must considerthe role and presence of the vulnerable roaduser. The conventional approach to roaddesign includes design speed choices of 30– 50 km/h for local roads and 50 – 80 km/hfor collector roads.

Arterials

• There are important differences between thecriteria applicable to low and high-speeddesigns. In general, because of thesedistinct differences, the upper limit for low-speed design is 70km/h and the lower limitfor high-speed design is 80 km/h.

• Urban arterials generally have runningspeeds of 30 to 70 km/h. It follows that theappropriate design speeds for arterialsshould range from 50 to 100 km/h. Thedesign speed selected for an urban arterialwill depend largely on the spacing of

signalized intersections, the selected typeof median cross section, the presence orabsence of curb and gutter along the outsideedges of the traveled way, and the amountand type of access to the street.

Expressways and Freeways

Commonly used design speeds for urbanfreeways in Canada vary between 90 km/h and120 km/h.

Design Speed Choices for RehabilitationProjects

Identifying the design speed is one of the initialsteps in the rehabilitation project designprocess. Rehabilitation projects are often referredto as 3R/4R projects – a term that refers to thevarious levels of rehabilitation (….) that areembraced by the more general term“rehabilitation”. The TAC Guide for 3R/4R28 notesthat there is often a poor relationship betweenthe 85th percentile speed, the original designspeed, and the posted speed. In many cases,in particular with older roads that have evolvedover time, a design speed may never have beenestablished or is not known. For 3R/4R projectsthe design speed should reflect actual operatingspeeds, not necessarily the legal speed limit,since drivers are more apt to accept a lowerspeed limit where a difficult condition is obviousthan where there is no apparent reason for it. Inmost cases, drivers adjust their speeds tophysical limitations and traffic.

The recommended practice for 3R/4R projectsis to define the design speed as the existing85th percentile speed on a roadway. Desirably,the 85th percentile speed should be measuredfor each project and the operational design speedprocedure shown in Figure 1.2.3.1 followed.Where this is not practical, system wide typicalvalues, for specific roadway classifications andother influencing characteristics such asroadway geometry, terrain and adjacent land usecan be utilized. In some cases, due to policy orthe desire to promote corridor continuity, forexample, it may be desirable to define the 3R/4R design speed as a value other than 85th

percentile speed.28, 29 Selection of design speedon this basis is part of context sensitive design33

or design for ambient conditions.30, 31, 32, 34



Figure 1.2.3.1 Operating Speed Approach for Design of Two-lane,Two-way Roadways

Design Controls



Page 1.2.R.1December 2007

REFERENCES

1. Hall, J.W. and Turner, D.S. Stopping SightDistance: Can We See Where We NowStand? in Transportation Research Record1208, TRB, National Research Council,Washington, D.C., 1989.

2. Woods, D.L. Sensitivity Analysis of the Fac-tors Affecting Roadway Vertical Curve De-sign. Presented at 68th Annual Meeting ofTRB, January 1989.

3. Conceptions Routiers: Normes, Ouvrages,Routiers. Quebec Ministere des Transports,1995.

4. A Policy on Geometric Design of Highwaysand Streets. AASHTO, Washington, D.C.,1994.

5. Harwood, D.W. and Glennon, J.C. Pass-ing Sight Distance for Passenger Cars andTrucks, in Transportation Research Record1208, TRB, National Research Council,Washington, D.C., 1989.

6. Sanderson, R.W. The Need for Cost-Effec-tive Guidelines to Enhance Truck Safety.Presented to the Transportation Associa-tion of Canada Conference, 1996.

7. Passing Manoeuvres and Passing Lanes:Design, Operational & Safety Evaluations.ADI Limited for Transport Canada SafetyDirectorate, Ottawa, Ontario, 1989 (unpub-lished).

8. Harwood, D.W., Mason, J.M., Glauz, W.D.,Kulakowski, B.T. and Fitzpatrick, K. TruckCharacteristics for Use in Roadway Designand Operation. FHWA-RD-89-226 and 227,1990.

9. Lamm, R., Choueiri, E.M., Hayward, J.C.and Paluri, A. Possible Design Procedureto Promote Design Consistency in HighwayGeometric Design on Two-Lane RuralRoads, in Transportation Research Record1195, TRB, National Research Council,Washington, D.C., 1988.

10. Lamm, R., Heger, R. and Eberhard, O. Op-erating Speed and Relation Design Back-grounds - Important Issues to be Regardedin Modern Highway Alignment Design, pre-sented at the IRF World Meeting, Toronto,ON, 1997.

11. Alexander, G.J. and Lunenfeld, H., DriverExpectancy in Highway Design and TrafficOperations. Report No. FHWA-TO-86-1,Federal Highway Administration, US De-partment of Transportation, Washington,D.C., 1986.

12. Design Vehicle Dimensions for Use in Geo-metric Design, by Lea Associates for theTransportation Association of Canada, Ot-tawa, ON, 1996.

13. Fitzpatrick, K., Shamburger, B. andFambro, D. Design Speed, OperatingSpeed and Posted Speed Survey, pre-sented at Transportation Research Boardmeeting, Washington, D.C., 1996.

14. Krammes, R.A., Fitzpatrick, K., Blaschke,J.D. and Fambro, D. Speed - Understand-ing Design, Operating and Posted Speed.Report No. 1465-1, Texas TransportationInstitute, 1996.

15. Garber, N.J. and Gadiraju, R. Factors Af-fecting Speed Variance and its Influence onAccidents, in Transportation ResearchRecord 1213, TRB, National ResearchCouncil, Washington, D.C., 1989.

16. Gambard, J.M. and Louah, G. VitessesPractiquees et Geometrie de la Route,SETRA, 1986.

17. Ogden, K.W. Safer Roads - A Guide toRoad Safety Engineering, Ashgate Publish-ing Co., Brookfield, VT, 1996.

18. Olson, P. Forensic Aspects of Driver Per-ception and Response. Lawyers & JudgesPublishing Company, Inc., Tucson, AZ,1996.

Design Controls

Page 1.2.R.2 December 2007

19. Highway Capacity Manual. Special Report209. Transportation Research Board, Na-tional Research Council, Washington, DC,1994.

20. Krammes, R.A., Brackett, R.Q., Shafer,M.A., Ottesen, J.L., Anderson, I.B., Fink,K.L., Collins, K.M., Pendleton, O.J.,Messer, C.J. Horizontal Alignment DesignConsistency for Rural Two Lane Highways,FHWA-RD-94-034, 1993.

21. Prisk, C.W. Passing Practices on RuralHighways. Proceedings of the Highway Re-search Board, Vol. 21, 1941, in A Policy onGeometric Design of Highways and Streets.AASHTO, Washington, D.C., 1994.

22. Shelbume, T.E. and Sheppe, R.L. SkidResistance Measurements of VirginiaPavements. Research Report No. 5-B.Highway Research Board, April 1948, in aPolicy on Geometric Design of Highwaysand Streets. AASHTO, Washington, D.C.,1994.

23. Moyer, R.A. and Shupe, J.W. Roughnesssand Skid Resistance Measurements ofPavements in California. Bulletin 37. High-way Research Board, August 1951, in aPolicy on Geometric Design of Highwaysand Streets. AASHTO, Washington, D.C.,1994.

24. Safety, Speed and Speed Management: ACanadian Review. IBI Group for TransportCanada, 1997.

25. Parker, M.R. Effects of Raising and Lower-ing Speed Limits on Selected RoadwaySections. FHWA-RD-92-084, Washington,D.C, 1997.

26. Synthesis of Safety Research Related toSpeed and Speed Management. FHWA-RD-98-154, Washington, D.C., 1998.

27. Managing Speed: Review of Current Prac-tice for Setting and Enforcing Speed Lim-its. Special Report 254, TRB, National Re-search Council, Washington, D.C., 1998.

28. Canadian Guide to 3R/4R. TransportationAssociation of Canada, Ottawa, 2001.

29. Highway Safety Design and OperationsGuide. ASSHTO, Washington, D.C., 1997.

30. Context Sensitive Highway Design. ASCE,1999.

31. Thinking Beyond the Pavement. AASHTO,1998.

32. Flexibility in Highway Design. FederalHighway Administration. Washington, D.C.,1997.

33. Kassoff, H., Contextual Highway Design:Time to be Mainstreamed. ITE Journal,December 2001.

34. Beyond the Green Book. TransportationResearch Board. Washington, D.C., 1987.

DesignConsistency

Chapter 1.4

Design Consistency

December 2007


Page 1.4.iDecember 2007

1.4 DESIGN CONSISTENCY

1.4.1 INTRODUCTION .......................................................................................... 1.4.1.1

1.4.2 CROSS SECTION CONSISTENCY............................................................. 1.4.2.1

1.4.3 OPERATING SPEED CONSISTENCY ........................................................ 1.4.3.1

1.4.3.1 Prediction of Operating Speeds .................................................... 1.4.3.1Horizontal Alignment Approach .................................................... 1.4.3.2Speed Profile Model 1 .................................................................. 1.4.3.2Horizontal and Vertical Alignment Approach ................................ 1.4.3.2Speed Profile Model 2 .................................................................. 1.4.3.3Horizontal and Vertical Model Special Aspects ............................ 1.4.3.5

1.4.4 DRIVER WORKLOAD CONSISTENCY ...................................................... 1.4.4.1

1.4.5 SAFETY AND CONSISTENCY .................................................................... 1.4.5.1

REFERENCES........................................................................................................ 1.4.R.1

TablesTable 1.4.3.1 Equations for Estimating Operating Speed on Various

Types of Speed-Limiting Curves ................................................ 1.4.3.7Table 1.4.3.2 Equations for Speed-Profile Model ............................................. 1.4.3.8

Figures

Figure 1.4.3.1 Variable Definitions for Speed-Profile Model .............................. 1.4.3.6Figure 1.4.5.1 Mean Collision Rate versus Mean Speed Reduction

Between Geometric Elements .................................................... 1.4.5.2Figure 1.4.5.2 3-year Collision Frequency versus Mean Speed

Reduction Between Geometric Elements ................................... 1.4.5.3

Design Consistency

Page 1.4.ii December 2007



1.4.1 INTRODUCTION

Chapter 1.2 describes how driver reactions aredetermined in large measure by how well theirexpectancies are satisfied. Suggestions aremade about how designers should respond toreduce/eliminate uncertainty or the unexpectedfor drivers. An important component of thisresponse is design consistency. The moreconsistent road designs are over a widegeographic area, the more effective thedesigner’s contribution will be to reducingcollision occurrence.

Many different classes of roads are required toserve different purposes, as described inChapter 1.3. Furthermore, terrain conditionsvary widely from one region to another acrossCanada, requiring different approaches todesign. However, the aim of Canadiandesigners must be to achieve consistency indesign of each classification of road, in eachtype of terrain, regardless of location. Thisobjective is a primary justification for theexistence of this guide.

As long ago as 1978, it was pointed out thatthere was a general lack of explicit criteria forthe contiguous combination of basic designelements or for the longitudinal variations ofsuch features as horizontal alignment, verticalalignment, and cross section.1

In this chapter, three principles are suggestedby which a designer can evaluate consistencyof a road design:

• Cross section consistency

• Operating speed consistency

• Driver workload consistency

However, the most valuable tool for evaluatingdesign consistency is actual collisionexperience, which should, for any existingfacility, be used as a basis for the designconsistency review.

When inconsistencies cannot be avoided atcertain isolated locations because of highconstruction cost, land use restrictions, orsevere environmental impacts, drivers shouldbe given advance warning and other visualcues to alert them to the inconsistency.

Designers should note that the research workson which these consistency measures arebased dealt only with two-lane rural highways.The same principles, however, can be appliedto other classes of roads.

The same research has also provided valuableguidance to designers regarding the potentialroad safety performance implications of designconsistency considerations. These aresummarized in Section 1.4.5 of this Chapter.

Design Consistency




1.4.2 CROSS SECTIONCONSISTENCY

For a given classification of road in given terrainconditions, cross section elements shoulddesirably be the same everywhere, but certainlyon any specific road.

Once cross section dimensions, such as laneand shoulder widths and clear zoneconfigurations have been established, theyshould be consistently applied. So too shouldother features of the cross section, such asmarker posts and roadside barriers.

A situation to be avoided is the creation ofincompatibilities between the road cross sectionand its horizontal and vertical alignments.2 Inthe case of road improvement, for example,upgrading cross section elements withoutcorresponding upgrading of alignment canresult in an erroneous and potentially hazardousillusion. This can result in the driver choosingto operate at speeds excessive for the criticalalignment conditions.

Sometimes, a sudden change in cross sectionconfigurations is unavoidable. One example ofthis would be where a narrow two-lane road isbeing reconstructed to four lanes in stages.Staging may be required because of constraintssuch as funding or property acquisition, but thisapproach can result in a sudden change in crosssection where a newly constructed section endsand the original road cross section isencountered.

Another example could occur throughsignalized intersections, where the number oftraffic lanes is increased or decreased over acomparatively short section of road, foroperational and/or capacity reasons. A similarsituation can also temporarily exist where aconstruction detour is introduced with narrowercross section dimensions than the precedingsection of road.

Other examples that violate driver’s expectancyinvolve tangential exits and narrow bridges.3 Atangential exit at the point of curve may causea driver to be ‘pulled-off’ the road by followingthe straight road alignment into the exit. Asimilar situation is a tangential roadside feature(for example, utility poles) that causes the driverto follow the road feature inadvertently.

Narrow bridges, where the width of thepreceding section of road is not reduced, alsorepresent an expectancy violation for the driver.This is especially true when the bridges arelocated on curves or dips, where they aredifficult to perceive.

In these circumstances, the designer should doeverything possible to mitigate the impact of theunexpected features or realign the road toeliminate the inconsistency. For example, crosssection change should be introduced gradually,with tapers as long as practically possible, andadvance signing should be used to warnapproaching drivers of what to expect.

Design Consistency



1.4.3 OPERATING SPEEDCONSISTENCY

The safety of a road is closely linked tovariations in the speed of vehicles travelling onit. These variations are of two kinds:

1. Individual drivers vary their operatingspeeds to adjust to features encounteredalong the road, such as curves,intersections, and accesses in thealignment. The greater and more frequentare the speed variations, the higher is theprobability of collision.

2. Drivers travelling substantially slower orfaster than the average traffic speed havea higher risk of being involved in collisions.

A designer can therefore enhance the safety ofa road by producing a design that encouragesoperating speed uniformity.

As noted in the discussion of speed profiles inChapter 1.2, simple application of the designspeed concept does not prevent inconsistenciesin geometric design. Traditional North Americandesign methods have merely ensured that alldesign elements meet or exceed minimumstandards, but have not necessarily ensuredoperating speed consistency betweenelements.

Practices used in Europe and Australia havesupplemented the design speed concept withmethods of identifying and quantifyinggeometric inconsistencies in horizontalalignments of rural two-lane highways. Inaddition, recent research work in Canada andthe United States has addressed designconsistency for combined (horizontal andvertical) alignments. The focus of this work ison two-lane rural highways. These methodshave not been perfected, particularly inpredicting the performance of a newly designedroad. Their effectiveness is greater in evaluatingexisting roads and identifying priorityimprovements to reduce collision rates.

December 2007

1.4.3.1 Prediction of Operating Speeds

In order to establish consistency of highwayalignment design for a proposed new road, it isnecessary to predict operating speedsassociated with different geometric elements,including isolated horizontal and vertical curves,and horizontal-vertical curve combinations.Limited information for Canadian conditions isgenerally available to assist designers withpredicting operating speed, but the materialpresented in this chapter may help, noting thelimited database used. As an alternative, somejurisdictions have local data on which to basespeed predictions.

Two distinct approaches to the assessment ofoperating speed design consistency arepresented here.

• The first – based on research carried out inthe US – considers only the horizontalalignment, and as a result, is somewhatsimpler to apply, but has been shown toprovide consistent and relevant results. Thismethod may have its best application at theplanning and preliminary design stages,when the vertical alignment may not be welldefined. It may also have good applicationat the detailed design stage in situations ofrelatively flat (non-rolling, non-mountainous)terrain. Software is available to assist in thistype of design consistency analysis.

• The second – also based on US data –considers both horizontal and verticalalignment. It is somewhat more complex toapply, but has also been shown to provideconsistent and relevant results. Thisapproach may be most applicable at thedetailed design stage when the verticalalignment is well defined, and where theterrain is rolling or mountainous. In suchareas - where elevation changes aresubstantial - this technique may providesomewhat more realistic predictions ofoperating speed than the method notedimmediately above.

Both methodologies have merit and are deemedto be applicable in the Canadian road designcontext.

Page 1.4.3.1

Design Consistency

Horizontal alignment approach: Introduction

In this approach, researchers collected datafrom five US states, measuring 85th percentileoperating speeds, under free flowing trafficconditions, on long tangents and horizontalcurves on rural two-lane highways. Longtangents (250m or more) are those lengths ofstraight road on which a driver has time toaccelerate to the desired speed beforeapproaching the next curve. The mean 85th

percentile speed on long tangents was foundto be 99.8 km/h on level terrain and 96.6 km/hin rolling terrain. It was noted that these speedswere probably constrained by the 90 km/hposted speed in force at the time.

On horizontal curves, the research foundconsistent disparities between 85th percentilespeeds, with the greater disparity on tighterradius curves. The 85th percentile speedexceeded the design speed on a majority ofcurves in each 10 km/h increment of designspeed up to a design speed of 100 km/h. Athigher design speeds, the 85th percentile speedwas lower than the design speed.

Using regression techniques, a relationship wasfound between 85th percentile speeds and thecharacteristics of a horizontal curve.

V85

= 102.45 + 0.0037L (1.4.1) - (2741.75 + 1.75L) / R

Where V85

= 85th percentilespeed on curve (km/h)

L = length of curve (m)

R = radius of curvature (m)

Speed Profile Model 1: The technique

The findings outlined in Subsection 1.4.3.1support the conclusion that there is no strongrelationship between design speed andoperating speed on horizontal curves.Consistency of horizontal alignment designcannot therefore be assured by using designspeed alone. A further check can be carried outby constructing a speed profile model, usingpredicted 85th percentile operating speeds for

Page 1.4.3.2

a92.2585V85VV2

TL2

22

12

fc

−−=

new road and measured 85th percentile speedsfor existing roads.

For the predictive model, it is necessary tocalculate the critical tangent length betweencurves as follows:

(1.4.2)

Where: TLC

= critical tangent length (m)

Vf

= 85th percentile desiredspeed on long tangents(km/h)

V85n

= 85th percentile speed oncurve n (km/h)

a = acceleration/decelerationrate, assumed to be0.85 m/s2

The calculation assumes that decelerationbegins where required, even if the beginning ofthe curve is not yet visible.

Each tangent is then classified as one of threecases, as shown on Figure 1.4.3.1, bycomparing the actual tangent length (TL) to thecritical tangent length (TL

C).

Having found the relationship between eachtangent length (TL) and the critical tangentlength (TL

C), the equations in Table 1.4.3.1 can

then be used, as appropriate, to construct thespeed profile model.

The speed profile model is used to estimatethe reductions in 85th percentile operatingspeeds from approach tangents to horizontalcurves, or between curves. Designers shouldnote that the research6,9 on which the model isbased dealt only with two-lane rural highways.The same principles, however, can be appliedto design of other classes of roads.

Horizontal and vertical alignment approach:Introduction

In this approach, researchers4 collected datain six states at 176 sites, again measuring

December 2007


operating speed (85th percentile speed) fordifferent types of curves on two-lane ruralhighways, and developing regression equationspredicting 85th percentile speeds for passengercars for most combinations of horizontal andvertical curves. The significant difference in thisresearch, was the inclusion of vertical alignmentcharacteristics in the predictive equations.

Table 1.4.3.1 summarizes the regressionequations for various types of curves. Theradius was the only significant variable inpredicting operating speed for all alignmentcombinations that included a horizontal curveon grade. The best form of the independentvariable in regression equation is 1/R, where Ris curve radius.

Operating speeds on horizontal curves are verysimilar to speeds on long tangents when theradius is greater than about 800 m. When thiscondition occurs, the grade of the section maycontrol the selection of operating speed and thecontribution of the horizontal curve radius isnegligible. For radii less than 800 m, operatingspeeds decrease as the radius decreases, anddrop sharply when the radius is less than about250 m.

For horizontal curve combined with either sagcurve or crest curve with limited sight distance(LSD), the radius of the horizontal curve wasthe best predictor of speed. For nonlimited sightdistance (NLSD) crest curves combined withhorizontal curves, the lower of the speedpredicted using the equation for horizontalcurves on grades or the desired speed shouldbe used.

For LSD crest curve on horizontal tangent,operating speed is predicted using the rate ofvertical curvature as the independent variable.The best form of the independent variable inthe regression equation is 1/K, where K is rateof vertical curvature.

The regression equations for vertical curves arebased on data collected on crest vertical curveswith initial upgrades followed by downgrade andsag vertical curve with initial downgradesfollowed by upgrades. Although these are thetypical vertical curves, other types of crest and

December 2007

sag curves exist and it is assumed that theyhave similar speed relationships. Whereavailable, local data for these types of curvesshould be used.

The analysis4 found that the use of spiral curvesdid not result in a significant difference in speedcompared with similar sites without spirals.

In addition, the analysis indicated that the trendfor trucks and recreational vehicles is generallysimilar to that for passenger cars. Therefore, adesign consistency evaluation based onpassenger cars is recommended.

Speed-Profile Model 2: The technique

The model used in this approach forestablishing the speed profile along acontinuous alignment involving both horizontaland vertical geometry is shown in Figure 1.4.3.1.The supporting equations are presented inTable 1.4.3.2. The model is based on earlierresearch work5 and subsequent modifications.6

The speed-profile model uses the operatingspeeds predicted in Subsection titled Horizontaland Vertical Alignment: Introduction on page1.4.3.2 for individual curves to determine theoperating speeds on the speed-change(acceleration and deceleration) segmentsbetween the curves. The speeds are then usedto establish the speed profile along the roadand evaluate design consistency.

The process involves the following four steps:

• Selecting a desired speed along theroadway.

• Predicting an operating speed for eachcurve.

• Calculating operating speeds on speed-change segments.

• Evaluating the design consistency.

Step 1: The desired speed, Vf, is the speed

which drivers select when not constrained bythe grade on a horizontal or vertical curve. Thespeeds observed on long tangents can serveas an assumed desired speed. A long tangent

Page 1.4.3.3

Design Consistency

is one that is long enough for the driver toaccelerate to, and for some distance sustain, adesired speed. The 85th percentile speedobserved on long tangents of two-lane ruralhighways ranges from 94 to 104 km/h.Therefore, a rounded value of 100 km/h is agood estimate for the desired speed on longtangents of two-lane rural highways with a speedlimit of 90 km/h.

Step 2: The operating speeds for various typesof curves are predicted using the equations ofTable 1.4.3.1. These speeds correspond to themidpoint of the curve. Based on a study inCanada for combined alignments,7 operatingspeeds at the point of curve (PC) and the pointof tangent (PT) were found to be greater thanthe speed at the middle point for both horizontalcurve-sag curve and horizontal curve-crestcurve combinations. The average differencewas about 2.0 km/h. This value is recommendedfor estimating operating speeds at the beginningand end of the combined curves.

For isolated horizontal curves, a constant speedalong the curve is assumed because data havenot shown a defined pattern of how speed variesthrough the curves.5 If the predicted speed on acurve is greater than the desired speed, thedesired speed should be used.

Step 3: Calculation of the operating speed onspeed-change segment, TL, involvesdetermining the following:

• Length of road for acceleration from curven speed to desired speed, X

1a

• Length of road for deceleration from desiredspeed to curve n+1 speed, X

1d

• Length of road for deceleration from curven speed to curve n+1 speed, X

2d

• Length of road for acceleration from curven speed to curve n+1 speed, X

3a

• Critical length of road needed toaccommodate full acceleration anddeceleration, TL

c

• Length of road available for speed changes,TL

Page 1.4.3.4

The critical length for speed changes betweencurves, TL

c, is the distance required to

accelerate from a curve speed to the desiredspeed and then decelerate to the next curvespeed. Thus,

TLc = X

1a + X

1d (1.4.1)

As noted in Table 1.4.2.2, the distances X1a

andX

1d are functions of the 85th percentile speeds

on curves n and n+1, Vn and V

n+1 (km/h),

respectively [predicted in Subsection titledHorizontal and Verticval Alignment: Introductionon page 1.4.3.2], V

f, a, and d, where a =

acceleration rate (m/s2) and d = decelerationrate (m/s2). Values of a and d equal to 0.54 m/s2 and 1.00 m/s2, respectively, are assumed.These values represent the comfortable,observed levels on two-lane rural highways.

The speed-change segment, TL, is a criticalcomponent of the speed-profile model. In thecase of an alignment with no vertical curves,TL is simply the tangent distance between PTof one horizontal curve and PC of the nextcurve. For combined alignments, however, TLis the distance between the speed-limitingcurves shown in Table 1.4.3.1. These curvesare the horizontal curves (whether on tangentsor combined with vertical curves) and the LSDcrest curves on horizontal tangents.

The findings to date4 demonstrate that speedon NLSD crest curves on horizontal tangentsand sag curves on horizontal tangents is notsubstantially influenced by the characteristicsof the curve. Therefore, these types of verticalcurves should be included in the length of roadavailable for speed changes. The operatingspeed on these curves equals desired speed.

By comparing the actual speed-change lengthTL and the critical length TL

c, and curve speeds

Vn and V

n+1, each length of speed-change

segment is classified as one of the five casesshown in Figure 1.4.3.1.

Note that in Case 2b, when the actual lengthequals X

2d, the driver will decelerate at the

assumed value of 1.00 m/s2. When the actuallength is less than X

2d, the driver will have to

use greater deceleration to reach the speed ofthe next curve. In this case, the designer should

December 2007


ensure that the resulting deceleration rate (d’in Table 1.4.3.2) does not exceed the maximumpractical rate (2.0 m/s2).

Similarly, in Case 3b when the actual lengthequals X

3a, the driver will accelerate at the

assumed value of 0.54 m/s2. When the actuallength is less than X

3a, a driver may use higher

acceleration rate than the assumed rate toreach the predicted curve speed V

n+1 or will enter

the next curve at a speed, Vn+1

a, lower than thatpredicted by the regression equation. It wouldbe unusual for a driver to accelerate past thecomfortable rate simply because of thegeometry. Therefore, the adjusted speed V

n+1

a

should be used in establishing the speed profile.

Step 4: To evaluate design consistency, thespeed profile established in the previous stepis used to estimate the reductions in 85th

percentile speeds between speed-changesegment and horizontal curves, or betweencurves. Large differences in operating speedsof successive elements would indicate thatinconsistencies exist in the alignment.

Some studies18 suggested that a difference of10 km/h or less represents good design and adifference between 10 km/h and 20 km/hrepresents fair design. A difference greater than20 km/h indicates the need for improvement.The research to date illustrated that predictingoperating speed for combined alignments iscomplex, and requires further studies.

Horizontal and Vertical Model Special Aspects

The speed-profile model assumes thatdeceleration begins where required, whichimplies that sight distance is always largeenough for the beginning of the next curve tobe visible. The model also assumes that allacceleration and deceleration take place beforeor after the speed-limiting curve.

In some situations, the speeds predicted withthe regression equations are higher than theassumed desired speed on the tangent. Forconsistency within the speed-profile model, themaximum speed on curves should be set equalto the desired speed on the tangent.

December 2007

The smallest radius in the field data used fordeveloping operating speed equations was100 m. For smaller radii, the regression mayproduce very small or negative speeds.Therefore, when the radius of a horizontal curveis less than 100 m, set the speed equal to60 km/h.

Overlapping horizontal and vertical curvesrequire special attention. If the curvescompletely or significantly overlap, then theyrepresent a single element for which one of theregression equations is used to predict speed.If the curves partially overlap, the designershould make a judgement regarding whetherthe overlapped curves should be treated as twoor more adjacent elements. For example, if avertical curve begins after PC and ends beforePT, three predicted speeds will result from thisalignment configuration. A suggested methodis to use the lowest speed predicted for anycondition throughout the horizontal curve.

Page 1.4.3.5

Design Consistency

Figure 1.4.3.1 Variable Definitions for Speed-Profile Model


Speed-Change Segment Curve

TL

X3a

85th percentile speed



nV

Vf

nV

nV

Vf



nV

nV

Vf

tV

Vf

X1a

nVVf

TL

TL

Vt

distance

n + 1V

distance

distance

n + 1V

n + 1V

X2d

TL

X1d

TL

distance

n + 1V

distance

n + 1V

CurveSpeed-Change Segment

distance

distance

distance

distance

distance

85thpercentile

speed

85thpercentile

speed

85thpercentile

speed

85thpercentile

speed

85thpercentile

speed

Case 1:

TL ≥ TLc

Case 2:

TL < TLc

Vn ≤ V

n+1

Case 2(a):

TL > X2d

Case 2(b):

TL ≤ X2d

Case 3:

TL < TLc

Vn ≤ V

n+1

Case 3(a):

TL > X3a

Case 3(b):

TL ≤ X3a


Table 1.4.3.1 Equations for Estimating Operating Speed on Various Typesof Speed-Limiting Curves5


Type Alignment Condition Equationa

1 Horizontal curve on grade: -9% ≤ G < -4% V85

= 102.10 -

2 Horizontal curve on grade: -4% ≤ G < 0% V85

= 105.98 -

3 Horizontal curve on grade: 0% ≤ G < 4% V85

= 104.82 -

4 Horizontal curve on grade: 4% ≤ G < 9% V85

= 96.91 -

5 Horizontal curve combined with sag verticalcurve V

85 = 105.32 -

6 Horizontal curve combined with non limitedsight distance crest vertical curve -b

7 Horizonal curve combined with limited sightdistance crest vertical curve (i.e., K ≤ 43m/%)c V

85 = 103.24 -

8 Vertical crest curve with limited sight distance(i.e., K > 43 m/%) on horizontal tangent V

85 = 105.08 -

3077.13R

3709.90R

3574.51R

2752.19R

3438.19R

3576.51R

149.69K

a V85 = 85th percentile speed of passenger cars (km/hr), K = rate of vertical curvature, R = radius of curvature (m), and G = grade (%).b Use lowest speed of the speeds predicted from Type 1 or 2 (for the upgrade) and Type 3 or 4 (for the downgrade).c In addition, check the speed predicted from Type 1 or 2 (for the upgrade) and Type 3 or 4 (for the downgrade) and use the lowest speed. This will ensure that the speed predicted along the combined curve crest vertical curve results in a higher speed.

Design Consistency


Case andCondition Sub-Condition Equationa

Case 1: X1a

= (Vf

2 – Vn

2)/25.92 a N.A.

TL > TLc

X1d

= (Vf

2 – Vn+1

2)/25.92 d

TLc = X

1a + X

1d

Case 2: Case 2(a) X2d

= (Vn

2 – Vn+1

2)/25.92 dTL > X

2d

Vt = {Vn2 + 25.92 [ ad/(a + d)] (TL – X2d)} 1/2

TL < TLc

Vn ³ V

n+1Case 2(b) For TL < X

2d, d’ + (V

n

2 – Vn+1

2) (25.92 TL)TL ≤ X

2d

Case 3: Case 3(a) X3a

= (Vn+1

2 – Vn

2)/25.92 aTL > X

3a

TL < TLc

Vt = {V

n+1

2 + 25.92 [ad/(a + d)] (TL – X3a

)}1/2

Vn < V

n+1

Case 3(b) For TL < X3a, V

n+1

a = [Vn

2 + 25.92 a TL]1/2

TL < X3a

a Vn and V

n+1 = 85th percentile speeds for curve n and n+1 (predicted using equations in

Table 1.4.2.1)

Table 1.4.3.2 Equations for Speed-Profile Model



1.4.4 DRIVER WORKLOADCONSISTENCY

Driver workload represents the demands placedon a driver by the road. If the workload a driverexperiences drops too low or rises too high, thecollision rate can increase. If workload is toolow, for example on long straight, flat stretchesof road in a rural setting, drivers may becomebored or tired. Their responses to unexpectedsituations may then be inappropriate or slow. Atthe other end of the scale, drivers may becomeconfused by situations that require very highworkload, such as fast-moving, heavy traffic inan urban setting that presents a plethora of signsand advertisements or construction work zones.In these situations drivers may overlook ormisinterpret an unexpected occurrence andeither not respond until too late or respondinappropriately.

Driver workload quantifies the criticality ofindividual features and the interacting effects ofcombination of features along road alignment.Consistent road geometry allows a driver topredict the correct path while using very littlevisual information processing, thus allowingattention to be dedicated to navigation andobstacle avoidance.

Some general principles have been establishedthat should be considered when an unusualdesign feature or combination of features is beingcontemplated11. Abrupt increases in driver

workload increase collision potential. Suchincreases can be caused by:

• The criticality of the feature beingapproached (an intersection or lane drop ismore critical than a change in shoulderwidth, for example)

• Limited sight distance on the feature

• Dissimilarity of the feature to the previousfeature (that might cause surprise to thedriver)

• Large percentages of drivers unfamiliar withthe road (for example, on a major arterialas opposed to a local road)

• A high demand on the driver’s attention aftera period of lesser demand (for example, asharp curve at the end of a long stretch ofstraight road).

Situations where most or all of these factorsare encountered simultaneously should beavoided.

Various methods8,11,12,13,14,15 have been proposedby which driver workload can be used to evaluatedesign consistency, however these researchapproaches have yet to mature to the pointwhere they can be considered to be sufficientlyrobust for application in the design environment.

Design Consistency




1.4.5 SAFETY ANDCONSISTENCY

As noted in Chapter 1.2, there is evidence thatthe risk of a collision is lowest near the averagespeed of traffic and increases for vehiclestravelling much faster or slower than the averagespeed. While this is true in relation to the generaldistribution of speeds in a stream of traffic, ithas also been found to apply when there is avariation in speed caused by the effects ofreduction in speed from one geometric designelement to the next.

This particular form of speed variation may beexperienced in situations such as the transitionfrom a tangent to a curve, or between curves. Inthe first of the design consistency researchprojects previously mentioned, researchers foundthat the mean collision rate increased in directproportion to the mean speed difference causedby the transition from one geometric element tothe other. The results are noted in Figure1.4.5.11 .

The numerical values from Figure 1.4.5.1 shouldbe used with caution, because of the databaseused. However, the principles show the effect ofa lack of horizontal alignment consistency onincreasing collision potential. Figure 1.4.5.1should not be interpreted to mean that collisionrate decreases as speed decreases.

In the second research study mentioned earlier,researchers4,10 found that the number ofcollisions that occurred on a horizontal curve ina three year period increased exponentially withthe increase in the speed reduction (SR) causedby the transition from one geometric element tothe next (Figure 1.4.5.2)2 .

The collision rate also increased in directproportion to the exposure, M. The exposurewas defined as the product of curve length (m)and AADT (veh/day) during a three-year period,a common exposure measure for road sections.The exposure is expressed in terms of millionsof vehicle-kilometres of travel.

The results of Figure 1.4.5.2 show that thepredicted collision experience is clearly sensitiveto speed reduction on a horizontal curve. Thissensitivity lends support to the use of speedreduction estimates as a design consistencymeasure.

Similar caveats apply to Figure 1.4.5.2 as werenoted for Figure 1.4.5.1 – including the fact thatthe numerical values from Figure 1.4.5.2 shouldbe used with caution, because of the databaseused. Wherever possible, designers should uselocal data.

Design Consistency


0 5 10 15 20 25 30 35 40

0

5

1

2

3

4

6

mean speed reduction (km/h)

mean collision rate per million

vehicle-kilometres

Figure 1.4.5.1 Mean Collision Rate versus Mean Speed Differencebetween Geometric Elements9

(i.e. reduction in speed from one geometric element to the next,e.g. tangent to curve)



Figure 1.4.5.2 3-year Collision Frequency versus Mean Speed ReductionBetween Geometric ElementsY = exp(-0.8571) M exp(o.0780 SR)

M = million veh-km of travel during a 3 year period

Design Consistency



Page 1.4.R.1December 2007

REFERENCES

1. Glennon, J. and Harwood, D. HighwayDesign Consistency and Systematic DesignRelated to Highway Safety. In TransportationResearch Record 681, TRB, NationalResearch Council, Washington, D.C., 1978.

2. Designing Safer Roads: Practices forResurfacing, Reconstruction, andRehabilitation. Special Report 214, TRB,National Research Council, Washington,D.C., 1987.

3. Alexander, G. and Lunenfeld, H. DriverExpectancy in Highway Design andOperations. FHWA-TO-86-1, FederalHighway Administration, Washington, D.C.,1986.

4. Fitzpatrick, K. et al. Speed Prediction forTwo-lane Rural Highways. FHWA-RD-99-171, Washington, D.C., 1999.

5. Fitzpatrick, K. and Collins, J. Speed- ProfileModel for Two-lane Rural Highways. InTransportation Research Record 1737, TRB,National Research Council, Washington,D.C., 2000, 42-49.

6. Easa, S. Geometric Design Guide Update:Design Consistency AssessmentMethodologies. Transportation Associationof Canada, Ottawa, Ontario, 2002.

7. Gibreel, G., Easa, S., and El-Dimeery, I.Prediction of Operating Speed on Three-Dimensional Highway Alignments. Journalof Transportation Engineering, ASCE,127(1), 2001, 20-31.

8. Fitzpatrick, K. et al. Alternative DesignConsistency Rating Methods for Two-laneRural Highways. FHWA-RD-99-172,Washington, D.C., 1999.

9. Polus, A. and Dagan, D. Models forEvaluating the Consistency of HighwayAlignment. In Transportation ResearchRecord 1122, TRB, national ResearchCouncil, Washington, D.C., 1987, 47-56.

10. Anderson, I., Bauer, K., Harwood, D., andFitzpatrick, K. Relationship to Safety ofGeometric Design Consistency Measuresfor Rural Two-Lane Highways. InTransportation Research Record 1658, TRB,national Research Council, Washington,D.C., 1999.

11. Messer, C., Mounce, J.M. and Brackett,R.Q. Highway Geometric DesignConsistency Related to Driver Expectancy.Final Report, Environmental Division, FHWA,U.S. Department of Transportation,Washington, D.C., 1981.

12. Krammes, R.A. and Glascock, S.W.Geometric Inconsistencies and AccidentExperience on Two-lane Rural Highways. InTransportation Research Record 1356, TRB,National Research Council, Washington,D.C., 1992.

13. Wooldridge, M.D. Design Consistency andDriver Error. In Transportation ResearchRecord 1445, TRB, national ResearchCouncil, Washington, D.C., 1994.

14. Krammes, R.A., Brackett, R.Q., Shafer,M.A., Ottesen, J.L., Anderson, I.B., Fink,K.L., Collins, K.M., Pendleton, O.J., andMesser, C.J. Horizontal Alignment DesignConsistency for Rural Two-lane Highways.FHWA-RD-94-034, Washington, D.C., 1993.

15. Hassan, Y., Gibreel, G., and Easa, S.Evaluation of Highway Consistency andSafety: Practical Application. Journal ofTransportation Engineering, ASCE, 126(3),2000, 193-201.

16. Wooldridge, M., Fitzpatrick, K., Kappa, R.,and Bauer, K. Effect of Horizontal Curvatureon Driver Visual demand. In TransportationResearch Record 1737, TRB, NationalResearch Council, Washington, D.C., 2000,71-77.

17. Easa, S. Distributing Superelevation toMaximize Highway Design Consistency.Journal of Transportation Engineering,ASCE, 2002 (in press).

Design Consistency

Page 1.4.R.2 December 2007

18. Lamm, R., Guenther, E., and Mailaender,T. Highway Design and Traffic SafetyEngineering Handbook. McGraw-Hill, NewYork, N.Y., 1999.

19. Morrall, J. and Talarico, R. Side FrictionDemanded and the Margins of Safety onHorizontal Curves. Transportation ResearchRecord 1435, Transportation researchBoard, Washington, D.C., 1994, 145-152.

20. Talarico, R. and Morrall, J. The Cost-Effectiveness of Curve Flattening in Alberta.Canadian Journal of Civil Engineering, 21(2),1994, 285-296.

GeometricDesignGuide forCanadianRoadsPart 2

TransportationAssociation ofCanada

September 1999Updated December 2007

December 2007

© Transportation Association of Canada, 19992323 St. Laurent BoulevardOttawa, Canada K1G 4J8Tel. (613) 736-1350Fax: (613) 736-1395Web site: www.tac-atc.caISBN 1-55187-131-9

The Transportation Association of Canada is a national association with a mission to promote theprovision of safe, secure, efficient, effective and environmentally and financially sustainabletransportation services in support of Canada’s social and economic goals. The association is aneutral forum for gathering or exchanging ideas, information and knowledge on technical guidelinesand best practices. In Canada as a whole, TAC has a primary focus on roadways and their strategiclinkages and inter-relationships with other components of the transportation system. In urbanareas, TAC’s primary focus is on the movement of people, goods and services and its relationshipwith land use patterns.

L’ATC est une association d’envergure nationale dont la mission est de promouvoir la sécurité, lasûreté, l’efficience, l’efficacité et le respect de l’environnement dans le cadre de la prestation deservices financièrement durables de transport, le tout à l’appui des objectifs sociaux et économiquesdu Canada. L’ATC est une tribune neutre de collecte et d’échange d’idées, d’informations et deconnaissances à l’appui de l’élaboration de lignes directrices techniques et de bonnes pratiques.À l’échelle du pays, l’Association s’intéresse principalement au secteur routier et à ses liens etinterrelations stratégiques avec les autres composantes du réseau de transport. En milieu urbain,l’Association s’intéresse non seulement au transport des personnes et des marchandises, maisencore à la prestation de services à la collectivité et aux incidences de toutes ces activités sur lesmodèles d’aménagement du territoire.

IndexChapter 4.3

Index

December 2007


Page 4.3.1December 2007

INDEX

A

Acceleration lane, see Lanes, accelerationAccess classification system, 3.2.2.2Access management, 3.2.1-3.2.10Access types, 3.2.2.1Aesthetics, 1.1.3.5, 2.1.4, 2.4.7.2-4Alignment

at intersections, 2.1.3.11-12, 2.3.2consistency, 1.4.3.1coordination, 2.1.4horizontal, 2.1.2interchange ramps, 2.4.6vertical, 2.1.3

Approach taper, see Taper, approachArterial roads, 1.3.2, 1.3.3, 1.3.4, 3.2.3.2Auxiliary lanes, see Lanes, auxiliary

B

Barriers, trafficanchored, 3.1.6.12, 3.1.6.20-22crash cushions, 3.1.6.1, 3.1.6.20-22end treatment, 3.1.6.20-22energy attenuation devices, 3.1.6.1, 3.1.6.22flare rate, 3.1.6.8-10flexible, 3.1.6.3-4, 3.1.6.15, 3.1.6.18lateral offsets, 3.1.6.6-8length of need, 3.1.6.9-12longitudinal, 3.1.6.1-2median, 3.1.6.15-19rigid, 3.1.6.3-4, 3.1.6.15, 3.1.6.18selection criteria, 3.1.6.6, 3.1.6.17semi-rigid, 3.1.6.3-4, 3.1.6.15, 3.1.6.18terrain effects, 3.1.6.8, 3.1.6.17transition, 3.1.6.19-20warrants, 3.1.6.1-6, 3.1.6.15-19

Bay taper, see Taper, bayBenefit Cost Analysis, 1.1.2.1-2Bikeways

alignment elements, 3.4.5.1cross section elements, 3.4.6.1delineators, 3.4.8.1-3design considerations, 3.4.4.1-2functional classification, 3.4.3.1-3horizontal clearance, 3.4.6.2-3intersection treatment, 3.4.7.1-10rider requirements, 3.4.2.1vertical clearance, 3.4.6.3

Borders, 2.2.6.1-3Boulevards, 2.2.6.1-4

Index

Page 4.3.2 December 2007

Brake reaction distance, see Sight distance, stoppingBraking distance, 1.2.5.2-5Breakaway supports, 3.1.3.6-9, 3.1.5.1-4Bridges

cross sections, 2.2.10.1-10horizontal clearances, 2.2.10.1-8railings, 3.1.6.19sight distance, 2.1.2.38-39, 2.3.3.14vertical clearances, 2.1.3.12-13

Building set-back guidelines, 3.2.1.5-6Bus, 1.2.4.9Bus bays, 2.2.3.3, 2.4.7.1-2, 3.5.2.1-4

C

Car, see Passenger carChannelization, 2.3.6.1-4Classification, see Road classificationClear runout area, 3.1.3.1-2Clear zone, 3.1.3, 3.1.4Clearance

at bridges, see Bridgesat bikeways, see Bikeways

Climbing lanes, see Lanes, climbingCollector roads, 1.3.2, 1.3.3, 1.3.4, 3.2.3.2-3Collision, 1.1.3.1-5Collision Modification Factor, 1.1.3.3Corner clearances

major intersections, 3.2.8.1-4minor intersections, 3.2.9.6-7

Corner radius, 2.3.4.4-9Critical length of grade, 2.1.8.1Cross sections

bikeways, 3.4.6.1-2bridges, 2.2.10.1-10consistency, 1.4.2.1roads, 2.2.1.1-5transition, 2.3.9.1-2typical, 2.2.13.1-13

Cross-slopesbikeways, 3.4.5.2intersections, 2.1.3.11-12, 2.3.2.4-17roads, 2.1.5, 2.2.8.4shoulders, 2.2.4.4sidewalks, 2.2.6.7, 3.2.9.4, 3.2.9.14

Crosswalk, 2.2.6.7-8, 2.3.14.1-2, 3.2.1.6, 3.3.2.3, 3.4.7.2Crown, 2.1.2.4-13, 2.1.2.25-28, 2.1.5.1-5Culs-de-sac, 3.2.10.1Culverts, 3.1.4.11-13Curbs and gutters, 2.2.7.1-3Curves

broken back, 2.1.2.24, 2.1.2.37, 2.1.3.9



circular, 2.1.2crest vertical, 2.1.3.5-7horizontal, 2.1.2.2lane widening on, 2.1.2.29-36minimum radii, 2.1.2.7-8, 3.4.5.2-3reverse, 2.1.2.24-25sag vertical, 2.1.3.7-9spiral, 2.1.2.20-25transition, 2.1.2.20-25vertical, 2.1.3.4

D

Deceleration lane, see Lanes, decelerationDecision sight distance, 1.2.5.7-8Departure taper, see Taper, departureDesign controls, 1.2.1.1Design domain, 1.1.5.1-2, 1.1.6.1Design Speed

bikeways, 3.4.5.1intersections, 2.3.2.1roads, 1.2.3.2, 1.2.3.5-6

Design vehicles, 1.2.4, 3.4.5.1Desired speed, 1.2.3.1-2Divided roadways, 1.3.2.1Drainage, 2.1.2.25, 2.1.3.3-4, 2.1.3.10-11, 2.2.8.1-2, 3.1.4.3-5, 3.1.4.11-14, 3.4.8.1Driver workload, 1.4.4.1Driveways, 3.2.9

E

Emergency vehicle crossings, 2.3.11.2-3Encroachments, 3.1.2.1-3, 3.1.3.3-4, 3.1.6.15-17Energy attenuation devices, see Barriers, trafficEntrance terminal, 2.4.6.9-17, 2.4.8.6-8Exit terminal, 2.4.6.5-9, 2.4.8.2-4Express-collector system, 2.1.6.5-6Expressways, 1.3.2, 1.3.3, 1.3.4, 3.2.3.1-2

F

False grading, 2.1.3.3, 2.1.5.2-3Fencing, 3.3.5.3-5Flared intersections, 2.3.5Forgiving roadside, 3.1.1.1Freeways, 1.3.2, 1.3.3, 1.3.4, 3.2.3.1Friction

coefficient of, 1.2.5.2-4, 2.1.2.17, 3.4.5.1lateral, 2.1.2.2-20, 3.4.5.2-4

Frontage road, see Service roads

Index


G

Grade, 2.1.3.1-4bikeway, 3.4.5.4critical length, 2.1.8.1driveways, 3.2.9.11-14maximum, 2.1.3.2-3minimum, 2.1.3.3, 2.1.3.10-11

Grading, see DrainageGuardrail, see Barriers, trafficGuiderail, see Barriers, trafficGutters, see Curbs and gutters

H

Height of eye, 1.2.4.4, 2.1.3.5, 2.3.3.14, 3.2.9.4, 3.4.5.2Horizontal alignment, see Alignment, horizontalHuman factors, 1.2.2.1-3

I

Inlets, see DrainageInterchanges

bus interface, 2.4.7.1-3cloverleaf, 2.4.5.3diamond, 2.4.5.2, 2.4.5.5location and spacing, 2.4.3.1-2parclo, 2.4.5.6-11, 2.4.8.9-13ramps, 2.4.6rotary, 2.4.5.10split diamond, 2.4.5.9system considerations, 2.4.4.2treatment for bicycles, 3.4.7.7-10treatment for pedestrians, 2.4.7.2trumpet, 2.4.5.10typical design features, 2.4.8.1-13warrants, 2.4.2.1-2

Intersectionsconfigurations, 2.3.1.1-3, 2.3.4.1-4, 2.3.5.1-5horizontal alignment, 2.3.2.1-4open throat, 2.3.4.1-13platform, 2.2.6.8realignment (retrofit), 2.3.1.18-21, 2.3.2.16-18roundabouts, 2.3.12sight distance, 2.3.3spacing, 2.3.1.11-17traffic conflicts, 2.3.1.4-9treatment for bicycles, 3.4.7.1-7treatment for pedestrians, 2.3.14.1-2vertical alignment, 2.3.2.4-13

Islands, 2.3.6.1-12



K

K value, 2.1.3.4

L

Landscaping, 2.4.7.2-4, 3.3.4.1-7Lane drop, 2.3.7.4, 2.3.7.6Lanes

acceleration, 2.2.3.2, 2.4.6.2, 2.4.6.11-12auxiliary, 2.1.6.2-5, 2.1.7.1-4, 2.1.9.1-8, 3.2.4.2, 3.2.5.1-11balancing, 2.1.6.2-5,climbing, 2.1.8, 2.2.3.1continuity, 2.1.7.1-4deceleration, 2.2.3.2, 2.3.5.4-6, 2.4.6.2, 2.4.6.7-9left turn, 2.2.3.1, 2.3.4.3, 2.3.8.1-26parking, 2.2.3.2passing, 2.1.9, 2.2.3.1right turn, 2.2.3.1, 2.3.4.3, 2.3.5.1-6, 2.3.6.3-4, 3.2.5.1-11service, see Service roadsspeed change, 2.3.5.4, 2.4.6.2transit, 2.2.3.2truck escape, 2.1.10two-way left turn, 3.2.6.1-5weaving, 2.2.3.2width, 2.2.2.1-4, 2.2.3.1-3, 3.4.6.1-2

Lateral friction, see Friction, lateralLeft-turn lane, see Lanes, left turnLevel of service, 1.1.3.1Lighting

bicycle facilities, 3.4.9.1pedestrian facilities, 3.3.5.3-4

Local roads, 1.3.2, 1.3.3, 1.3.4, 3.2.3.3Luminaire supports, 3.1.5.3-4

M

Medians, 2.2.5.1introduction of, 2.3.8.7, 2.3.8.10on arterial roads, 2.2.5.2-4, 3.3.4.6-7on freeways and expressways, 2.2.5.1-5openings, 2.3.11.1-3, 3.2.15widths, 2.2.5.1

N

Normal crown, see Crown

O

Object height, 1.2.5.1-2Operating speed, 1.2.3.3

Index


consistency, 1.4.3.1prediction, 1.4.3.1-5

Outer separations, 2.2.5.4-6, 3.2.7.1-10, 3.3.4.6-7Overpasses, see Bridges

P

Passenger car, 1.2.4.1-4Pavement edge drop, 2.2.4.3Pavement widening, see Curves, lane widening onPedestrians

accommodation and safety of, 3.2.1.6, 3.3.2.1-5crossings, 2.3.14.1-2, 3.2.9.14, 3.3.4.7ramps, stairs and handrails, 3.3.3.1-2

Persons with disabilities, 2.1.5.2, 2.2.6.7-8, 2.3.14.2, 3.2.1.6, 3.2.9.14, 3.3.2.1Posted speed, 1.2.3.4-5Public lanes (alleys), 1.3.2, 1.3.3, 1.3.4

R

Railway grade crossings, 2.3.13.1-5, 3.4.7.10Ramps

alignment, see Alignment, interchange rampsbikeway, 3.4.7.7-10metering, 2.4.7.1sidewalk, 2.2.6.7-8terminals, see Entrance terminals and Exit terminalstruck escape, 2.1.10width, 2.4.6.5

Recovery zone, 3.1.3.1Retrofit, 1.1.4.1

driveway spacing, 3.2.9.7-8horizontal alignment, 2.1.2.39, 2.1.2.44intersection, 2.3.1.1, 2.3.1.12, 2.3.1.18-21, 2.3.2.18pedestrian accommodation, 3.3.2.5roadway widening, 2.1.2.35-36vertical alignment, 2.1.3.12

Right turn lanes, see Lanes, right turnRight-turn taper, see Taper, right turnRoad classification

characteristics, 1.3.4.1-3factors, 1.3.3.1-3purpose, 1.3.1.1system, 1.3.2.1-2

Road network, 1.3.4.1-3Road Safety Audits, 1.1.4.2Roadside safety, 3.1.1.1-2, 3.1.2.1-4Roadside slopes, 3.1.4.1-10Roadside vegetation, see VegetationRotary intersection, see Intersections, roundaboutsRoundabouts, see Intersections, roundaboutsRumble strips, 2.2.4.3-7Running speed, 1.2.3.3-4



S

Safety Performance Function, 1.1.3.1-4Service (frontage) roads, 2.2.3.3, 2.2.5.4-6, 3.2.1.5, 3.2.7.1-10Severity Index, 3.1.2.2Shoulders, 2.2.4.1-10Shy-line offset, 3.1.6.8Side slopes, see Roadside slopesSidewalks, 2.2.6.1-9, 3.3.1.2, 3.3.2.1, 3.3.3.3-4, 3.3.5.1, 3.3.5.3Sight distance

at bridge structures, 2.1.2.38-39, 2.3.3.14criteria used, 1.2.5.1crossing, 2.3.3.5-6decision, 1.2.5.7-8, 2.3.3.12-14driveways, 3.2.9.2-4intersection, 2.3.3.1-17no control, 2.3.3.4-5passing, 1.2.5.5-7railway crossings, 2.3.13.4-5signal control, 2.3.3.12stop control, 2.3.3.5-12stopping, 1.2.5.2-5, 3.4.5.1turning, 2.3.3.8-9yield control, 2.3.3.5

Sight triangle, 2.3.3.1-4Signals, see Traffic signalsSigns

streetscaping, 3.3.5.3supports, 3.1.5.1-3

Slopescross, see Cross-slopesnon-recoverable, 3.1.3.1-9, 3.1.4.2-10recoverable, 3.1.3.1-9, 3.1.4.2-10roadside, see Roadside slopes

Slot left-turn lanes, see Lanes, left turnSnow, 2.1.3.11, 2.2.9.1-3, 3.3.2.1Speed, 1.2.3.1-9Speed-change lane, see Lanes, speed changeSpeed profile model, 1.4.3.1-4Speed-radius relationship, 2.1.2.2-3Spiral parameter, 2.1.2.21-25Staged construction, 2.2.12.1-5, 3.2.3.1-2Stopping sight distance, see Sight distance, stoppingStorage length, 2.3.8.1-7Street hardware and furniture, 3.3.5.1-5Superelevation

bikeways, 3.4.5.2intersections, 2.3.2.11-18roadways, 2.1.2.2-45, 2.1.5.6-7

Index


T

Taperapproach, 2.3.8.1bay, 2.3.5.2, 2.3.8.3, 2.3.8.5departure, 2.3.5.1, 2.3.8.1-9, 3.5.2.2right-turn, 2.3.5.1-5yield, 2.3.7.4-5

Traffic circle, see Intersections, roundaboutsTraffic signals

control, 2.3.3.1-13, 2.3.4.4spacing, 2.3.1.15-17, 3.2.4.2supports, 3.1.5.4

Transit lanes, see Lanes, transitTrees, 3.3.4.1-7Truck climbing lanes, see Lanes, climbingTrucks, 1.2.4.1-11Turning paths, see Vehicle turning pathsTurning roadways, 2.3.7.1-7, see also Lanes, left turn, Lanes, right turn, and Ramps, alignmentTwo-way left-turn lanes, see Lanes, two-way left turnTurnout, 2.1.9.2

U

Underpasses, see BridgesUndivided roadways, 1.3.2.1Utility placement, 2.1.2.2, 2.2.1.3, 2.2.11.1-3Utility pole placement, 2.2.6.1, 2.2.11.1-3, 3.1.5.5

V

Value engineering, 1.1.4.1-2Vegetation, 3.1.5.5-6, 3.3.4.1-7Vehicle height, 1.2.4.4Vehicle turning paths, 1.2.4.10Vertical alignment, see Alignment, verticalVertical curves, see Curves, vertical

W

Weaving, 2.1.7.4-7, 2.2.3.2Weaving lane, see Lanes, weaving

X

Y

Z

Z mid-block bike path crossing, 3.4.7.1

Geometric Design Guide for Canadian Roads

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