1_272_RR114 Review of Geometric Design Standards_DC_1987

8/11/2019 1_272_RR114 Review of Geometric Design Standards_DC_1987

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IA review of some recent geometric roadstandards and their application todevelopingcountriesby D Kosasih, R Robinson and J Snell


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Digest RR114 1987

A REVIEW OF SOME RECENT GEOMETRIC ROAD STANDARDS AND THEIRAPPLICATION TO DEVELOPING COUNTRIES

by

D Kosasih, R Robinson and J Snell

COMPARISON OF STANDARDS

Since 1980, Australia, Britain and the United States have all made major modifications to theirrecommendations for geometric design standards for rural roads NAASRA 1980, Department ofTransport 1981, AASHTO 1984 . This report reviews the research that formed the basis of thecurrent standards under the headings of design speed, sight distance, horizontal and verticalalignment, and cross-section.

The three standards are all based on the concept of design speed, but the application of this differsconsiderably between the standards. The AASHTO method of determining design speed is based on aqualitative assessment of traffic volume and terrain conditions. It has the objective of achievingconsistency of standards commensurate with the function of the road, and a balance betweenconstruction and operating costs. NAASRA introduces the concept of speed environment related toterrain and range of horizontal curvature along an alignment. The design speed of individualgeometric elements is related to the speed environment and, on successive elements, should not differby more than 10 km/h. The British design speed standard is based on overall alignment constraintsand roadside friction values. Relaxation of standards is allowed on cost grounds, but these stillprovide acceptable levels of safety and operating condit ions.

The key design chart for the NAASRA standard is shown in Figure 6 from the report and Figure 7shows the key chart for the British TD 9/81 standard.

lm, I , , A ,1 1

-,

,20

1 0

.r z . a ( curve , s, ( m)

F ,g . 6 NAAS RA r .l al io n$ ht ps f or mi nim um c ut va r ti. $

DEVELOPING COUNTRIES

,o~:o-02,6810 1214161820

. l, , me . . s( ,, , , AC k m( fi f ., 0 ., C/ w, ,, . 66 . 8 / 0,,,,. Clw.,$ = ,2- ,s, (60 , 28/45

F ig , 7 TD 9{ 81 De si gn ch ar t

Standards that have traditionally been applied in developing countries are alsothat traffic requirements, road safety and network considerations are different

discussed. It is notedin developing countries

I

,,


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and that, in order to develop local standards, it is convenient to define the objectives of road projectsin terms of three levels of development. These are:

Level 1: to provide access;Level 2: to provide additional capacity;Level 3: to increase operational efficiency.

For roads whose objective is to provide fundamental access Level 1 , absolute minimum standardscan be used to provide an engineered road. The choice of standards will be governed only by suchissues as traction requirements, turning circles and any requirement for the road to be all weather.

If the object of the project is to provide additional capacity for the road Level 2 , then decisions willneed to be taken on whether or not it should be paved and on what is an appropriate structuralstrength. Road width will normally be governed only by the requirement that vehicles should be ableto pass each other. It may be appropriate to design a variable width road where the cross-section isnarrow on straights, but is increased on bends or where other restrictions on sight distance apply.

It is only when the objective of a road is to increase the operational efficiency of a route Level 3that standards such as those developed by AASHTO, NAASRA or the UK Department of Transportbecome relevant. It is not normally practicable to apply standards such as these to roads at Levels 1or 2. Because the requirements of roads in developing countries are different to those in theindustrialised countries where these standards were developed, the three standards should only beapplied with caution in developing countries, even to Level 3 roads.

APPLICATION OF STANDARDS

Before American, Australian or British standards are applied to Level 3 roads in developing countries,it is necessary to review the assumptions on which the standards have been based to determine wherethey are appropriate for conditions found in individual countries. To assist with this task, this reportreviews the principal assumptions in the three standards to determine which aspects of each might beappropriate in developing countries.

Guidance is thereby given on how to adapt standards from the industrialised countries for use untilsuch time as specific standards have been developed that are appropriate for use in developingcountries.

REFERENCES

AASHTO, 1984. A policy on geometric design of highways and streets. Washington DC: AmericanAssociation of State Highway and Transportation Officials.

DEPARTMENT OF TRANSPORT, 1981. Road layout and geometry: highway link design.Departmental Standard TD 9/81. London: Department of Transport.

NAASRA, 1980. Interim guide to the geometric design of rural roads. Sydney: National Association

of Australian State Road Authorities.

The work described in this Digest forms part of the programme carried out by the Overseas Unit Unit Head: Mr J S Yerrell of TRRL for the Overseas Development Administration, but the viewsexpressed are not necessarily those of the Administration.

If this information is insufficient for your needs a copy of the ful[ Research Report RR114, may beobtained, free of charge, pre-paid by the Overseas Development Administration on written requestto the Technical Information and Library Services, Transport and Road Research Laboratory, OldWokingham Road, Cro wthorne, Berkshire, United Kingdom.

Crown Copyright. The views expressed in this Digest are not necessarily those of the Department ofTransport. Extracts from the text may be reproduced, except for commercial purposes, provided thesource is acknowledged.


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TRANSPORT AND ROAD RESEARCH LABORATORY

Department of Transport

RESEARCH REPORT 114

A REVIEW OF SOME RECENT GEOMETRIC ROADSTANDARDS AND THEIR APPLICATION TO

DEVELOPING COUNTRIES

by

D KosasihR Robinson

J Snell

Institut Teknologi Bandung IndonesiaOverseas Unit Transport and Road Research Laboratory

Department of Transportation and Highway EngineeringUniversity of Birmingham

The work described in this Report forms part of the programme carried out forthe Overseas Development Administration, but the views expressed are notnecessarily those of the Administration

Overseas UnitTransport and Road Research LaboratoryCrowthorne, BerkshireUnited Kingdom1987

ISSN 0266-5247


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CONTENTS

Abstract

1. Introduction

2. Design speed

2.1 Driver behaviour and expectation

2.2 AASHTO

2.3 NAASRA

2.3.1 Speed environment

2.3.2 Selection of speedenvironment

2.3.3 Speeds on curves

2.3.4 Side friction factor

2.3.5 Curve design speed

2.4 TD9/81

I2.4.1 Background to the standard

2.4.2 Determining the design speed

2.4.3 Relaxation of standards

2.5 Comments on design speed

3. Sight distance

3.1

3.2

3.3

3.4

3.5

Basic considerations

Stopping sight distance

3.2.1 Recommended values

3.2.2 Driver reaction time

3.2.3 Coefficient of longitudinal

friction

3.2.4 Effect of gradient

3.2.5 Effect of trucks

Passing sight distance

3.3.1 Critical factors

3.3.2 Recommended values

Eye and object heights

Comments on sight distance

Page

1

1

1

1

3

3

3

4

4

5

5

6

6

7

7

8

10

10

10

10

10

13

14

14

14

14

15

17

17

4. Horizontal alignment

4.1

4.2

4.3

4.4

4.5

4.6

Alignment, user costs and accidents

Vehicle movement on a circularcurve

Minimum curve radius

4.3.1 Fundamental relationship

4.3.2 AASHTO

4.3.3 TD9/81

4.3.4 NAASRA

Transition curves

4.4.1

4.4.2

4.4.3

4.4.4

Shortts method

Superelevation run-offmethod

Rate of pavement rotationmethod

Other considerations

Pavement widening on curves

Comments on horizontal alignment

5. Vertical alignment

5.1 Gradient

5.2 Vertical curves

5.3 Comments on vertical alignment

6. Cross-section

6.1

6.2

6.3

6.4

6.5

Road width

Shoulder width

Pavement crossfali

Shoulder crossfall

Comments on cross-section

7. Application of standards in developingcountries

7.1 Available standards

Page

18

18

19

21

21

22

22

22

23

24

24

25

25

26

26

27

27

29

30

32

32

32

33

33

34

34

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7.2

7.3

7.4

Considerations for developingcountries

7.2.1 Level of development

7.2.2 Traffic requirements

7.2.3 Road safety

7.2.4 Network considerations

Development of local standards

Review of assumptions

7.4.1 Design speed

7.4.2 Sight distance

7,4.3 Horizontal alignment

7,4.4 Vertical alignment

7.4.5 Cross-section

8. Summary

9. Acknowledgements

10. References

Page

35

35

35

36

36

36

37

37

37

37

38

38

38

38

39

@ CROWN COPYRIGHT 1987

Extracts from the text may be reproduced,except for commercial purposes, provided the

source is acknowledged.


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A REVIEW OF SOME RECENT GEOMETRIC ROADSTANDARDS AND THEIR APPLICATION TODEVELOPING COUNTRIES

ABSTRACTSince 1980, Australia, Britain and the United Stateshave all made major modifications to theirrecommendations for geometric design standards forrural roads. This report reviews the research thatformed the basis of the current standards under theheadings of design speed, sight distance, horizontaland vertical alignment, and cross-section. Standardsthat have traditionally been applied in developingcountries are also discussed.

It is noted that traffic requirements, road safety andnetwork considerations are different in developing

countries and that, in order to develop localstandards, it is convenient to define the objectives ofroad projects in terms of three levels of developmentof the road network. These are:

Level 1: to provide access;

Level 2: to provide additional capacity;

Level 3: to increase operational efficiency.

It is only when the objectives of the road are at Level3 that standards such as those developed inAustralia, Britain and the United States are relevantand the principal assumptions in these standards are

reviewed to assist in their adaptation to roads indeveloping countries.

1 INTRODUCTION

Geometric design is the process whereby the layoutof the road in the terrain is designed to meet the

needs of the road users. The principal geometricfeatures are the horizontal alignment, verticalalignment and road cross-section. The use ofgeometric design standards fulfills three objectives.Firstly, the standards ensure minimum levels ofsafety and comfort for drivers by the provision ofadequate sight distances, coefficients of friction androad space for vehicle manoeuvres; secondly, theyensure that the road is designed economically; and,thirdly, they ensure uniformity of the alignment. Thedesign standards adopted must take into account theenvironmental conditions of the road, trafficcharacteristics and driver behaviour. Theinterdependence between these factors and thegeometric features is summarised in Table 1.

Since 1980, Australia, Britain and the United Stateshave all made major modifications to theirrecommendations for geometric design standards forrural roads. This report reviews the current standardsunder the headings of design speed, sight distance,horizontal and vertical alignment, and cross-section.The Australian standards were published byNAASRA (1980) as an interim guide, the British codewas produced as Departmental Standard TD 9/81(Department of Transport 1981) with subsequentbackground information (Department of Transport19W) and amendments, and the American standardswere published as a policy document by AASHTO(19M).

A study to develop appropriate geometric designstandards for use in developing countries is beingundertaken by the Overseas Unit of TRRL. As a firststep in this work, a comparison between the recentAmerican, Australian and British standards has beencarried out. This Report describes the findings of thispreliminary study and discusses the potential forapplying these industrialised country standards indeveloping countries.

2 DESIGN SPEED

2.1 DRIVER BEHAVIOUR AND

EXPECTATION

In design guides, a design speed for a particular roadclassification is usually selected according to theterrain and traffic volume. To provide consistency ofthe design elements, general controls for thehorizontal alignment, the vertical alignment and thecombination between them are given. It is alsorecommended that the design speed chosen shouldbe consistent with the speed a driver is likely to

expect. It is this issue which causes difficulty whenapplying the standards since, except for reference totypical speed distributions of a general nature onsimilar facilities already built, designers generally haveinsufficient information available to them to takeaccount of the actual behaviour and speedexpectations of drivers on the different alignmentelements along a section of road.

Designing according to the design elementspermitted by a specified design speed does notnecessarily ensure alignment standards consistentwith driver behaviour. This is because drivers tend tovary their speeds along the road especially whennegotiating different horizontal curves.

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TABLE 1

Driver, vehicle and road characteristics in geometric design standards

Geometric design standardDriver characteristics Vehicle characteristics Road characteristics

considered considered considered

Minimum safe stopping Perception reaction time Layout of controls,distance

Skid resistance of roadbraking systems, tyre surface, design speedcondition, tread pattern

Minimum safe passing Judgement of gap Acceleration capability Design speeddistance availability and vehicle

capability

Driver eye height Physiology Dimensions

Object height Dimensions for passing

Horizontal geometry

Superelevation (e,X) Consistency of steering Urban/rural environment,effort on successive climatic conditions, opencurves highway lintersection,

degree of curvature

Coefficient of friction Comfort Skid resistance of road(fmax) surface, open highwayl

intersection

Radius (R~i) Design speed, openhighway lintersection

Transition curves Behaviour on entering Appearance ofcurves, comfort carriageway edges, design

speed

Phasing Response to visual defects Appearance, creation ofand hazards visual defects and

hazards, design speed

Vertical geometry

Crest curves

Sag curves

Gradients

Speeds during night-timecompared with day-timecomfort

Comfort

Behaviour on approach togradients

Headlight height,proportion of stoppingdistance illuminated byheadlights

Headlight height, beamdivergence, distanceilluminated by headlights

Passenger car and truckperformance,powerlweight ratio ofdesign vehicle, dimensions

Drainage, appearance ofroad, design speed

Drainage, appearance ofroad, design speed

Crawler lanes provideovertaking opportunity,design speed

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TABLE 1 continued

~ rive;;:= ehic:;;::zristicsoad characteristicsconsideredCross-section

Number of lanes

Lane width

Lateral clearance

Shoulder width

Median width

Crossfall

Vertical clearance

Comfort, ability tomanoeuvre in trafficstream and maintaindesired speed

Sensitivity to restrictedwidth

Sense of restriction


Sense of well-being


2.2 AASHTO

AASHTO continues to use the conventionaldefinition of design speed: the maximum safe speedthat can be maintained over a specified section ofhighway when conditions are so favorable that thedesign features of the highway govern. Since thestandard caters for freeways, rural and urban arterialroads, collector roads and streets, and local roadsand streets, a range of design speeds is used. Designspeeds recommended for local rural roads range from20 to 50 mph whilst, for rural collectors, the range is20 to 60 mph, both dependent on terrain and trafficvolume. Rural arterials should have design speeds of50, 60 or 70 mph in mountainous, rolling or levelterrain respectively. For rural freeways the normaldesign speed is 70 mph which may be reduced to 60or 50 mph in difficult terrain, this being consistentwith driver expectancy.

AASHTO recommends that a design speed of70 mph should be used on main roads to ensure anadequate design in the future should the current55 mph speed limit in US be removed.

In recommending the above values, the standardmakes the following points:

(i) Speed is governed by the traffic volume andphysical limitations of the road, not theimportance of the road.

Dimensions of designvehicle


Vehicle/barrier collision


Urban/rural environment,design speed

Nature of lateralobstruction

Urban/rural environment,type of facility

Type of facility, terrain,urban/rural environment,appearance ofcarriageway edges

Drainage, type of facility

Future resurfacing

(ii)

(iii)

(iv)

2.3

Higher traffic volumes may justify higherstandards, since savings in operating costs canoffset the increased construction costs.

Design speed establishes minimum standardsfor safe operation, but there should be norestriction on the use of more generous designsif they are justified economically.

A relevant consideration in selecting designspeeds is the average trip length. Standardsprovided on long lengths of a highway forlonger trip lengths, should be as consistent aspossible throughout and provide a goodlevel-of-service.

NAASRA

2.3.1 Speed environment

To some extent, these standards are based on a fieldstudy of speeds on curves. The study was directedat investigating the relationship between vehiclespeeds and the geometric properties of horizontalcurves on two-lane rural roads (McLean and ChinLenn 1977, McLean 1978 a, b, c, 1979). In order totake driver behaviour into consideration in thestandards, two different speeds were recognised;namely speed environment and design speed. Speedenvironment is the desired speed of the 85thpercentile driver and, as such, is the 85th percentile

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speed on the longer straights or large radius curvesof a section of road where the speed isunconstrained by traffic or alignment elements.Design speed is defined as the 85th percentile speedon a particular geometric element, which is used forexample to correlate curve radius, superelevation,friction demand, etc. Design speed varies along theroad depending on the speed environment, the

ho~izontal curve radius and, to some extent, on thegradients.

Select nominal P

speedenvironment 1I I

I I

nII

DETERMINECHECK Should be of

TRIALthe same order unless

ALIGNMENT

very long straights occur

I1 I

I I

oredict 85th percentile

curve speeds

= curve design speeds

aheck consistencyand sight distance

cModify ifnecessarv

natisfactoryalignment

I

I

NAASRA introduced an iterative process in thegeometric design as shown in the flow chart inFigure 1. The most important part of this process isthe consistency checks which ensure that the designspeeds of successive geometric elements should notdiffer by more than about 10 km/h. This agrees withthe recommendation by Leisch and Leisch (1977) thatthe change should not be more than 10 mph

(15 km/h). On two way roads, consistency ischecked for travel in both directions.

2.3.2 Selection of speed environment

The speed at which a driver will choose to travel asection of road is generally a compromise betweenthe maximum speed at which he would be preparedto travel to reach his destination and the perceivedlevel of risk which is seen to increase with increasedspeed. On straight open roads, road features willpresent little risk and the choice of speed will bedetermined largely by drivers preference and vehiclecapabilities. The presence of features which a driverperceives as contributing to risk tends to restrict thespeed of travel chosen. Such restrictions might arisefrom horizontal curvature, gradient, pavement widthand condition, and the volume and nature of othertraffic. Desired speeds of travel, and hence speedenvironment, therefore, whilst being defined in termsof unconstrained geometric elements, will be affectedby overall standards of geometry and the terrainthrough which the road passes. Speed environmentsrecommended by NAASRA for single carriagewayroads are given in Table 2. These reflect the lowerspeed environment values associated with moredifficult terrain resulting in higher values of bendiness

on sections of road.

TABLE 2

NAASRA speed environment value as a function ofoverall geometric standards and terrain type forsingle carriageway rural roads for use when geometryis constrained.

Approximaterange of

horizontal curveradii (metres)

Less than 7575300

150500Over 300500Over 600700

Speed environment (km/h)

Terrain type

Flat I Undulating \ Hilly

7590 85

100 95115 110120

Mountainous

70

* The more consideration givento consistency at the trialalignment stage, the fewer willbe the modif icat ions requiredlater

Fig. 1 NAASRA alignment selection procedure

4

2.3.3 Speeds on curves

The NAASRA standards are based on the following

research results. From field observations, a good


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correlation was found between curve speeds andapproach speeds of individual vehicles. Curve speedis defined as the speed at the mid point of the curve,whereas approach speed is the speed measured 100to 400 metres before the entrv tangent point. Ingeneral, the vehicles approaching at high speedsshowed a greater speed reduction on curvescompared to the vehicles approaching at low speeds.

There are at least two reasons for this. FirstIV, driversadopt much higher speeds on tangent sections thanthe design speed; and secondlv, drivers are notconfident of negotiating curves at high speeds. Atvpical relationship between curve and approachspeeds of individual cars is given in Figure 2 (McLean1978 a).

//

/ /

oL-~o 60 80 100 120

VA = approach speed (km/h)

Fig. 2 TVpical relationship from Australia betvveencurve speed and approach speed for cars

For curves with speed standards (as defined below)greater than about 90 kmlh, the 85th percentile caroperating speeds on curves tended to be less thanthe curve speed standards. For curves with lowerspeed standards, the 85th percentile car operatingspeeds tended to be in excess of the curve speedstandards. This is shown in Figure 3 (McLean1978 a). The curve speed standard is defined as themaximum speed (Vd) at which vehicles can negotiatethe curve without exceeding the earlier NAASRA(1970) side friction factors according to:

e+f = Vd2

127R

where e = superelevationf = side friction factorvd = maximum (design) speed km/hR = curve radius, metres.

Considering the findings above, for curve speedstandards less than 90 km/h, drivers tended to travelat speeds which are much faster than the designspeed on the tangent sections and still above thedesign speed on the curves. For curve speedstandards greater than 90 km/h, drivers might travelat about the design speed on the tangent sections,but they reduced their speeds below the design

speed when entering the curves. This suggestedthat, for design speeds greater than 90 km/h, driverbehaviour tended to be more conservative relative tothe design assumptions. Hence the earlier NAASRAcurve standards were retained for design speeds inexcess of 90 km/h in order to provide a high level ofsafetv and comfort for drivers.

2.3.4 Side friction factor

It could be deduced from Figure 3 that, for the 85thpercentile curve speeds less than about 90 km/h, thecorresponding side friction factors were in excess ofthe values assumed previously for design purposes;

whereas for speeds higher than 90 kmlh, the sidefriction factors were less than assumed for design, asshown in Figure 4 (McLean 1978 a).

140

mc.-% 80kn0

Vc (85) = vd

L

8

.* *

//

I

I,0 L-- I 1 1 1 I I 1 1 I I

0 40 60 80 100 120 140

Curve speed standard (km/h) Vd

Fig. 3 Relat ionship between observed 85th percentilecurve speed and curve speed standard for carsin Australia

The proposed design values for side friction factorsderived from Figure 4 are discussed in more detail inSection 4.3

2.3.5 Curve design speed

The variation in observed 85th percentile speed oncurves was explained bv the following regressionequation:

V8S=53.8+ 0.464 VE3.26C+0.0848C2where VE5= 85th percentile curve speed km/h

VE = speed environment km/hC = curvature (1000/radius, R) (metres)1

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However for curves of radius below 70 metres (C>approximately 14), this equation was not asatisfactory representation of the observedrelationship. An improved representation wasprovided by using four separate linear regressionequations of speed on curvature for the data groupedaccording to four speed environment ranges. Thesefour equations were then used as a basis to derive

the family of curves relating 85th percentile carspeeds to speed environment and curvature asshown in Figure 5 (McLean 1978 b). This family of

.

.. .

.... .. .

. . . .: : 0 :

. . :

.AA5RA(1,~=~._Side frict ion factor . ~ .::

Speed design relationship n. . . .

o 20 40 60 80 100 120

85th percenti le car speed (km/h)

Fig. 4 Relationship beween f85 and 85th percentilespeed used as basis for NAASRA design criteria

1

c

; 100uII

Designspeed

5060.708090

100110120130

curves can be combined with superelevation ratesand maximum side friction factors to give values of85th percentile curve speed for specified speedenvironments and curve radii shown in Figure 6.

2.4 TD9/812.4.1 Background to the standard

This standard is based on a speed-flow-geometrystudy which led to the development of both speeddistribution curves and a relationship between meanoperating speed and geometric features. The conceptof desig~ speed is still used, but in a more flexibleway than previously.

120 V85=115 3.94 c

;F< ;;a: ;6

=~ o c70 69-0.715 C

b60~

60-0,360 C

I Desired speed (km/h)~ o~

5 10 15 20 2

C = curvature = 1000/R (m-l)

Fig. 5 Relationships used for predicting curve speedsin Australia

I 20

Max. side friction s uperelevation Q ~

coefficient (sealedpavement)

0.350.330.310.260.180,120.120.11 A0.11 A

110-

-1 Speed environment, km/h

I7/ v 90- --- -80

Y- -. Example:

-70

---- Speed environment 100 km/hMax. superelevation 0.08

~___ -/

Curve design speed 84 km/h-60 Min. radius 183m

y I 1 1 I I I I I I I 1 1 I 1 I I 1 I I I 1 1 1

--40 50 60 70 80 90 100 150 200 300 400 500 600 700

Horizontal curve radius (m)

Fig. 6 NAASRA relationships for minimum curve radius


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Observations suggested that mean operating speedwas a function of traffic volume and geometricfeatures. In order to derive geometric designstandards, speeds of light vehicles at the nominaltraffic volume of 100 vehicles per hour were used.

These mean free operating speeds on dual and singlecarriageways were expressed by the following

equations:

V~(Dwet)=103.4* B + HF

10 4

VL(S Wet) =73.6g+ l.l CW+VISi5

-{

2+1

-}

-:-{%++} w+ Cw

where VL( D wet) = mean free operating speed oflight vehicles on dualcarriageways in wetconditions (km/h)

V~(S wet) = mean free operating speed oflight vehicles on singlecarriageways in wetconditions (km/h)

I = number of intersections, lay-bys and non-residentialaccesses (total for both sides)per km

B = bendiness (degrees/km)CW = carriageway width (metres)VW = verge width (metres, including

metre strips)VISI = harmonic mean visibility

HF = sum of the falls (metres/km)HR = sum of the rises (metres/km)

H =total hilliness (HR + HF)(metres/km)

NG = net gradient (HR HF)(metres/km)

These equations were rationalised to the followingsingle equation:

VL~(wet) = 110 Ac Lcwhere VLW(wet) = mean free operating speed in

wet conditions.Ac = alignment constraint (see 2.4.2)

Lc = layout constraint (see 2.4.2)

The effect of hilliness is excluded from initialassessments of design speed and specificadjustments can be applied during the detaileddesign of the road.

In the standard, this equation is presented in theform of a chart as shown in Figure 7. From thespeed distribution curves in Figure 8 (Kerman 1980),

it was found that the ratios 99th/85th, 85th/50thpercentile spee~s were approximately constant at thevalue of about {2 for each road type. Nominaldesign speeds were then arranged on the basis ofthis ratio and the values of 120, 100, 85, 70, etc,km/ h were adopted. Since the 85th percentile speedis normally adopted as the design speed, an increaseor decrease of one design speed step means that the

design is based on the 99th or 50th percentile speedrespectively. For example, on a rural singlecarriageway road with a nominal 85th percentiledesign speed of 85 km/h, provision for 100 km/hgeometries would cater for the 99th percentile speed,whilst provision for 70 km/h geometries would caterfor only the 50th percentile speed. Hence theimplication of raising or lowering design speed for aparticular geometric element is clear.

2.4.2 Determining the design speed

TD 9/81 adopts an iterative approach to design. Thefirst step is to design a trial alignment for anassumed design speed. For this design the alignmentconstraint (Ac) is determined from:

For dual carriageways: AC= 6.6 + B

10For single carriageways: AC= 12 VISI + 26

60 45

The layout constraint, Lc, is then determined fromTable 3.

Mean free operating speed, and hence design speed(ie the 50th and 85th percentile speeds under wet

road conditions), are determined by entering thesevalues of AC and LC into Figure 7. There are twocategories A and B, for each design speedrepresenting upper and lower bands. Whilstrelaxation of standards for a given design speed ispermitted for both categories on individual elementsof the design, there are restrictions on the relaxationsin category A because of the lower values ofalignment and layout constraints and hence higher85th percentile speeds.

The trial design speed and that determined fromFigure 7 are then compared to identify locationswhere elements of the initial design may be relaxedto achieve cost or environmental savings, orconversely where the design should be upgraded tomatch the calculated design speed.

The design speed is then used to determine thedesign standards from Table 4, which showsdesirable and absolute minimum values for eachthe main elements.

2.4.3 Relaxation of standards

of

For dual 3 lane motorways; lower basespeedsare applicablefor dual 2 lane motoways and for dual2 and 3 laneallpurposeroads.

Desirable minimum values generally cater for vehiclesat the 85th percentile speed at the normally acceptedhigh levels of safety and driver comfort, whilst


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V5Q ~~~ 85 wet

r100

90

80

70

60

Layout constraint Lc km/h Design speedJ 120~

A

-_ 120km/h

% -w-~

100

\

\100km/h

\ \\

*85

85km/h

straight roads or where 70

70km/h

1 1 11 1 1 I I I I I B60 o 2 4 6 8 10 12 14 16 18 20

Alianment constraint AC kmlh for Dual Clwavs = 6.6 + BI1O.Single ClwaVs = 12 VIS1/60 + 20145

Fig. 7 TD9/81 Design chart

E

30 50 70 90 110 130 150

V = Speed (km/h)

Fig. 8 Distributions of car journey speeds in UK

absolute minimum values for a particular designspeed are identical to desirable minimum values for

the next lower design speed step.

For the higher vehicle speeds, relaxation of standardsto absolute minimum levels can imply design levels ofsafety and comfort below what has normally beenaccepted for design in the past. However theresearch leading to TD9/81 has shown that thesehigh design levels of safety can be lowered to alimited extent without affecting accident rates.Further departures below absolute minimum levelsmay be allowed in exceptional circumstances. Thesefurther departures require detailed consideration ofsafety implications since, whilst they will not createhazards, the margin between what is considered safeand hazardous will, in these cases, be significant.

2.5 COMMENTS ON DESIGN SPEEDAASHTO recommends ranges of design speeds forthe different road classifications, depending mainlyon terrain and traffic volume, and that the standardsused should be consistent on long lengths of road.Consideration should be given to the economic trade-offs between the increased construction costs ofhigher standards and the savings in operating costswhich result. Most of these savings in operatingcosts will be savings in travel time from higherspeeds of travel.

In general, time and vehicle operating costs can berepresented by the following equation:

C=a+ (b+d) +c~

v

where C =v=

a, b,c=d=

unit operating cost per kmoperating (travel) speed, km/hcoefficients of vehicle operating costscoefficient representing the value oftime per vehicle

The above equation can be used to determine aminimum operating cost speed and will give verydifferent values of this speed depending on whethertime is valued in the road appraisal. It would seemlogical to provide standards which encourage free-flow speeds in the vicinity of these minimumoperating cost speeds.

The new approaches in both NAASRA and TD 9/81involve a check of initial designs against an overallmeasure of speed to achieve consistency and reflect

8


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TABLE 3

TD9/81 Layout constraint Lc (km/h)

Road type S2 WS2 D2AP D3AP D2M D3M

dual dual dual dualCarriageway width (excl. metre strips) 6m 7.3 m 10 m 7.3 m 11 m 7.3 m* 11 m*

Degree of access and junctions H M M L M L M L L L L

Standard verge width 29 26 23 21 19 17 10 9 6 4 0

1.5 m verge 1311281251231

0.5 m verge 33 30

Notes: L = Low access numbering 2 to 5 per kmM = Medium access numbering 6 to 8 per kmH = High access numbering 9 to 12 per km* = Hard shoulder is recommended

S2 = Two-lane single carriageway

WS2 = Two-lane wide single carriagewayD2AP = Two-lane all purpose dual carriagewayD3AP = Three-lane all purpose dual carriageway

D2M = Two-lane motorway dual carriagewayD3M = Three-lane motorway dual carriageway

TABLE 4

TD9/81 Design Standards

Design speed kmlh

STOPPING SIGHT DISTANCE m

Al Desirable MinimumA2 Absolute Minimum

HORIZONTAL CURVATURE m

61 Minimum R * without elimination of AdverseCamber and Transitions

62 Minimum R * with Superelevation of 2.5%B3 Minimum R * with Superelevation of 3.5%B4 Desirable Minimum R with Superelevation of 5%B5 Absolute Minimum R with Superelevation of 7%B6 Limiting Radius with Superelevation of 7V0 at

sites of special difficulty (Category B DesignSpeeds only)

VERTICAL CURVATURE

Cl FOSD Overtaking Crest K ValueC2 Desirable Minimum * Crest K ValueC3 Absolute Minimum Crest K ValueC4 Absolute Minimum Sag K Value

OVERTAKING SIGHT DISTANCE

D1 Full Overtaking Sight Distance FOSD m

120

295215

2880

204014401020

720510

*

18210037

*

100

215160

2040

14401020

720510360

* Not recommended for use in the design of single carriageways

4001005526

580

85

160120

1440

1020720510360255

285553020

490

70

12090

1020

720510360255180

200301720

410

60

9070

720

510360255180127

142171013

345

50

7050

510

36025518012790

10010

6.59

290

@lR

5

7.071014.142028.28

9


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driver expectations along a section of road. This thenallows some variation in design speed or standardsfor the individual elements to achieve cost effectivedesigns, at the same time taking into accountobserved driver behaviour on the individual elements.

[n the NAASRA standards, speed consistency isprovided by the concept of a speed environment

related to the terrain and range of horizontalcurvature along the road. A family of relationshipshas been developed between speed environment anddesign speed for horizontal curves which, togetherwith the criterion that design speed on successiveelements should not differ by more than 10 km/h,ensures overall consistency with regard to driverexpectations and safe efficient design of individualelements.

Similarly, in TD 9/81, an overall design speed isdetermined from the alignment and layout frictionsalong a road and variation in provision of standardson individual elements is allowed to a prescribedextent. By developing a unique relationship betweendesign speed steps and speed distributions, and fromstudies of accident rates within the margin that existsbetween the traditional interpreta tions of safe andunsafe design, relaxations of standards are nowpossible which are acceptable in both safety andlevel-of-service terms.

3 SIGHT

3.1 BASIC

DISTANCE

CONSIDERATIONSThe drivers ability to see ahead contributes to safeand efficient operation of the road. Ideally, geometricdesign should ensure that at all times, any object onthe pavement surface is visible to the driver withinnormal eye-sight distance. However, this is notusually feasible because of topographical and otherconstraints, so it is necessary to design roads on abasis of lower, but safe, sight distances.

There are two principal sight distances which are ofparticular interest in geometric design.

Stopping sight distance: If safety is to be built into

the road, then sufficient sight distance should beavailable for drivers to stop their vehicles prior tocolliding with an unexpected object on thepavement.

Passing sight distance: If operational efficiency isto be built into the road, for higher traffic volumes,then lengths of road with sufficient sight distancemay have to be provided for drivers to overtakeslower vehicles safely.

3,2 STOPPING SIGHT DISTANCE3.2.1 Recommended values

It s important that, on all engineered roads,

sufficient forward visibility is provided for safestopping on vertical and horizontal curves throughoutthe length of the road. The derivation of stoppingsight distance is based on assumed values for totaldriver reaction time and rate of deceleration, thelatter expressed in terms of the coefficient oflongitudinal friction.

D, = R,.V = $3.6 2%.f

where Ds = stopping sight distance, metresRT = total driver reaction time, secondsV = design speed, kmlhf = coefficient of longitudinal friction.

Tables 5, 6, and 7 show the minimum stopping sightdistances recommended by AASHTO, TD 9/81 andNAASRA respectively.

The three standards employ different stopping sightdistances depending on the values of total driverreaction time, coefficient of longitudinal friction and

vehicle speed assumed. These values are usuallydetermined from experimental studies related tocriteria such as safety, comfort and economics.

3.2.2 Driver reaction time

Driver reaction time consists of two components:perception time and brake reaction time. Perceptiontime is the time required for the driver to perceivethe hazard ahead and come to the realisation thatthe brake must be applied. This depends on thedistance to the hazard, the physical and mentalcharacteristics of the driver, atmospheric visibility,types and condition of the road and colour, size and

shape of the hazard.

Brake reaction time is the time taken by the driver toactuate the brake after the decision to brake. Thisdepends on the physical and mental characteristics ofthe driver, the driver position and layout of thevehicle controls.

Johansen (1977) made a detailed study of driverreaction time. He defined total driver reaction time asthe time which elapses from the moment a signal isperceived until the moment the driver initiatespreventative action. He described the psychological

and physiological processes involved as illustrated inFigure 9. However, quantitatively, whilst it isrelatively easy to carry out controlled experimentsunder alert laboratory conditions to measure driverreaction time, the relationship between this time andthat which would obtain under non-alerted roadconditions, where the perception of hazards on theroad ahead is but one of a number of driver tasks, isdifficult to determine. Also, it is easier to observetotal reaction time rather than to measure separatelyits component processes.

Most of the limited number of field studies haveshown that total driver reaction time varies from

about 0.5 to 1.7 seconds. At high speeds, the values

10


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TABLE 5

Stopping sight distances recommended by

DesignSpeed

(mph)

2025303540455055606570

AASHTO

Assumed Brake ReactionSpeed forCondition ~me

(mph) (see)

2020 2.52425 2.52830 2.53235 2.53640 2.54045 2.5450 2.548-55 2.55260 2.55565 2.55870 2.5

Driver eye height (feet)Object height (feet)

Distance

(ft)

73.3 73.388.0 91.7

102.71 10.0117.3 128.3132.0146.7146.7165.0161.3183.3176.0201.7190.7 220.0201 .7238.3212.7256.7

designspeed(km/h)

50607085

100120

Coefficientof Friction

f

0.400.380.350.340.320.310.300.300.290.290.28

total driverreaction time

(seconds)

2.02.02.02.02.02.0

driver eye height (metres)object height (metres)

StoppingSight

Distancefor Design

(ft)

1251 25150150200200225-250275325325400400475450550525650550725625850

3.50.5

TABLE 6

Stopping sight distances recommended by TD 9/81

stopping sight distancecoefficientof friction desirable min. * absolute mint

fwet (metres) (metres)

0.25 70 500.25 95 700.25 120 950.25 160 1200.25 215 1600.25 295 215

1.052,00 1.052.00

0.262.00 0.262.00

Notes:

* Based on 85th percentile speeds (design speeds)tBased on 50th percentile speeds (one step down from the given design speeds) or based on a coefficient offriction of 0.375

of this time are less than those at low speeds. This isbecause fast drivers are usually more alert. It is alsoexpected that drivers will be more alert on roads indifficult terrain and so the reaction time in thissituation is likely to be less than that in rolling orlevel terrain. Johansen suggested a total driverreaction time of about 0.5 seconds in situationswhere drivers are keenly attentive and a time of 1.5

seconds for normal driving.

The AASHTO standard for total driver reaction timeis 2.5 seconds which represents the time used bynearly all drivers under the majority of roadconditions. Total driver reaction time recommendedin TD 9/81 standards is 2.0 seconds which providesa limited margin of safety over the field study figure.NAASRA recommends total driver reaction time of2.5 seconds as a standard value and 1.5 seconds as

an absolute minimum value. The latter value should

11


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designspeed

(km/h)

5060708090

100110120

TABLE 7

Stopping sight distances recommended by

NAASRA

driver eye height (m)object height (m)

coefficientof friction

fwe,

0.650.600.550.500.450.400.370.35

stopping sight distance (metres)

normal design

R,=2.5 sec

D,506585

105140170210250(1)

1.150.20

1.4 Ds7090

120150200240290350(2)

1.150

constrained situations

R,=2 sec.

45607595

120

(3)

1.150.20

Rt=l.5 sec.

355065

(4)

1.150.20

Notes:(1) = Standard values for stopping sight distance(2) = Values used in less constrained budget situations or in easier terrain

(3) = Adopted as manoeuvre sight distance(4) = Absolute minimum values for stopping sight distance

Visible Psychological Physiological Psychological Physiologicalsignal process process process process

x v a b c d e f 9

Attention Sensation Perception Movement Initiationand decision

Total driver reaction timecommonlv defined

Total driver reaction time

better defined x : Stimulus (signal, obstacle) visible (audible, etc) t o a normal driver

V : Driver attention to the stimulusa : Stimulation of the sense organ

b : Transmission of the sensation bv the senorv nerve and theini tiat ion of the brain processes

c : Identification o f the obstacled : Interpretation of the obstaclee : Decision-making to avoid the obstaclef : Transmission of brain impulses bv the motor nerves

9 : Stimulation of the muscles and the initiation of movements

Fig. 9 Total driver reaction time

12


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be used only if drivers are expected to be driving inconditions which lead to alertness and thecarriageway is sufficiently wide to provide reasonablespace for evasive action.

NAASRA also recommends the use of manoeuvresite distance to achieve cost effective designs ofvertical curves in difficult situations. This distance

ensures that the driver can perceive a hazard on theroad ahead in sufficient time to take evasive actionthrough lateral manoeuvring, rather than stopping thevehicle. Reasonable manoeuvre times fromobservation vary from about 3 seconds at ahorizontal alignment design speed of 50 km/h to 5seconds at 100 km/h. The resulting manoeuvre sitedistances, which are shown in Table 8, are close tovalues for stopping sight distances based on a driverreaction time of 2.0 seconds.

TABLE 8

Manoeuvre sight distances recommended byNAASRA

design speed(km/h)

derived manoeuvre manoeuvre sighttime (seconds) distance (metres)

5060708090

100

3.23.63.94.34.85.6

45607595

120155

The NAASRA manoeuvre site distance can becontrasted with the AASHTO decision sightdistance which has been introduced to allow forsituations where the normal stopping sight distancesare inadequate. This may be appropriate in situationswhen complex or instantaneous decisions,unexpected or unusual manoeuvres are required, andwhen information is difficult to perceive. Suchlocations might be at interchanges, intersections,changes in cross-section, or where drivers in heavytraffic need to perceive information from a variety ofcompeting sources. The provision of the longer sightdistance at these critical locations will ensure thatdrivers can safely

(a) detect and recognise these hazards or informationsources,

(b) decide and initiate an appropriate response and

(c) manoeuvre his vehicle accordingly.

Times for these components of decision sightdistances range from (a) 1.5 to 3.0 seconds. (b) 4.2to 7.0 seconds. (c) 4.0 to 4.5 seconds, resulting indecision sight distances ranging from 10.2 to 14.5seconds depending on design speed. These distancesare at least twice the normal stopping sight

distances.

3.2.3 Coefficient of longitudinal friction

The determination of design values of longitudinalfriction (f) is complicated because of the manyfactors involved. It is, however, known that f valuesare a decreasing function of vehicle speed, exceptunder the most favorable road surface textures.Coefficient of longitudinal friction is measured usingeither the sideways-force machine (SCRIM) or the

portable skid pendulum. The main factors affectingthe friction between the tyres and the road surfaceare:

(i)

(ii)

(iii)

(iv)

Road surface macrotexture: rough macrotextureis required to maintain skidding resistance athigher speeds.

Road surface microtexture: harsh microtexturesof surfacing materials are important to providegood skid resistance as they will puncture anddisperse the thin film of water remaining afterremoval of the bulk water by the macrotextureand tyre tread.

Road surface condition: wet pavements areassumed when deriving values for designpurposes.

Tyres: a good tread pattern provides escapechannels for bulk water and a radial plyincreases contact area; tyre stiffness is also afactor.

If comfort for vehicle occupants is considered to bethe sole criterion, f values greater than about 0.5should not be used as decelerations (f. g) of 0.5gresult in unrestrained passengers sliding from their

seats. In normal driving, such values of f would onlybe generated in emergency braking. For designpurposes, it is important that no loss of control ofthe vehicle occurs during stopping and lower f valuesare therefore desirable.

The design values of f used by AASHTO, shown inTable 5, are generally conservative since they includemost of the curves shown in Figure 10(b). The rangeof speeds assumed for design in Table 5 are basedon average running speeds for low traffic volumeconditions (AASHO 1965) at the lower extreme, anddesign speed at the higher extreme. This reflects

current observations that many drivers travel as faston wet pavements as on dry.

Constant f values were adopted in the TD 9/81standards with a value of 0.25 for desirable minimumand 0.375 for absolute minimum stopping sightdistances for the 85th percentile vehicle speed. The fvalue of 0.25 is slightly less than the minimum targetvalue of pavement skidding resistance of 0.30 forstraight sections and large radius curves proposed byTRRL (Salt and Szatkowski 1973) and shown inTable 9, whilst the f value of 0.375 is consideredacceptable for retaining vehicle control in stopping

on wet normally textured surfaces.

13


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0%

1 o%

40%

60%

90%

1 00%

o 10 20 30 40 50 60

Speed of vehicle (mph )

(a) Skid resistanceof pavement(basedonGermanvalues)

Test data--- Extrapolated

II

o.a

O.J

0.6

0.5

0.4

0.3

0.2Lo 30 40 50 60 JO

Fig. 10

14

Speed of vehicle (mph)

(b) Skid resistanceor varioustyre andpavementrenditions

Variation in coefficient of friction with vehicle

speed in United States

NAASRA adopted relatively high f values as shownin Table 7. These were based on the tests conductedby the Australian Road Research Board (McLean1978 c), ranging from 0.65 at 50 km/h design speeddown to 0.35 at 120 kmlh, due consideration havingbeen given to road surface polishing, the reduction inwet skidding resistance with increasing speed, andthe need for vehicle control in stopping.

3.2.4 Effect of gradient

Shorter braking distances are required on uphillgrades and longer distances on downhill grades asfollows:

Braking distance = ~ metres

254(f~G)

where V = design speed, kmlhf = coefficient of longitudinal frictionG = gradient, Yo, positive if uphill

negative if downhill

For two-lane roads, sight distances are longer onmany downgrades than on upgrades, so that theabove correction is provided automatically.

3.2.5 Effect of trucks

Trucks generally require longer distances to stop fora given speed than cars, but this is offset by thehigher eye height of truck drivers and hence theirbetter visibility and earlier perception of potentialhazards. Truck speeds on crest curves are alsogenerally lower than the speeds of cars. Noadjustment of the stopping sight distance standardsis normally considered for trucks. However, wherethere is a combination of steep downhill grade andhorizontal curvature, higher values than the minimumstandards should be used.

3.3 PASSING SIGHT DISTANCE3.3.1 Critical factors

Factors affecting passing sight distance are thejudgement of overtaking drivers, the speed and sizeof overtaken vehicles, the acceleration capabilities ofovertaking vehicles, and the speed of oncomingvehicles. Driver judgement and behaviour areimportant factors which vary considerably amongdrivers. For design purposes, the passing sightdistance selected should be adequate for the majorityof drivers.

Passing sight distances are determined empiricallyand are usually based on passenger car requirements.On average, heavy commercial vehicles take aboutfour seconds longer than cars to complete theovertaking manoeuvre. Nevertheless, it is unusual forpassing sight distance to be based on commercialvehicle needs, except when the proportion of trucksin the traffic stream is very high. Apart from the


22/47


23/47

TABLE 10

Passing sight distances recommended by

the overtaking manoeuvre (approximately twothirds of the time the passing vehicle occupiesthe opposing lane).

To determine safe passing sight distances, AASHTOassumes that the speed of the overtaken vehicle isequal to the average running speed at intermediatevolumes of traffic where overtaking occurrences are

most likely. The speeds of the overtaking andoncoming vehicles are considered to be 10 mph(16 km/h) faster than that of the overtaken vehicle.

The TD 9/81 standard for passing sight distance (fullovertaking sight distance FOSD) was based on astudy carried out by the Transport and RoadResearch Laboratory (Simpson and Kerman 1982),the results of which are summarised in Figure 11.This shows the distribution of overtaking durationsfor a typical road at an approximate design speed of85 km/h. It can be seen that the time taken for mostovertaking manoeuvres to be completed wasbetween 3 and 15 seconds, with 85 per cent ofdrivers overtaking in less than 10 seconds. The 10second value was therefore adopted for designpurposes. The standard assumes that the overtakingvehicle starts to overtake at a speed two designspeed steps below the nominal design speed andaccelerates to the design speed over the duration ofthe overtaking manoeuvre, whilst the oncomingvehicle travels at the design speed.

In determining lengths of road available for safepassing, TD 9/81 assumes that such lengthsterminate when only an equivalent Abort SightDistance (equal to FOSD/2) is available. This

distance is that required for an overtaking driver tocomplete a manoeuvre in the face of oncomingvehicles from when it is abreast of the overtaken

AASHTO

Assumed Speeds

Passed PassingVehicle Vehicle(mph) (mph)

20 3026 3634 4441 5147 5750 6054 64

DesignSpeed(mph)

20304050606570

Minimum PassingSight Distance (ft)

(Rounded)

8001,1001,5001,8002,1002,3002,500

driver eye height (feet)object height (feet)

3.504.25

TABLE 11

Passing sight distances recommended byTD 9/81

DesignSpeedkmfh

Overtaking ManoeuvreTime

(seconds)

Minimum PassingSight Distance

(metres)

10.010.010.010.010.0

2900607085

100

345410490580

driver eye height (metres)object height (metres)

1.052.001.052.00

vehicle.

TABLE 12

Passing sight distances recommended by NAASRA

Continuation Distanceassing Sight Distance

DesignSpeed

(km/h)

OvertakenVehicle Speed

(km/h)

Timegap

(see)

Sightdistance

(metres)

Timegap

(see)

Sightdistance

(metres)

5060708090

100110120130

43516069778694

103111

13.614.615.716.918.219.721.323.125.2

350450570700840

1010121014301690

4.55.05.46.47.68.39.1

10.011.0

165205245320410490580680800

driver eye height (m)object height (m)

1.151.15

1.151.15

16


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100

80

60

40

20

0

-- -- __....... . ......

-85%

115%

T = Duration of overtaking manoeuvres (sees)

Fig. 11 Distribution of overtaking duration inUnited Kingdom

Passing sight distances recommended by NAASRA

were based on studies carried out by Troutbeck(1981 ), which used a gap-acceptance approach. Thepassing manoeuvre was assumed to be in threephases.

Phase 1 is the distance travelled from the point atwhich the opposing lane is entered to when thevehicle is alongside that being overtaken.

Phase 2 is the distance from the end of Phase 1 towhen the vehicle, still in the opposing lane, is clearof that being overtaken.

Phase 3 is the distance from the end of Phase 2 tothe point at which the vehicle is entirely back in itsown lane.

The minimum distance which is adequate toencourage a given proportion of drivers tocommence an overtaking manoeuvre is known as theestablishment sight distance and is the sum ofPhases 1 to 3. The sum of Phases 2 and 3 wasconsidered as the Continuation distance which wouldenable an overtaking driver to either complete safelyor abort a manoeuvre already underway.

3.4 EYE AND OBJECT HEIGHTSThe, higher the driver eye height and object height,the longer will be the sight distance available over avertical crest curve. On sag curves, obstructions canoccur where an overbridge crosses the alignment.Msibility on horizontal curves depends on whetherthe sight line falls outside the right-of-way limits, Forsight lines on horizontal curves within the right-of-way, eye and object heights are not generallysignificant, except where cutting slopes, bridgeparapets, etc, obstruct the line of sight. Sightdistances outside the right-of-way are much moredependent on eye and object heights. For example,

in determining the value of Alignment Constraint AC

(see para 2.4.2),TD 9/81 requires estimation of theharmonic mean visibility between eye and object,both with assumed heights of 1.05 m.

For geometric design purposes, driver eye and objectheights reflect conditions found in practice. Drivereye height depends largely on vehicle characteristicsand to some extent on driver posture. It is generally

accepted that provision of visibility to the roadsurface, ie an object height of zero, for a distanceequal to that required for safe stopping is not cost-effective. Selection of a higher object height fordesign is thus a compromise between possiblereduced safety and savings in construction costs.Object heights must also be related to whether thevehicle is stopping or passing.

Driver eye and object heights proposed by the threestandards are given in Tables 5, 6 and 7. TheAASHTO driver eye height of 3.50 ft reflects theobserved reduction that was taken place in the lasttwenty-five years in average passenger car and drivereye heights. The AASHTO object height for stoppingof 6 is derived from economic considerations asabove. This height was the lowest which could beconsidered a hazard and perceived by the driver asrequiring him to stop. The AASHTO object height forpassing of 4.25 ft represents the current averagepassenger car height.

TD 9/81 proposes an envelope of clear visibility forstopping involving driver eye heights between 1.05and 2.00 m. The lower bound represents the heightexceeded by 95 per cent of driver heights in UK,whilst the upper bound is a typical eye height for

heavy goods vehicle drivers. For object height, thelower bound of the visibility envelope is 0.26 m, withan upper bound of 2.00 m. For passing, an envelopeof clear visibility between points 1.05 and 2.00 mabove the road surface over the full passing sightdistance is required.

Studies of driver eye heights in Australia haveresulted in the NAASRA recommendation of 1.15 mand 1.8 m for car and heavy vehicle driver eyeheights. NAASRA adopts an object of 0.2 m forstopping under normal design, but allows an objectheight of zero on the approaches to causeways andfloodways subject to flood water residues orwashouts. For passing, an object height of 1.15 m isused.

3.5 COMMENTS ON SIGHT DISTANCEDesign standards for stopping sight distance arelargely dependent on assumed values of tota l driverreaction time and longitudinal coefficient of friction.There is consistency between the three standards inchoice of total driver reaction time, with normalvalues in the range 2.0 to 2.5 seconds representingmost drivers and road conditions. However, it shouldbe recognised that a simple criterion is being used,

based on limited field studies, and which is assumed

17


25/47

to represent the whole population of drivers. TheNAASRA standards allow a lower reaction time of1.5 seconds where drivers are alert but, whilst thismay be attractive in order to achieve constructioncost savings, further investigation is needed and amore detailed specification of such situations wouldbe helpful.

Driver eye heights in the three standards are broadlysimilar, all reflecting trends that have occurred invehicle design and, to some extent, driver position.The concept of an envelope of clear visibility used inTD 9/81 is useful in ensuring safe design for allvehicle types using the road.

There is some variation between the three standardsin assumed design values of wet coefficient oflongitudinal friction. However, other importantfactors to be considered are those related toquestions of vehicle maintenance (tyre and brakecondition) and whether the skid resistance of theroad surface is adequate to provide the likely requireddeceleration, in addition to the need to maintainvehicle control during stopping in wet conditions.

The studies for TD 9/81 have indicated increasingaccident rates with reducing sight distances,particularly where the latter are sub-standard.

Nevertheless, the standard allows consideration ofdepartures from design values in difficult situationson road sections without accesses or junctions.

The NAASRA manoeuvre site distance standard willachieve cost effective designs in terms ofconstruction costs and would appear attractive forlow volume roads, given the assumed driverbehaviour of lateral manoeuvring instead of stopping.

The AASHTO decision sight distances provide moregenerous standards where the decision and initiationof appropriate reponses to perceived information is

more complex. Whilst this may be desirable inidentified locations, it has been introduced here as anexample of where the search for cost effectivestandards based on studies of driver behaviour, andthe safety implications of altering standards, canresult in a recommendation for higher standards thanotherwise.

There are considerable differences in the threestandards for passing sight distance due to differentassumptions about the component distances of thestandard, different assumed speeds for themanoeuvre and, to some extent, driver behaviour.

Nevertheless, the standards are based on studies ofdriver overtaking behaviour in all three countries. Anadditional important practical consideration would bethe siting of overtaking sections. If faster vehicles areconstrained to follow slower ones over a particularsubsection of road, then it is desirable to follow thiswith an overtaking subsection. Otherwise, increaseddriver frustration results and drivers will attempt toovertake at increased risk on more dubiousovertaking alignments. The principle in TD 9/81 ofusing sharper non-overtaking bends and longerstraights where overtaking is safe is a welcome movein this regard, and moves away from the provision oflonger and larger radii curves in flowing alignments

where overtaking can only be carried out at risk.

18

Choice of object height is a compromise between

safety and savings in construction cost. The use of aminimum object height for stopping of 0.26 m inTD 9/81 has brought UK standards broadly in linewith those in US and Australia. Design objectheights must be related to the likely occurrence ofhazards on the pavement surface, which may berelated to the problems of routine maintenance ofroads.

4 HORIZONTAL ALIGNMENT

4.1 ALIGNMENT, USER COSTS ANDACCIDENTS

Horizontal alignment usually consists of a series ofintersecting tangents and circular curves, with orwithout transition curves. The alignment should bedesigned to be as direct as possible in order toreduce road user costs, but will be constrained bytopography in hilly terrain, land use, availability ofroad materials and crossing points. Theenvironmental effects of roads and traffic havebecome increasingly important considerations inmany developed countries, where the non-

quantifiabie costs and benefits of road schemes inaddition to construction, operating and maintenancecosts are taken into account. Many of theseenvironmental effects are related to choice ofalignment.

The layout of the horizontal alignment significantlyaffects the total cost of the road. Mean operatingspeed is a decreasing function of overall horizontalcurvature, so that the road user costs of fuel andtime, which are functions of vehicle speed, will beaffected by the horizontal curvature. Constructioncost normally increases with increasing horizontal

radius, especially in hilly terrain.

The effects of horizontal curve radius on accidentrates have been studied in the United Kingdom(Shrewsbury and Sumner 1980) and are shown inFigure 12. This study showed that accident ratesincreased with reducing horizontal curve radius, butmore rapidly below a value of about QO m. A morecomprehensive study was carried out earlier in theUnited Kingdom (Road Research Laboratory 1965),the results of which are summarised in Table 13. Thestudy showed that inconsistency of the horizontalalignment of a road significantly increased accidentrates, which were affected not only by individual

curve radius and average horizontal curvature, but


26/47

also by the combination of the two. A sharp curveradius on an otherwise straight alignment wouldcause a higher accident rate than that on analignment with a high degree of bendiness.

1

I1 1 1 1 1

0 200 400 600 aoo 1000 1200

Horizontal radius (m)

Fig. 12 Relationship between accident rate and curveradius in United Kingdom

TABLE 13

UK non-intersection injury accidents on straights andcurves on a 9 metre roadway with different levels of

average curvature

AverageCurvature(deg/km)

O2525505075>75

Injury Accidents/10G veh-km

Curve Radius (m)Straightsand Radius 610 to 305 to>1520m 1520 610

0.7 0.7 0.60.6 0.6 0.60.4 0.3 0.60.2 0.3 0.6

4.2 VEHICLE MOVEMENT OCIRCULAR CURVE

1

t


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monotony for drivers and to avoid glare from vehicle The NAAS RA standard does not specify a methodheadlights or the setting sun. Drivers are also found for determining superelevation and side frictionto have difficulty in judging oncoming vehicle factors for the range of intermediate curve radii.speeds, and hence overtaking opportunities, on long Figure 6 is in terms of maximum friction values andstraight sections. superelevation rates from which a minimum curve

\R = Radius of curve (ft)

3

4

.10 fV=80

.08

,06

.04

.02

0

0 5 10 15 20 25D = Degree of curve

Fig. 13 Design superelevation rates for e ~ax = 0.10 recommended by AASHTO

0.04

0 200 400 600

Fig. 14

20

800 1000 1200 1400 1600 1800 2000

Radius (m)

Superelevation of curves recommended by TD9/81


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TABLE 14

Maximum degree of curvature and minimum radius determined for limiting values of e and f by AASHTO

DesignSpeed(mph)

2030405060

20304050606570

20304050606570

20304050606570

Maximume

.04

.04

.04

.04

.04

.06

.06

.06

.06

.06

.06

.06

.08

.08

.08

.08

.08

.08

.08

.10

.10

.10

.10

.10

.10

.10

Maximumf

.17

.16

.15

.14

.12

.17

.16

.15,14.12.11.10

.17

.16

.15

.14.12.11.10

.17

.16

.15

.14

.12

.11

.10

Total(e+f)

.21

.20

.19

.18

.16

.23

.22

.21

.20.18.17.16

.25

.24

.23

.22

.20

.19

.18

.27

.26

.25

.24

.22

.21

.20

MaximumDegree of

Curve

44.9719.0410.176.173.81

49.2520.9411.24

6.854,283.452.80

53.5422.8412.31

7.544.763.853.15

57.8224.7513.388.225.234.263.50

RoundedMaximumDegree of

Curve

45.019.010.0

6.03.75

49.2521.011.256.754.253.52.75

53.522.7512.257.54.753.753.0

58.024.7513.258.255.254.253.5

MinimumRadius

(ft)

127302573955

1,528

116

273

509

849

1,348

1,637

2,083

107

252

468

764

1,2061,5281,910

99231432

694

1,0911,3481,637

NOTE: In recognition of safety considerations, use of e~,~= 0.04 should be limited to urban conditions

8160

5760

tl080 :XStraight and nearlvstraight o/taking sections(both directions)

+Over~aking

section

+I Radii NOT recommended

llo.o~

wesign speed (km/h)

Fig. 15 Horizontal cuwe designrecommendedby TD9/81

radius is determined for a given curve design speedand speed environment. However the standard notesthat radii greater than minimum, together withsuperelevation and friction less than maximumvalues, are usually adopted; and that curves aredesigned so that, over the range of speeds likely tooccur, drivers will be required to turn their steering

wheels towards the centre of curvature to generatethe necessarv side friction.

4.3 MINIMUM CURVE RADIUS4.3.1 Fundamental relationship

For a given design speed, the minimum curve radiuscan be calculated from the equation e + f = V21127. Rusing maximum values of e and f.

A maximum e value of 0.10 has generally beenaccepted for rural roads where ice and snowproblems do not occur. A superelevation rate of 0.12

is sometimes used in verv hilly terrain, but other

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factors should also be taken into account, such asthe proportion of slow vehicles, the stability of highladen commercial vehicles, appearance of the roadand the need to match levels at junctions andentrances.

The minimum curve radii recommended by AASHTO,TD 9/81 and NAASRA are given in Table 14, Table 4

and Figure 6 respectively.

Maximum f values depend on a number of factorssuch as driver comfort, vehicle speed, types andcondition of the road surface, types and condition oftyres and expected weather. Various studies haveshown a decrease in friction values for an increase invehicle speeds.

4.3.2 AASHTO

Maximum f values recommended by AASHTO arebased on comfort and safety criteria. The comfortcriterion was determined by limiting the residualsideways force on the vehicle, which is related to theside friction factor. The safety criterion was satisfiedby adopting smaller values than those observed fromvarious experimental studies, as shown in Figure 16,varying from 0.17 at 20 mph to 0.10 at 70 mph,

0.22

0.2C

0,18

& 0.16:c.-,: 0,14&

fm

0.12

0.10

0.08

. . . . . HR8 lg40 Mover and Berry

.. MeVer 1949 aea e.... Arizona

--- HRB 1936 Barnett

HRB 1940 Stonex and Noble

..-.

.

Assumed for curve design

i I I I10 20 30 40 50 60 70

Speed (mph)

Fig. 16 AASHTO maximum safe side friction factors

4.3.3 TD 9/81

As mentioned in Section 4.2, TD 9/81 maximumvalues of f were based on the need to limit grosslateral acceleration to 0,22 g, a level establishedsome fifty years ago from safety and comfortconsiderations. A study of driver discomfort due tolateral forces on curves (Leeming 1944, Leeming andBlack 1950) confirmed this by suggesting that, when

maximum superelevation (e= 0.07) is allowed for, themaximum design value of f should be 0.15. Byrequiring the 99th percentile speed vehicle togenerate this f value on curves of absolute minimumradius, the corresponding 85th percentile speed grosslateral acceleration of 0.16 g on a curve of absoluteminimum radius will require an f value of about 0.09to be generated. Desirable minimum values of radiushave been established using a limit on the grosslateral acceleration at design speed of 0.11 g, ie halfthe maximum value. Wth the same proportions ofgross lateral acceleration taken by superelevation(45Yo) and by friction (55%), this results in a

desirable f value of about 0.06, and a desirablemaximum superelevation rate of 0.05 (5Yo).

Studies for TD 9/81 have shown that drivers usespeeds that are reduced more on curves of lowerradii compared with their approach speeds. However,the resulting calculated values of gross lateralacceleration that would be used by drivers on thesecurves were greater than the maximum value of0.22 g used for design, indicating that a relaxation ofstandards below absolute minimum radius could beconsidered at very difficult sites. In these cases,values of limiting radius have been established with amaximum superelevation rate of 0.07, requiring fvalues of 0.15 to be generated by vehicles at designspeed.

4.3.4 NAASRA

The proposed NAASRA design values for sidefriction factors were introduced in Section 2.3.4. Thecurve in Figure 17 was derived from the followingconsiderations:

(i) For design speeds up to about 50 kmlh, thecurve recommended by Kummer and Meyer(1967) was adopted. This curve was based onthe minimum recommended skid numbers forAmerican roads which were a function of meanoperating speed. Skid number can be regardedas being approximately equal to the wet sidefriction factor multiplied by 100. Although thiscurve was not adjusted to the 85th percentilespeed (design speed), the curve was of morethan two standard deviations below the meanside friction factors on horizontal curvesmeasured in Australia and was thereforeadopted as a lower bound estimate of theminimum pavement friction likely to beencountered and, as such, was regarded as the

upper limit for plausible design f values.

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(ii) For design speeds between 50 and 90 km/h,the data in Figure 4 was used to derive two setsof points based on:

(a) the upper 85th percentile confidence bandof a linear regression on the data (Method1, Figure 17) and

(b) grouping of the data in 10 km/h ranges to

form the cumulative distributions shown inFigure 18 and then taking the 85thpercentile value of each curve. (Method 2,Figure 17).

0.40

0.30

fg5

0.20

0,10

0

Derivation ofdata points

Method 1

O Method 2A Method 3

\

Kummer and Meyerrecommended minimumpavement friction

,o

p

ProposedA design values

o

A

~

P *.NAASRA ?0 o \values A.O

1 I I 1 I 1

0 20 40 60 80 100 120 140

85th percenti le curve speed (km/h)

Fig. 17 Relationship between 85th percentile car sidefriction factor and 85th percentile car speedfor Australia

A third set of points (Method 3) was calculatedfor 85th percentile side friction factors from theNAASRA 1973 curve speeds and side frictionfactors using the rela tionship between curvespeed standards and 85th percentile curvespeeds shown in Figure 3.

From the three sets of points proposed designvalues were derived using the best fitting curveto the sets of points and giving a smoothtransition to the recommended values below

50 kmfh and above 90 kmlh.

(iii) For design speeds in excess of 90 km/h, thevalues recommended bv NAASRA (1970) wereretained. These values were higher than thoseutilised bv drivers, but were adopted to providea high level of safetv and comfort.

Observed operating

(/~speed grouping

0.1 0.2 0,3 0.4 0.5 0.6 0

Side friction factor

Fia. 18 Distribution of side friction factors computedfrom observed car speeds grouped bv 85th

percentile speed for Australia

The NAASRA curvelspeed studv found that thepaths of vehicles transversing curves varied. On smallradius curves, drivers tended to utilise the availablelane width such that the vehicle path radiusincreased and the f value utilised was below thatimplied bv the assumed circular path. For longer,large radius curves, however, drivers tended todecrease the radius of the vehicle path, utilising an fvalue greater than assumed. This study also

suggested that there were many drivers who wereprepared to tolerate a high degree of discomfort onhorizontal curves. In addition, the higher f valuesutilised bv drivers could also be due to improvementsin road surfaces, tvres and vehicle performance sincethe earlier studies were carried out. Compared withthe AASHTO standards, the NAASRA maximumvalues for f are higher, particularly for the lowerdesign speeds, ranging from 0.35 at 50 km/h to 0.11at 130 km/h.

4.4 TRANSITION CURVESTransition curves are inserted between tangents andcircular curves, or between circular curves ofsubstantially different radius for the followingreasons:

(i)

(ii)

(iii)

(iv)

to provide a gradual increase or decrease in theradial acceleration when a vehicle enters orleaves a circular curve.

to provide a length over which thesuperelevation can be applied.

to facilitate pavement widening on curves.

to improve the appearance of the road byavoiding sharp discontinuities in alignment at

the beginning and end of circular curves.

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The type of transition curve which is normallv usedin practice is the euler spiral, or clothoid. This spiralis defined bv the degree of curvature at anv point onthe spiral being directly proportional to the distancealong the spiral. There are several methods ofdetermining the length of transition curves.

4.4.1 Shortts method

This method was derived for the gradual increase inradial acceleration on railway curves. The equationused is

L= = VB13.63 C R

where LS = length of transition curve, metresU = design speed, krnlhR = circular curve radius, metresC = rate of increase of radial acceleration,

metres/second3

TD 9/81 adopts this equation to derive lengths of

transition curves. It was recommended that the valueof C should not normally exceed 0.3 m/secBalthough, in difficult cases, it could be increased upto 0.6 m/sec3.

A modified equation could also be used which takesaccount of the superelevation (e) on modernhighways. This leads to much shorter lengths oftransition:

This equation implies that, for a driver at the hands-

Off speed for a particular curve radius andsuperelevation, then L,= O, which is theoreticallycorrect.

Leeming (1944) observed that there was notheoretical justification for any particular length oftransition curve, since driver comfort in negotiatingsuperelevated curves was dependent on the value oflateral acceleration itself and not on its rate ofincrease, the latter being the basis for the aboveequations.

AASHTO suggests that roads do not need the same

degree of precision in computing length of transitioncurve using either of the above equations. A morepractical control was adopted known as thesuperelevation run-off method.

4.4.2 Superelevation run-off method

Superelevation run-off is defined as the length ofroad required to achieve the change in superelevationfrom a normal cross section on a tangent to the fullvsuperelevated cross section required on the circularcurve, or vice versa. This length is determined suchthat the slope of the pavement edges (ie the edge ofpavement profiles) over the transitional lengthcompared with the centreline slope or profile should

not exceed a maximum value. These maximumrelative gradients are usuallv established fromconsiderations of appearance of the road. The valuesrecommended by AASHTO are given in Table 15.

Superelevation run-off is directly proportional to thetotal superelevation, which is the product of the lanewidth and the summation of the normal crossfall and

superelevation

L. =

and L, =

where L~ =L, =b=m=

e =e=

rate:

~ (e+e)

superelevation run-off, metreslength of spiral, metreslane width, metresrelative gradientnormal crossfall of pavementsuperelevation of the curve

Table 16 shows the values recommended bvNAASRA to obtain a smooth visual appearance.

TD 9/81 stipulates that the edge of carriagewayprofile gradients should not differ bv more than 1 percent with respect to the line of rotation to ensuresatisfactory appearance.

TABLE 15

Relative gradients between pavement edgeand centre-line for two-lane roads

recommended by AASHTO

design speed maximum relative grade(mph) (%)

20304050606570

0.750.670.580.500.450.410.40

TABLE 16

Relative gradients between pavement edgeand centre-line for two-lane roads

recommended by NAASRA

design speed maximum relative grade(km/h) (Ye)

40 or under 0.9060 0.6080 0.50

100 or over 0.40

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4.4.3 Rate of pavement rotation method

This method adopted by the NAASRA standards wasbased on driver comfort as well as road appearancecriteria. The rate of pavement rotation (n) is definedas the change in crossfall divided by the time takento travel along the length of superelevation transitionat a given design speed. NAASRA recommends thatthe rate of pavement rotation should not exceed

0.025 radians per second of travel time for designspeeds greater than 80 km/h, and 0.035 radians persecond of travel time for design speeds up to70 kmlh.

Thus

L,=fi3.6 n

and L.= L,+*3.6 n

where V = design speed, kmlh

4.4.4 Other considerations

The superelevation run-off method in Section 4.4.2can result, in some instances, in unacceptably shorttransition lengths, particularly for low superelevationrates and with higher design speeds. AAS HTO

therefore recommends minimum lengths, regardlessof superelevation, ranging from 100 ft to 200 ft fordesign speeds of 30 and 70 mph respectively. Thesedistances are approximately those travelled in twoseconds at the design speed.

The TD 9/81 standard recommends that, on sharpcurves, the length of transition should be limited

such that the shift (p) of the circular curve is notgreater than one metre

L 2iep=


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such curves, a vehicle can follow a transitional pathwithin its own lane without the provision of atransition curve. In addition, the effects of suchcurves on appearance is negligible.

4.5 PAVEMENT WIDENING ONCURVES

On horizontal curves, vehicle path width is largersince the rear wheels track inside the front wheels. Inaddition, there is a tendency for drivers to shy awayfrom the edge of the carriageway as they traverse acurve. Therefore, pavements are sometimes widenedon curves to provide a safe clearance betweenopposing vehicles.

The amount of widening required depends on

curve radius,basic lane width on straight sections,vehicle dimensions, andrequired safe clearance (empirical value).

An empirical derivation for pavement widening oncurves was developed by AASHTO and is

reproduced in Figure 19. For practical reasons, andbecause of the empirical nature of the extra widthderivation, it is recommended that design values forwidening should be multiples of one half-foot and theminimum value should be two feet. Table 17 givesthe values recommended by AASHTO assuming arigid chassis design vehicle.

Neither the NAASRA nor TD 9/81 standard relatewidening specifically to design speed, unlike theAASHTO standard. These standards for widening areshown in Tables 18 and 19.

4.6 COMMENTS ON HORIZONTALALIGNMENT

For the detailed design of horizontal curves, bothsafety and driver behaviour considerations have beenimportant in establishing standards. Lower curvestandards do give the designer more flexibility in

difficult areas and can help reduce construction andland-take costs, but the safety and driver implicationsshould also be considered in these cases.

TABLE 17

AASHTO calculated and design values for pavement widening on open highway curveswith two-lane pavements, one-way or two-way.

Widening, in feet, for 2-lane pavements on cuves for width of pavement on tangent of:

24 feet 22 feet 20 feetDegree

of Design speed, mph Design speed, mph Design speed, mph

curve 30 40 50 60 70 80 30 40 50 60 70 30 ~ 50 60

1 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.5 0.5 1.0 1.0 1.5 1.5 1.5 2.O2 0.0 0.0 0.0 0.5 0.5 0.5 1.0 1.0 lo 1.5 1.5 20 2.O 2.O 2.53 0.0 0.0 0.5 0.5 1.0 1.0 1.0 1.0 1.5 1.5 2.O 2.O 2.O 2.5 2.5

4 0.0 0.5 0.5 1.0 1.0 1.0 1.5 1.5 2.0 2.0 2.0 2.5 2.5 3.05 0.5 0.5 1.0 1.0 1.5 1.5 2.0 2.0 2.5 2.5 3.0 3.06 0.5 1.0 1.0 1.5 1.5 2.0 2.0 2.5 2.5 3.0 3,0 3.5

7 0.5 1.0 1.5 1.5 2.0 2.5 2.5 3.0 3.5

1_272_RR114 Review of Geometric Design Standards_DC_1987

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Transcript of 1_272_RR114 Review of Geometric Design Standards_DC_1987