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January 2002

Road Planning and Design Manual Chapter 9: Sight Distance

9Chapter 9

SightDistance

Table of Contents

9.1 General 9-1

9.2 Visibility Restrictions 9-1

9.2.1 Crest Vertical Curve Restrictions 9-1

9.2.2 Sag Vertical Curve Restrictions 9-5

9.2.3 Horizontal Curve Restrictions 9-5

9.2.4 Other Visibility Considerations 9-5

9.3 Types of Sight Distance and Their Application 9-7

9.3.1 Manoeuvre Sight Distance 9-7Derivation 9-7Reaction Time (RT) 9-7Evasive Action Distance (S) 9-7Manoeuvre Sight Distance 9-8Pavement Widths 9-8

9.3.2 Stopping Sight Distance 9-8Derivation 9-8Reaction Time (RT) 9-9Longitudinal Deceleration (d) 9-9Stopping Sight Distance 9-9Stopping Sight Distance on Unlit Roadways 9-10

9.3.3 Overtaking Sight Distance 9-10

9.3.4 Other Types of Sight Distance 9-13

9.3.5 Bicycles 9-13

References 9-13

Relationship to Other Chapters 9-13

Example 9A 9-14

Example 9B 9-14

January 2002

Chapter 9: Sight Distance Road Planning and Design Manual

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Chapter 9 Amendments - January 2002

Revision Register

Issue/ Reference Description of Revision Authorised DateRev No. Section by

1 First Issue Steering JuneCommittee 2000

2 9.3.1 Modification to Table 9.3 W. Semple Jan 20013 9.2.1 Additional sentences on unlit roadways.

Modifications to Figures 9.1 and 9.39.2.4 Additional dot point. Modifications to Table 9.29.3.2 Additions to ‘Longitudinal Deceleration’ on unsealed Steering July

pavements Committee 20019.3.3 Modifications to Figure 9.5New Relationship to other chapters

4 9.2.2 Figure 9.2 - Top of the headlight beam assumed to bedirected upward at 1°.

9.2.3 Vegetation on benches - Designers required to design forin-service conditions.

9.3.1 Widening for manoeuvre sight distance - Volume warrant Steering Janremoved; extent of widening added; Table 9.3 modified to Committee 2002exclude 1.5secs reaction time.

9.3.2 Reaction time - Section on Reaction time replaced withnew text - 1.5secs removed from Table 9.4.

9.3.2 Stopping Distance for trucks - Additional text on providingfor trucks and new table for Truck stopping distance.Table 9.5B added.

9.3.3 Overtaking SD for MCV conditions - Additional text andTable 9.6B added (in accordance with Western Queensland Best Practice Guidelines)

9.3.5 Bicycles - New section cross referencing Chapter 5,Section 5.5.4.

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9.1 General

This section discusses required sight distancesalong roadways excluding intersections. Forrequired sight distances at intersections includingroundabouts, refer to Chapter 13, Intersections,and Chapter 14, Roundabouts.

Sight distance is defined as the distance, measuredalong the carriageway, over which visibilityoccurs between a driver and an object (singlevehicle sight distance) or between two drivers atspecific heights above the carriageway in theirlane of travel. For safety on the road, sufficientsight distance must be provided to enable driversto control their vehicles to avoid collisions withother vehicles or objects on the road. For thisreason, minimum sight distances must be obtainedat all points along the roadway for the chosendesign speed. However, the longest sight distancepossible commensurate with economy should beprovided and only in isolated sections should thesight distance approach the minimum using the85th percentile speed (i.e. where the cost ofproviding sight distances greater than theminimum is too great).

9.2 Visibility Restrictions

Restrictions to visibility may occur on verticalcurves and on horizontal curves. There are 6general conditions where these restrictions occurand these are shown as Types C1-C2, S1-S2, andH1-H2 in Figures 9.1 to 9.3 and are discussedbelow:

9.2.1 Crest Vertical CurveRestrictions

Adequate sight distance needs to be provided overcrests to allow a driver to take evasive action oncethe hazard intrudes into the drivers field of view.Figure 9.1 shows types of restrictions on crestvertical curves (Types C1 and C2). For roadwaysconsisting of one way travel per lane, sightdistance must be provided for Type C1 restrictionsto visibility. On unlit roadways consisting of oneway travel per lane, it is desirable to provide sightdistance for type C2 restrictions to visibilitywithin the limits of the illumination capacity ofpassenger cars. It is required that at least stoppingdistance be achieved for C2 type restrictions whenmeasured to the tail light height (0.6m) for casesbeyond these limits. This is achieved with theminimum radius curves provided for C1conditions. Vertical height parameters used for thecalculation of sight distance are shown in Table9.1.

Table 9.1 Vertical Height Parameters

Vertical Height Parameter Height (m)

Height of Eye of Driver1. Passenger Car 1.15

2. Commercial Vehicle 1.8

Headlight Height 0.75

Object cut-off Height1. Stationary object on road (a) 0.2*

(b) 0*

2. Vehicle tail light/stop light 0.6

3. Approaching Vehicle 1.15

* 0.2 m object height to be used in most situations oncrest curves.

0 m object height can be used on crest curves atintersections where it is necessary to see roadmarking. 0 m is also used for headlight sight distancein sags.

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Chapter 9

Sight Distance

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Figure 9.1 Crest Vertical Curve Restrictions to Sight Distance

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Figure 9.2 Sag Vertical Curve Restrictions to Sight Distance

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Figure 9.3 Horizontal Curve Restrictions to Sight Distance

9.2.2 Sag Vertical CurveRestrictions

On unlit roadways, visibility is restricted to thearea of road illuminated by the vehicle'sheadlights as illustrated in Figure 9.2 (Type S1). Itis desirable to provide for this condition in thedesign of sag vertical curves but it is necessary toconsider the physical limits of the vehicle lightingsystem to provide adequate illumination for thefull stopping distance at the higher speeds.Application of this principle is further developedin Chapter 12, Vertical Alignment.

If an overhead obstruction is placed in a sag e.g. abridge, then adequate sight distance must beprovided as shown by Type S2 restriction tovisibility in Figure 9.2.

Care should be taken in the design of landscapingin these circumstances to avoid the creation of avegetation canopy that restricts sight distance in asimilar way to overhead bridges. Where such acanopy exists, the sight distance requirements areto be dealt with in the same way as at overheadbridges.

9.2.3 Horizontal CurveRestrictions

Visibility may be restricted on horizontal curvesdue to an obstruction on the inner side of thecurve. This is shown by restrictions to visibilityType H1 in Figure 9.3. Sight distance in this casemust be provided. If the obstruction is a cut batterslope then benching may need to be employed toprovide adequate sight distance. Benching is thewidening of the inside of a cutting on a curve toobtain the specified sight distance. It usually takesthe form of a flat table or bench over which adriver can see an approaching vehicle or an objecton the road.

The height of the bench must allow the line ofsight to pass over it unobstructed. Therefore, theplane of the bench must be set at least 300mm (orthe maintenance intervention level, whichever isthe greater) below the line of sight to allow forgrowth of grass on the bench to this height. Grasson benches must be low growing varieties and /or

kept mowed below this level and other vegetationmust not be allowed to be established on thebench.

Designers must take account of the likely serviceconditions for the road. If a line of sight cannot beguaranteed, then the design cannot assume that itwill exist and an alternative treatment must beapplied (e.g. larger radius curve).

Where a line of sight must be preserved, thisshould be included in the maintenancerequirements for that road.

In plan view, the benching is fixed by theenvelope formed by the lines of sight. The driverand the object being approached are assumed tobe in the centre of the inner lane and the sightdistance is measured around the centre line of thelane, along the path the vehicle would follow inbraking.

Figure 9.3 shows the restriction to sight distanceon horizontal curves on unlit roadways (Type H2).In many cases, it is impractical to obtain singlevehicle stopping sight distance around horizontalcurves on unlit roadways because of thelimitations of the vehicle’s headlights. For thisreason, this criterion is generally not used in roaddesign.

9.2.4 Other VisibilityConsiderations

Other visibility considerations include thefollowing:

• If the line of sight is outside the road reserve,additional land will be required toaccommodate the line of sight.

• Guard fencing, noise barriers, bridge handrails,median kerbs and similar obstructions canrestrict the visibility available at horizontal andvertical curves.

• There is a sizable difference between the lengthof sight distance available to a driver dependingon whether the curve ahead of the driver is tothe left or it is to the right.

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Table 9.2 Relevant Types of Sight Distances to be adopted for given road types

NOTES:- indicates that the type of sight distance shown is not appropriate to be used with the roadway type.* indicates that the type of sight distance shown is only to be used in extremely isolated or constrained cases.

(minimum total seal widths apply).ø overtaking sight distance is generally not practicable on crests (see Section 12.3.3).

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• An “intermediate sight distance” equal to twicethe stopping distance and measured from1.15m to 1.15m may be appropriate in somecircumstances where two way travel may occurin the same path, e.g. low volume rural roadswith no line marking. Since this approachwould increase the length of road subject torestricted sight distance, providing stoppingdistance only, together with a wider, markedpavement would be preferred.

Examples considering the combined effect ofhorizontal and vertical restrictions are shown inExample 9A.

9.3 Types of Sight Distanceand Their Application

The various types of sight distances and wherethey should be used is given below. These sightdistance types are listed from the least desirable tobe used (manoeuvre sight distance - used only inextreme circumstances) to the most desirable(overtaking sight distance). Table 9.2 lists thevarious types of sight distance and the roadwaytypes to which they are appropriate.

9.3.1 Manoeuvre Sight Distance

Manoeuvre sight distance allows for a driver toreact to a hazard and slow down to an acceptablespeed in which to manoeuvre around theobstruction. Manoeuvre sight distance, while notrecommended as a prime design parameter, is auseful measure when examining variations ofgeometry in extremely restricted situations.Adequate sealed shoulder widths need to beprovided when using manoeuvre sight distance toallow vehicles ‘room to evade’.

Manoeuvre sight distance has been successfullyused on isolated vertical curves on a straight orlarge radius horizontal curve where lowering ofthe gradeline would mean expensive excavationinto hard rock materials. Manoeuvre sightdistance criterion is not to be used for restrictionson smaller radius horizontal curves unless a cleararea is provided on the outside of the curve. An

evasive action to the inside of a small radiushorizontal curve is very difficult to undertake.

Manoeuvre sight distance is not to be used fordesign speeds above 100 km/h.

Derivation

Manoeuvre sight distance, for a single vehicle tomanoeuvre around an obstruction is the sum oftwo components:

1. The distance travelled during the reactiontime = RTv

2. The distance travelled during themanoeuvring procedure = Si.e. MSD = RTv + S =

(9.1)

where

MSD = manoeuvre sight distance (m)RT = driver reaction time (sec)V = initial speed of vehicle (km/h)v = initial speed of vehicle (m/s)S = evasive action distance (m)

Values of RT and S must be assumed in order tocompute the values of MSD appropriate to aspecified initial speed.

Reaction Time (RT)

Values of reaction time for calculating manoeuvresight distance are the same as those for calculatingstopping sight distance. These are shown in Table9.4 with the exception that the 2.5 sec reactiontime is not used.

Evasive Action Distance (S)

Evasive action distance is the distance a driverrequires to undergo an evasive manoeuvre. Theevasive manoeuvre consists of braking to anacceptable speed followed by a swervingmanoeuvre to avoid the object. The values givenin Table 9.3 appear to provide reasonable evasiveaction distances based on experience gained fromroads in Australia.

S3.6

VRT +=

Manoeuvre Sight Distance

Using Equation (9.1) with substituted values ofRT and S, the manoeuvre sight distance for arange of design speeds can be calculated and areshown in Table 9.3.

Combinations of design speed and reaction timesnot shown in Table 9.3 are generally not used.

Table 9.3 Manoeuvre Sight Distance

Design Evasive Manoeuvre SightSpeed Action Distance (m)

Distance (m)* RT = 2.0 S

50 15 4560 25 6070 35 7580 50 9590 70 120

100 100 155>100 - Do not use

* Derived from Austroads “Geometric Design of RuralRoads”.

Pavement Widths

Where manoeuvre sight distance is all that can beprovided, an adequate total width of sealedpavement (including shoulders) is required toallow adequate space for the evasive action to takeplace. The minimum width required is:

1. Desirable minimum - 12m.

2. Absolute minimum - 9m.

This may require widening of the pavement overthe restricted sight distance zone.

Factors to be considered in reaching a decision onthe width to adopt are:

• traffic volumes;

• roadside hazards;

• traversability of batter slopes adjacent to theshoulder edge.

In the case of a two lane, two way divided road,the sealed width on each side of the median is tobe 6m.

The full width of the widened formation should beavailable at the point where the sight distance

reduces to the manoeuvre sight distance. Tapersare required on the approach to this point at a rateof 1 in 50.

9.3.2 Stopping Sight Distance

Stopping sight distance allows for a driver to reactto a hazard and completely stop prior to thehazard. Stopping sight distance is generally theminimum sight distance that would be provided inmost cases.

Derivation

The lengths of stopping sight distance derivedbelow are for a single vehicle stopping prior to ahazard.

Stopping sight distance is the sum of twocomponents:

1. The distance travelled during the totalreaction time = RT x v

2. The distance travelled during the brakingtime from the design speed to fully stopped

i.e.

(9.2)

where

SSD = stopping distance (m)

d = coefficient of longitudinal deceleration

g = acceleration due to gravity (9.8 m/s²)

RT = driver reaction time (sec)

V = initial speed of vehicle (km/h)

v = initial speed of vehicle (m/s)

a = longitudinal grade/percent (+ve uphill,-ve downhill)

0.01a)254(dV

3.6VR 2

T+

+=

0.01a)2g(dvvRSSD

2

T ++=

=+v

2g(d 0.01a)

2

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Values of RT and d must be assumed in order tocompute the values of SSD appropriate to aspecified initial speed.

Reaction Time (RT)

Reaction time is the time for a driver to perceiveand react to a particular stimulus and takeappropriate action. This time depends on thecomplexity of the decision or task involved.Chapter 5 discusses this issue in detail.

Based on research and observation, most driverscan react simply to a clear stimulus in less than2.5 secs in an emergency. Research has shownthat a minimum reaction time of 2.5 secs isrequired for older drivers at intersections, and notless than 2.0 secs in other situations where driverscan be expected to be alert.

Table 9.4. summarises the usual situationsencountered.

Table 9.4 Driver Reaction Time

Reaction AlignmentTime (s) Type

Few intersectionsAlerted driving situations

2.0 Restricted low speed urban areasConsistently tight alignmentsVery built-up areas - high trafficvolumesUnalerted driving situationsAt all intersections

2.5 Roads with high speed environmentsRoad with isolated geometricfeatures

Longitudinal Deceleration (d)

For sealed pavements, the values of longitudinaldeceleration used for design purposes allow forthe degradation of pavement skid resistance whenwet, and for a reasonable amount of surfacepolishing. The coefficient of longitudinaldeceleration values are given in Tables 9.5A andB. The lower values assumed for the higherspeeds reflect the reduction in wet pavements skidresistance with increasing speed and the need for

lateral vehicle control over the longer brakingdistances.

Unsealed road surfaces are highly variable andvery little research has been undertaken toquantify friction coefficients under variousclimatic conditions for these surfaces. The valuesprovided in Tables 9.5A and B may be used forunsealed roads but designers will need to makeallowance for reductions in friction factordepending on the type of material on the surface,the moisture environment and vehicle types.These factors, in combination with the likelyoperating speeds for the conditions may have animpact on the sight distance required (ARRB,1993).

Stopping Sight Distance

Using Equation (9.2) with substituted values of RTand d, the single passenger vehicle stopping sightdistances for a range of design speeds for levelgrades can be calculated and are shown in Table9.5A. Truck stopping distances can be calculatedusing the coefficient of longitudinal decelerationshown in Table 9.5B.

To balance between the costs and benefits formaking provision for trucks, rural roads are to bedesigned to cater for cars. Truck stoppingdistances should be used for checking purposes atlocations that could be potentially hazardous fortrucks. Note that the operating speed of trucks isgenerally lower than that of cars on the same road(see Table 6.3).

Potentially hazardous locations include:

• Intersections;

• Railway crossings;

• Speed change areas;

• Merge areas such as lane drops;

• Sag vertical curves, particularly at underpasses;

• Horizontal curves - trucks required extrabraking distance on horizontal curves anddesigners should avoid locating featuresrequiring braking on curves (e.g. intersections).

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Table 9.5A Stopping Sight Distance on LevelGrade (Cars)

Design Coefficient Stopping Sight DistanceSpeed of (m)(km/h) Longitudinal RT = 2.5s RT = 2.0s

Deceleration50 0.52 4560 0.48 6570 0.45 8580 0.43 115 10590 0.41 140 130100 0.39 170110 0.37 210120 0.35 250130 0.33 300

Note: Combinations of design speed and reaction timesnot shown in this table are generally not used.

Table 9.5B Stopping Sight Distance on LevelGrade (Trucks)

Design Coefficient Stopping Sight DistanceSpeed of (m)(km/h) Longitudinal RT = 2.5s RT = 2.0s

Deceleration50 0.29 68 6160 0.29 90 8270 0.29 115 10580 0.29 142 13190 0.29 172 159100 0.28 209 195110 0.26 258 243

Stopping Sight Distance on UnlitRoadways

On unlit roadways, sight distance is confined tothe range of a vehicle’s headlight beam. Thelimitations of headlights on high beam of modernvehicles restrict the sight distance that can besafely assumed for visibility of an object on aroadway, to about 120 - 150m. This correspondsto satisfactory stopping distance up toapproximately 90 km/h. This shortfall in vehiclelighting, however, cannot be provided for in roaddesign and is not a design consideration. Thiscriterion does not apply to roads which have streetlighting to the standards prescribed by the S.A.APublic Lighting Code, AS 1158, or on roads with

high traffic volumes where it is necessary to keepheadlights on dipped beam for a relatively highpercentage of the travel time.

9.3.3 Overtaking Sight Distance

Overtaking sight distance is the minimum sightdistance that must be available to enable the driverof one vehicle to overtake another vehicle safelyand comfortably, without interfering with thespeed of an oncoming vehicle travelling at the85th percentile speed should it come into viewafter the overtaking manoeuvre is started.Overtaking sight distance is considered only on 2lane undivided roads. Overtaking sections shouldbe provided as frequently as possible.

The extent of overtaking opportunities along aroad will depend on the topography and thealignment superimposed on it. It is thereforedifficult to establish rigid rules about thefrequency of these overtaking opportunities.However, it is desirable for an overtakingopportunity to be available every five kilometresif possible. If this is not possible, a spacing greaterthan ten kilometres should be avoided.

The designer should be constantly aware of thedesirability of such sections and should nothesitate to improve the design to incorporateovertaking sections, more particularly where onlya slight improvement is required and/or the extracost, if any, is relatively small. In cases where it isimpracticable or uneconomical to provide theovertaking sight distance something less thanstandard is of value and is better than nothing. Itmust be remembered that the 85th percentile speedis not the speed of all vehicles and something lessthan the overtaking sight distance provides for alarge percentage of drivers. Overtaking becomes adesign consideration through its effect on capacity.

With two-lane two-way roads, the overtaking ofslower moving vehicles is only possible whenthere is a suitable gap in the oncoming trafficaccompanied by sufficient sight distance andappropriate line marking. Overtaking demandincreases rapidly as traffic volume increases,while overtaking capacity in the opposing lanedecreases as volume increases. That is, sections of

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road with suitable sight distance and line markingfor overtaking no longer provide the same level ofovertaking opportunity and motorists are forced toreduce their individual desired speeds and followother vehicles until there is another section of roadthat is suitable for overtaking. This is why thelevel of service provided by two-lane two-wayroads is dependent upon traffic flow, theproportion of road that provides potentialovertaking opportunities and the interval betweenovertaking opportunities.

In practice, overtaking zones will usually be thefortuitous result of road alignment and crosssection. Because of the large sight distancesinvolved, it is often not practical to achieveovertaking zones through design. However, gooddesign practice will include a check on theovertaking zones that are provided and may resultin cases where an overtaking zone can beachieved through a practical refinement of thedesign. More commonly though, the proportion ofroad that provides overtaking is used inconjunction with traffic volume to assess the levelof service provided by a section of road and hencedetermine whether overtaking lanes arewarranted.

In determining whether a section of road providessufficient sight distance for overtaking, theovertaking model is based on research intoovertaking operation on Australian roads. This isdescribed in more detail in AUSTROADS 1989and Troutbeck 1981. A potential overtaking zonefor one direction of travel is established whensufficient sight distance is available to allow themajority of drivers to assess that there is sufficientlength of road available to overtake. This is calledthe Establishment Sight Distance and Table9.6A lists values for a range of design speeds.

The longer the length of road that provides at leastthe establishment sight distance, the greater thenumber of drivers that are able to commence anovertaking manoeuvre (phase 1 in Figure 9.4),especially if there is the need to wait for anoncoming vehicle to clear.

Once a driver has commenced to overtake, alesser value of sight distance is needed to safelycontinue with the overtaking manoeuvre (phases 2

and 3 in Figure 9.4). This distance is called theContinuation Sight Distance and Table 9.6Alists values for a range of design speeds.Continuation sight distance allows drivers tocontinue or abort the overtaking manoeuvre withsafety, assuming that an on-coming vehicle justcame into view at the most critical time, the pointof no return (Troutbeck 1981). The point of noreturn is taken to be the time when the rear of theovertaking vehicle is alongside the rear of thevehicle being overtaken.

Where the overtaken vehicle is a multi-combination vehicle (MCV), the continuationsight distance required is larger. Table 9.6B liststhe values for design speeds normallyencountered.

Table 9.6A Overtaking Sight Distances (1.15 m to1.15 m) for Determination of Start and Finish ofOvertaking Zones (Passenger Vehicles)

Establishment ContinuationDesign Overtaken Time Sight Time SightSpeed Vehicle Gap Distance Gap Distance(km/h) Speed (sec) (m) (sec) (m)*

(km/h)

50 43 12.8 330 4.5 16560 51 13.6 420 5.0 20570 60 14.4 520 5.4 24580 69 15.5 640 6.0 300

100 86 17.8 920 7.2 430110 94 19.4 1100 7.9 500120 103 21.0 1300 8.7 600130 111 22.4 1500 9.5 700

* Including 50 m to 60 m clearance at completion.Note: Time gaps used in derivation of distances alsoshown.

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Table 9.6B Overtaking Sight Distances (1.15 m to1.15 m) for Determination of Start and Finish ofOvertaking Zones (MCVs)

Design Assumed Establishment Continuation SightSpeed MCV Sight Distance (m)(km/h) Speed Distance B- Type 1 Type 2

(km/h) (m) Double Road RoadAll Veh. Train Train

70 60 520 350 400 47080 69 640 430 470 58090 77 770 520 590 710

100 86 920 620 730 870110 94 1100 750 880 >1000

Source: Main Roads (2000).

Normally, the overtaking zone will commence atthe point where establishment sight distance isattained and end where the sight distance fallsbelow the continuation sight distance. However,there is a limit to the length of the “continuationsight distance component" of the overtaking zonesince most drivers that are prevented from

overtaking at the start of the zone because ofoncoming traffic will not be encouraged toovertake unless establishment sight distance isregained; hence the term re-establishmentdistance is applied to this limit to the length forwhich continuation sight distance can apply. Re-establishment distance is assumed to be about 3minutes of travel, which is equal to a distance ofDesign Speed/20 km.

Figure 9.5 shows an example of using a sightdistance diagram to determine overtaking zones.In determining the proportion of road thatprovides overtaking in accordance with theovertaking model, the total length of theovertaking zones for both directions is divided bytwice the length of the road being considered.

The overtaking model is not used as a warrant forbarrier lining. Even though a section of road willnot have sufficient sight distance to satisfy theovertaking model, there may be sufficient sight

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Figure 9.4 Overtaking Manoeuvre

Figure 9.5 Overtaking Zones

distance to allow for the safe overtaking of veryslow vehicles. Hence barrier lining is used toindicate areas where it is almost always unsafe fordrivers to overtake. For barrier line warrants, seethe Manual of Uniform Traffic Control Devices.

9.3.4 Other Types of SightDistance

Sight distance requirements at interchanges andintersections (including roundabouts) are dealtwith in the relevant chapters on those elements.These provisions must be met when designinglandscaping and noise amelioration devices in thevicinity, or within the confines of intersectionsand interchanges. (For detailed background to thelandscaping and noise requirements, refer also tothe Road Landscape Manual and the Road TrafficNoise Management Code of Practice).

9.3.5 Bicycles

Sight distance requirements for bicycles arediscussed in Chapter 5, Section 5.5.4.

References

Australian Road Research Board (ARRB) (1993):Unsealed Roads Manual - Guidelines to GoodPractice.

Austroads (1989): Guide to the Geometric Designof Rural Roads.

Queensland Department of Main Roads (1997):Manual of Uniform Traffic Control Devices.

Queensland Department of Main Roads (2000):Traffic of Western Queensland - WQ34 (WesternQueensland Best Practice Guidelines).

Troutbeck, R.J., (1981): Overtaking Behaviour onAustralian Two-lane Rural Highways, ARRBSpecial Report 20.

Relationship to OtherChapters

• Provides input for Chapter 11 - HorizontalAlignment;

• Provides input for Chapter 12 - VerticalAlignment;

• Establishes the parameters for use in Chapters13, 14 and 16 (Intersections, Roundabouts andInterchanges).

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Example 9A

It sometimes happens that earthworks in theparticular case of a horizontal curve with visibilitybenching can often be reduced by the simpleexpedient of increasing the radius, therebyreducing the bench offset and the earthworks. Insome cases benching may be completelyeliminated. Every case requiring benching shouldbe so examined for optimising cost.

As well as providing lateral offset to the bench,the bench will be required to be at a nominalheight depending on the vertical alignment.Example 9A in Figure 9.6 shows an overlappinghorizontal curve and crest vertical curve with theline of sight not over the crest but off theformation. Cutting down the crest on thepavement will not increase sight distance as theline of sight is clear of the pavement. The heightof the bench is however dependent on the verticalalignment.

If a very small radius crest curve has been usedthen the line of sight may be below the pavementsurface. The height of the bench needs to belocated 300mm below the line of sight to allow forgrowth of grass (long grasses should not beplanted on the bench). Regular maintenance willbe required to limit maximum height of grass to300 mm.

An alternative to this would be to use a largerradius horizontal curve so that the line of sightremains within the formation but this will tend toincrease the 85th percentile speed. Eachindividual case will need to be considered on itsown merits to determine the more economicalsolution.

Example 9B

Example 9B in Figure 9.6 shows an overlappinghorizontal curve and sag vertical curve with theline of sight off the formation. The line of sight isabove the pavement surface because of the sagvertical curve. The bench in this case would beconstructed in the location as shown.

It should be pointed out that cost of earthworksshould not be allowed to weigh too heavilyagainst a good vertical alignment, as benching,not done now say, but carried out at a later date,will improve the sight distance, provided thevertical alignment is good, but a low standardvertical alignment can only be improved byregrading.

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* 300mm allowance for vegetation growth.

Figure 9.6 Examples 9A and 9B

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