Overtaking Lanes

April 2002

Road Planning and Design Manual Chapter 15: Auxiliary Lanes

15Chapter 15

AuxiliaryLanes

Table of Contents

15.1 General 15-115.2 Speed Change Lanes 15-1

15.2.1 Acceleration Lanes 15-1

15.2.2 Deceleration Lanes 15-1

15.3 Overtaking Lanes 15-115.3.1 Overtaking Demand 15-2

15.3.2 Overtaking Opportunities 15-2

15.3.3 Warrants 15-2

15.3.4 Length, Location, and Spacing 15-4

15.4 Climbing Lanes 15-615.4.1 General 15-6

15.4.2 Warrants 15-6

15.4.3 Length 15-6

15.4.4 Partial Climbing Lanes 15-8

15.5 Descending Lanes 15-815.6 Passing Bays 15-815.7 Runaway Vehicle Facilities 15-9

15.7.1 General 15-9

15.7.2 Types of Escape Ramps 15-9

15.7.3 Location of Runaway Vehicle Facilities 15-10

15.7.4 Design of Arrester Beds and Escape Exits 15-10

15.7.5 Brake Check and Brake Rest Areas 15-14

15.8 Geometry of Auxiliary Lanes 15-1415.8.1 Starting and Termination Points 15-14

15.8.2 Tapers 15-16

15.8.3 Cross Section 15-17

15.9 Overtaking Lanes and Accesses or Turnouts on the Right 15-1715.10 Linemarking and Signing 15-20

15.10.1 Overtaking Lanes 15-20

15.10.2 Climbing Lanes 15-23

15.10.3 Descending Lanes 15-24

15.10.4 Passing Bays 15-24

References 15-24Relationship to Other Chapters 15-24

April 2002

Chapter 15: Auxiliary Lanes Road Planning and Design Manual

15

Chapter 15 Amendments - April 2002

Revision Register

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

1 First Issue. Steering AugCommittee 2000

2 15.3.3 Table 15.1 revised; clarification of text made.

15.3.4 Additional paragraph added to provide for road train routesto adopt the normal maximum as the minimum;�Absolute minimum�, �Desirable Minimum� terms introduced.

15.4 Figure 15.1 - note added regarding the use of VEHSIM formore accurate results.

15.6 Maximum length added in the fIrst paragraph; details of design clarified.

15.7 Consistent symbol used for �grade� and consistent definitionadopted; appropriate changes to formulae made;second last paragraph of 15.7.5 amended.

15.7.4 Note added to section �Arrester Beds�; added sentencere need to �fluff up� the material in the arrester beds.

15.8.1 1st paragraph - eye height changed to 1.15m;eye height - change back to 1.15m to be consistent with Steering Febthe rest of the manual; sight distance to start and end points Committee 2002- Additional material and changes to Table 15.8 (now 15.8A and 15.8B).

15.8.2 Formula for taper length should have a divisor of 3.6;Merge taper - formula changed to precise figures andchanges made to Table 15.9.

15.8.3 Figure 15.6 modified; additional dot point provided to allowoverlapping overtaking lanes and giving conditions;Figure 15.6 - Approach taper shoulders modified -no extra widening opposite the start of the diverge;additional note on Figure 15.6 reinforcing the need toadopt line marking from �Guide to Pavement Markings�.

15.9 Right turns at overtaking lanes - New section to provide guidance on design of right turns associated withovertaking lanes.

15.9.1 Additions after 1st paragraph.

15.10.1 Signing and marking - Additional words to strengthen the(old 15.9.1) section and to address marking between the two directions.

New Relationship to Other Chapters.

3 15.3.4 Table 15.3 revised. Steering Apr15.4.3 Table 15.9 revised. Committee 200215.8.3 Figure 15.6 modified.

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15.1 General

Auxiliary lanes are those added adjacent to thethrough lanes to enhance traffic flow and maintainthe required level of service for the road inquestion. They are usually of relatively shortlength and can be referred to as speed changelanes, acceleration lanes, deceleration lanes,overtaking lanes, climbing lanes, descendinglanes and passing bays. In addition, emergencyescape ramps (runaway vehicle facilities) areincluded in this category. In this text, weavinglanes are not treated as auxiliary lanes but as partof the required cross section of a motorway whereweaving conditions occur (see Chapter 4).

Auxiliary lanes are used to remove traffic that iscausing disruption to the smooth flow of traffic inthe through lanes to a separate lane to allow thethrough traffic to proceed relatively unhinderedby the disruption. They are a means of separatingthe elements of the traffic stream on the basis ofthe speed difference between them, therebyimproving the safety of the road as well as itscapacity and the level of service provided.

15.2 Speed Change Lanes

15.2.1 Acceleration Lanes

Acceleration lanes are provided at intersectionsand interchanges to allow an entering vehicle toaccess the traffic stream at a speed approaching orequal to the 85th percentile speed of the throughtraffic. They are usually parallel to and contiguouswith the through lane with appropriate tapers atthe entering point. Acceleration lanes are almostalways on the left-hand side of the through lanesalthough in certain circumstances, they can be onthe right (seagull intersections, direct entry rampsat interchanges).

Details of the requirements for acceleration lanes

are given in Chapter 13 Intersections, and Chapter16 Interchanges.

15.2.2 Deceleration Lanes

Deceleration Lanes are provided at intersectionsand interchanges to allow an exiting vehicle todepart from the through lanes at the 85th percentilespeed of the through lanes and decelerate to a stop,or the 85th percentile speed of the intersecting road,whichever is appropriate for the circumstances.

At intersections, the deceleration lane can beplaced on either the right or the left of the throughlanes, depending on the type of turn beingeffected. At interchanges, it is preferred that theexit be from the left side for most ramps and thedeceleration lane will therefore be on the left inmost cases.

Details of the requirements for deceleration lanesare given in Chapter 13 Intersections and Chapter16 Interchanges.

15.3 Overtaking Lanes

Two lane two-way roads can only operatesatisfactorily in most practical circumstances ifadequate opportunities for overtaking areprovided. These opportunities may occur throughthe geometric design providing adequate sightdistance but as traffic increases, theseopportunities gradually disappear andincreasingly long queues (bunches) occur.

Overtaking lanes are provided to break upbunches and improve traffic flow over a section ofroad. They provide a positive overtakingopportunity and are sometimes the only realchance for overtaking to occur.

Chapter 15

Auxiliary Lanes

April 2002 15-1


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15.3.1 Overtaking Demand

The demand for overtaking occurs each time avehicle catches up with another and the driverdesires to maintain the speed of travel. Providedthere is no approaching traffic, this manoeuvrecan occur at will where there is adequate sightdistance. As traffic volume increases, theapproaching traffic will restrict the availableplaces where overtaking can occur and these willbe further limited by the geometry of the road.

If demand is not met, enforced following occurs,queues or bunches form and drivers experiencefrustration and delay. The proportion of traveltime spent following in bunches is a usefulmeasure of quality of service as seen by the driver.

Typical situations that cause this to occur includeslow trucks (frequently on grades) and that part ofthe traffic stream desiring to travel at a speed lessthan the other drivers do. Types are:

� Vehicles with fairly high desired speed of travelbut which slow down appreciably on grades;

� Vehicles with low desired speeds, not affectedby grades; and

� Vehicles with average speeds, but which areseen to be slow by those wishing to travelfaster.

The type of slow moving vehicle influences thenature of the overtaking demand. Some vehiclescan be overtaken easily anywhere along the route,while for others, an up grade opportunity isdesirable. In deciding on the appropriate form ofovertaking lane, the type of slow vehiclesinvolved must be considered, as well as whetherthe overtaking demand is continuous along theroute or whether it is confined to a specificlocation.

15.3.2 Overtaking Opportunities

On two lane roads overtaking opportunitiesdepend on sight distance available as well as gapsin the approaching traffic stream. As opposingtraffic volume increases, the availability of gapsdeclines and the chance of coincidence of the gap

with an appropriate sight distance becomesincreasingly remote. The apparent number ofopportunities can also be reduced by the size ofthe vehicle in front obscuring the sight line,particularly on left-hand curves.

On an existing road, overtaking opportunities canbe improved by improving the alignment or byproviding an auxiliary lane. The latter is often themost cost effective, particularly if the additionallane can be constructed in an area of lowerconstruction costs. In addition, even if thegeometry is improved to increase theopportunities, the opposing traffic may preventthem being realised. Simulation studies by ARRBshowed that providing auxiliary lanes at regularspacing often led to greater improvements inoverall traffic operations than even majoralignment improvements.

A two lane road with overtaking lanes provides alevel of service intermediate between that of twolanes and four lanes. The overtaking lanetherefore offers an economical and practicalmethod of improving traffic flow and thusdeferring the need for more extensive up gradingto dual carriageways. Additional lanes aresometimes required on dual carriageways wheretraffic volumes have reached a level where thelevel of service is unduly affected by theperformance of trucks and other vehicles ongrades.

15.3.3 Warrants

In deciding on whether an overtaking lane iswarranted, the analysis should be undertaken overa significant route length and not be isolated to theparticular length over which the additional lanemay be constructed. Overtaking opportunitiesoutside the particular length can affect the resultconsiderably. On multi lane roads, this may notapply since the reason for the extra lane willusually be confined to a specific location.

The following guidelines have been taken fromAustroads (1989) and are based on researchundertaken by ARRB using traffic simulation andcost benefit analysis. An alternative method is tobase the decision on an analysis of level of

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service. A more detailed analysis can beundertaken by using traffic simulation (TRARR)and applying the Main Roads Cost BenefitAnalysis Manual for Road InfrastructureInvestment.

Traffic data for use in the simulation model mustbe in terms of hourly volumes at the minimum andin terms of half hourly volumes if possible toensure that the peak conditions are properlymodelled. In addition, this will give betterknowledge of the directional split of traffic at thecritical times of the analysis. It is common to usethe hundredth highest hour as the design hour inthese analyses but the actual pattern of trafficdistribution should be examined to determinewhether a different hour would more closelyreflect the operations being considered.

These factors create the delays to driversconveniently measured by the �percent timefollowing� � the parameter used as the basis forassessing the level of service on the road. TRARRmodelling will produce the expected level ofservice for the road being analysed and ajudgement can be made as to the acceptability ofthat level of service.

Table 15.1 sets out the level of service for two-lane two-way roads that corresponds to �percenttime following� .These level of service criteria aretaken from the Highway Capacity Manual (TRB2000). They are also very close to the criteria thathave been used for TRARR studies since 1998(see Cox 1999, 2001). For rural roads, the desiredlevel of service would be �C� for the peakconditions with a level of service of �B� for therest of the day. For National Highways, level ofservice B should be provided for the 100th highesthourly volume in the design year.

When two-lane two-way roads have overtakinglanes or short sections of divided road that arenominally spaced at less than 10km, theproportion of the road with additional lanes startsto become significant. Certainly, this is the casewhen the nominal spacing is about 5km. At thesame time, bunching is not the appropriatecriterion for measuring level of service within thesections with an additional lane. Cox (2001)describes how a combined (or weighted) measure

may be used for these roads. Typically, thecombined (or weighted) measure will yield abetter level of service and/or show that the level ofservice can be sustained over a larger trafficvolume increase.

In the absence of a study based on the TRARRmodelling, the approach described in Austroads(1989) may be used. In this case, the basis foradopting an overtaking lane is the traffic volume,the percentage of slow vehicles (including lighttrucks and cars towing) and the availability ofovertaking opportunities on adjoining sections.The percentage of road allowing overtaking isdescribed in Chapter 9 Sight Distance.

Table 15.1 Percent Time Following Limits forLevel of Service

Level Class 1 Roads Class 2 Roadsof (a), (b) (a), (b)

Service % Time Average % TimeFollowing Travel FollowingTRB 2000 Speed TRB 2000

(km/h)

A ≤35 >90 ≤40

B ≤50 >80 ≤55

C ≤65 >70 ≤70

D ≤80 >60 ≤85

E ≤80 ≤60 >85

(a) For Class 1 roads, both the % time following andthe average speed criteria must be met for a givenlevel of service. This means for example if the %time following is in the range of level of service Bbut the average speed is in the range of level ofservice C, then the level of service will be C. ForClass 2 roads, only the % time following criterionapplies.

(b) The classes of two-lane two-way roads closelyrelate to their functions. Most National Highwaysand State Strategic Roads will be Class 1. MostRegional Roads and District Roads will be Class 2.However, the primary determinant of the Class of aroad will be the motorists� expectations. Thesemay not agree with the formal classification. Forexample, a State Strategic road that passesthrough an area of rugged terrain may beconsidered to be Class 2 if motorists recognizethat a high-speed route is not feasible.

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Table 15.2 gives the current year volumes atwhich overtaking lanes would normally bejustified. They are based on short, low costovertaking lanes at a spacing of 10 to 15 km ormore along a road in a given direction. If thespacing is less than this, a specific cost benefitanalysis will be needed to justify the constructionat the shorter spacing.

Table 15.2 Recommended Traffic VolumeGuidelines for Providing Overtaking Lanes

Overtaking Opportunities Current Year Designover the preceding 5km Volume (AADT)

(a)Description % Length Percentage of Slow

Providing Vehicles (c)Overtaking (b) 5 10 20

Excellent 70�100 5670 5000 4330Good 30�70 4330 3670 3330Moderate 10�30 3130 2800 2470Occasional 5�10 2270 2000 1730Restricted 0�5 1530 1330 1130Very Restricted 0 930 800 670(d)(a) Depending on road length being evaluated, this

distance could range from 3 to 10km(b) See Chapter 9(c) Including light trucks and cars towing trailers,

caravans and boats(d) No overtaking for 3km in either direction

15.3.4 Length, Location, andSpacing

Length

The length of the overtaking lane must besufficient to allow at least one overtakingmanoeuvre and desirably allow queues to bedissipated. Experience shows that most of theovertaking occurs at the beginning of theovertaking lane and there is little point in makingthe lane longer than that required to accommodatethis overtaking. Table 15.3 gives a range of lanelengths that are appropriate for all grades.

The minimum lengths provide for the majority ofsingle overtaking manoeuvres but will not

accommodate multiple overtaking or overtakingwhen there is only a small difference in speed.The desirable minimum values provide formultiple overtaking and the maximum lengthsprovide for extending the lanes to fit in with theterrain. There is little point, from an overtakingpoint of view, to extend the length beyond thesemaximum values. However, in areas where roadtrains operate, the normal maximum length shownin Table 15.3 should be used as the minimum.

It is generally more cost effective to construct twoshorter lengths of overtaking lanes than expend allof the available funds on a single long lane.Several overtaking lanes at closer spacing willallow queues to dissipate before they become toolong.

The length of overtaking lanes on grades isusually determined by the location of appropriatestart and termination points. The start points mustbe clearly visible to the approaching drivers andthe end point should be located so that the slowervehicles will have accelerated to the 85thpercentile speed of the traffic stream. This willminimise the speed differential between vehiclesin the traffic stream.

These requirements could result in lanes longerthan the maxima shown in Table 15.3. This willcause the cost to be larger than envisaged in thedevelopment of warrants in Table 15.2 and aspecific study (TRARR) and cost benefit analysiswill be required. In these circumstances, theovertaking lane may take on more of thecharacteristics of a climbing lane and should beconsidered as such (see discussion below).

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Table 15.3 Overtaking Lane Lengths

Design Total Overtaking Lane LengthSpeed Taper Including Tapers (m)(km/h) Length Absolute Desirable Normal

(a) (m) (b) Min. Min. Max. (c)50 130 200 350 45060 160 250 400 55070 185 300 500 65080 210 400 600 85090 240 500 700 1000

100 265 600 800 1200110 290 700 900 1350120 315 800 1000 1500

(a) For the section on which the overtaking lane isconstructed.

(b) See �Geometry� below.(c) Adopt as minimum where road trains operate.

Location

The location of overtaking sites should bedetermined after considering the following:

� Strategic planning of the road in question andthe long term objectives of that link � thespacing and consequently, expenditure, must bein accord with the strategy to obtain the bestuse of funds over the whole network;

� Nature of traffic on the section of road � ifqueuing occurs all along the route, thenovertaking lanes at any location will be useful;if they occur at specific locations where slowvehicles cause the queue, the specific locationsshould be chosen;

� Location of grades � may be more effective totake advantage of the slower moving vehicles;

� Costs of construction of the alternative sites �may get a more cost effective solution bylocating on the sites where construction ischeapest;

� Geometry of the road � when the sites are noton grades, sections with curved alignment andrestricted sight distances may be preferable tolong straight sections. These locations willmake the location appear appropriate to thedriver. However, sections with curves with

reduced safe speeds are not suitable forovertaking lanes;

� Conflict with intersections/access - see Section15.9.

If the conclusion is that the overtaking lane shouldbe located on a grade, the length will be tailoredto fit the grade. If the costs of the lane on the gradeoutweigh the benefits of being on the grade, thelane should be located to minimise the costs.Alternatively, a partial climbing lane could beconsidered (see �Climbing lanes� below).

Spacing

The factors already discussed must be taken intoaccount in deciding the spacing of the overtakinglanes on a section. An analysis of the operatingconditions over the whole link in the network,combined with the strategy for that link willestablish the desired locations and therefore thespacing of the overtaking lanes. In general, if noauxiliary lanes exist, establishing the first ones ata larger spacing will provide better service thanplacing two lanes in close proximity.

In the first instance, a spacing of up to 20km maybe appropriate, depending on the availableovertaking opportunities. A more desirablespacing would be from 10 to 15km with theobjective of providing overtaking opportunitiesevery 5km in the long term. The intermediatelanes will be provided between the initialinstallations as required as the traffic grows.

There may be cases where the spacing is closerbecause of the proximity of long grade sectionsrequiring treatment. A further case where thespacing may be close is where two partialclimbing lanes are provided on the same longgrade to reduce the total costs involved. In allthese cases, the availability of overtakingopportunities on adjacent sections must be takeninto account.

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15.4 Climbing Lanes

15.4.1 General

Climbing lanes can be considered as a specialform of overtaking lane but they are onlyprovided on up grades. Where they are provided,they form part of the network of overtakingopportunities and will therefore have an effect ondecisions on the location of other overtakinglanes. Unlike overtaking lanes, the location of aclimbing lane is dictated by the specificconditions at a specific location i.e. an up grade.

On multi lane roads, there is no need to takeaccount of the overall overtaking situation, as theeffect is limited to the specific location of thegrade in question. The decision on whether to adda climbing lane is based on level of serviceconsiderations only. Climbing lanes on multilaneroads are specifically provided for slow movingvehicles and are therefore treated differently forsigning and linemarking (see Section 15.9).

15.4.2 Warrants

Climbing lanes are warranted where:

� Truck speeds fall to 40km/h or less; and/or

� Traffic volumes equal or exceed those in Table15.4.

In addition, climbing lanes should be consideredwhere:

� Long grades over 8% occur;

� Accidents attributable to the effects of the slowmoving trucks are high;

� Heavy trucks from an adjacent industry enterthe traffic stream on the up grade; and

� The level of service on the grade falls twolevels below that on the approach to the upgrade or to level �E� (see Highway CapacityManual, TRB 2000).

Table 15.4 Recommended Volume Guidelines forProviding Climbing Lanes

Overtaking Opportunities Current Year Designover the preceding 5km Volume (AADT)

(a)Description % Length Percentage of Slow

Providing Vehicles (c)Overtaking (b) 5 10 20

Excellent 70�100 4500 4000 3500Good 30�70 3500 3000 2600Moderate 10�30 2500 2200 2000Occasional 5�10 1800 1600 1400Restricted 0�5 1200 1000 900Very Restricted 0 700 600 500(d)(a) Depending on road length being considered, this

distance can range from 3 to 10km.(b) See Chapter 9.(c) Including light trucks and cars towing trailers,

caravans and boats.(d) No overtaking for 3km in either direction.

15.4.3 Length

The length of the grade and the start and endpoints of the lane dictate the length of theclimbing lane. The theoretical start point is takenas the point at which the speed of the truck falls to40km/h and decelerating. The end of the lane isdetermined by the point at which the truck hasreached a speed equal to the design speed less15km/h and accelerating. The starting point of thelane should be clearly visible to driversapproaching from that direction.

Truck speeds on grades can be assessed using thecurves included in Figure 15.1 and thelongitudinal section of the road. These curvesassume an entrance speed to the grade of 80km/h.This is conservative as modern trucks can operateat highway speeds approaching those of cars. Ifmore precise design is required, the conditionsshould be analysed using software designed tosimulate truck performance and using entrancespeeds based on the actual speed profile at thesite.

The starting point should be located at a pointbefore the warrant is met to avoid the formation of

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April 2002 15-7


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Figure 15.1 Determination of Truck Speeds on Grades

queues and possibly hazardous overtakingmanoeuvres at the start of the lane (see Section15.8 Geometry).

If the length of climbing lane exceeds 1200m, thedesign should be reconsidered. Options include:

� Partial climbing lane;

� Passing bay(s) in extreme conditions;

� Overtaking lane prior to the grade (where thedelays on the grade are not excessive);

� Retention of the climbing lane where trafficvolumes are sufficiently high.

15.4.4 Partial Climbing Lanes

While climbing lanes should preferably bedesigned to span the full length of the grade, theremay be circumstances where it will be satisfactoryto use a shorter lane on part of the up grade. Thepartial climbing lane may be appropriate if thetraffic volumes are low or the construction costsare very high.

If a partial climbing lane is used, care must betaken to provide adequate sight distance andsigning at the start and merge points to avoidunexpected merge locations. The minimum sightdistance should be stopping distance for thedesign speed.

15.5 Descending Lanes

On steep down grades, the speed of trucks will beas low as on equivalent up grades with a similareffect on traffic flow if overtaking opportunitiesare not available. A descending lane will beappropriate in these circumstances.

If adequate sight distance is available and thetraffic volumes are low enough, overtaking willbe readily accomplished and a descending lanewill not be needed. Similarly, if a climbing lane isprovided in the opposite direction, and the sightdistance is adequate, overtaking slower down hillvehicles can be safely done. A deceleration lanewill not be needed.

Where the down grade is combined with tighthorizontal curves, a descending lane will beappropriate to provide satisfactory trafficoperation.

Design details are similar to those of climbinglanes and are discussed in Section 15.8 Geometry.

15.6 Passing Bays

On steep grades where truck speeds can reduce toa �crawl� speed less than 20km/h, and a fullclimbing lane can not be provided, passing baysmay provide an improvement to traffic flow. Apassing bay is a very short auxiliary lane (of theorder of 100m but not more than 160m plustapers) that allows a slow vehicle to pull aside toallow a following vehicle to pass. The passing bayprovides for the overtaking of the slowest vehiclesand is only appropriate if all of the followingconditions are met:

� Long grades over 8%;

� High proportion of heavy vehicles;

� Low overall traffic volumes; and

� Construction costs too high for partial climbinglanes.

Passing bays must be properly signed to ensuretheir effectiveness. 300m advance warning of thelocation of the bay is required to allow heavyvehicle drivers to prepare for the overtakingmanoeuvre and to alert other drivers to theapproaching facility.

Details of the design of tapers for passing bays areprovided in Section 15.8.2. Sight distance to thetermination points should be in accordance withSection 15.8.1.

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15.7 Runaway VehicleFacilities

15.7.1 General

Where long steep grades occur it is desirable toprovide emergency escape ramps at appropriatelocations to slow and/or stop an out-of-controlvehicle away from the main traffic stream. Out-of-control vehicles result from drivers losingcontrol of the vehicle because of loss of brakesthrough overheating or mechanical failure orbecause the driver failed to change down gears atthe appropriate time. Experience with theinstallation and operation of emergency escaperamps has led to the guidelines described in thefollowing paragraphs for such ramps.

15.7.2 Types of Escape Ramps

Figure 15.2 illustrates four types of escape ramps.

Figure 15.2 Types of Vehicle Escape Ramps

The sand pile types are composed of loose, drysand and are usually no more than 130m in length.The influence of gravity is dependent on the slope

of the surface of the sand pile. The increase inrolling resistance to reduce overall lengths issupplied by the loose sand. The decelerationcharacteristics of the sand pile are severe and thesand can be affected by weather. Because of thesecharacteristics, the sand pile is less desirable thanthe arrester bed. It may be suitable where space islimited and the compact dimensions of the sandpile are appropriate.

Descending grade ramps are constructed paralleland adjacent to the through lanes of the highway.They require the use of loose aggregate in anarrester bed to increase rolling resistance andtherefore slow the vehicle. The descending-graderamps can be rather lengthy because thegravitational effect is not acting to help reduce thespeed of the vehicle.

For the horizontal-grade ramp, the effect of theforce of gravity is zero and the increase in rollingresistance has to be supplied by an arrester bedcomposed of loose aggregate. This type of rampwill be longer than those using gravitational forceacting to stop the vehicle.

The ascending-grade ramp uses both the arrestingbed and the effect of gravity, in general reducingthe length of ramp necessary to stop the vehicle.The loose material in the arrester bed increases therolling resistance, as in the other types of ramps,while force of gravity acts downgrade, opposite tothe vehicle movement. The loose beddingmaterial also serves to hold the vehicle in place onthe ramp grade after it has come to a safe stop.Designs on an ascending grade ramp without anarresting bed are not encouraged in areas ofmoderate to high commercial vehicle usage asheavy vehicles may roll back and jack-knife uponcoming to rest.

Each one of the ramp types is applicable to aparticular situation where an emergency escaperamp is desirable and must be compatible with thelocation and topography.

The most effective escape ramp is an ascendingramp with an arrester bed. On low volume roadsof less than approximately 1000 vehicles per dayclear run off areas without arrester beds areacceptable.

Highway

Highway

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C. HORIZONTAL GRADE

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15.7.3 Location of RunawayVehicle Facilities

Runaway vehicle facilities should not beconstructed where an out of control vehicle wouldneed to cross oncoming traffic. On dividedroadways where adequate space is available in themedian, safety ramps can be located on either sideof the carriageway with adequate advancewarning signs prior to the safety ramp exit. (SeeQueensland Department of Main Roads -�Manual of Uniform Traffic Control Devices� forsigning requirements.)

For safety ramps to be effective their location iscritical. They should be located prior to or at thestart of the smaller radius curves along thealignment. For example an escape ramp after thetightest curve will be of little benefit if trucks areunable to negotiate the curves leading up to it.Because brake temperature is a function of thelength of the grade, escape ramps are generallybest located within the bottom half of the steepersection of the alignment.

Because of terrain a lack of suitable sites for theinstallation of ascending type ramps maynecessitate the installation of horizontal ordescending arrester beds. Suitable sites forhorizontal or descending arrester beds can also belimited particularly if the downward direction ison the outside or fill side of the roadwayformation.

15.7.4 Design of Arrester Bedsand Escape Exits

An arrester bed is a safe and efficient facility usedto deliberately decelerate and stop vehicles bytransferring their kinetic energy through thedisplacement of aggregate in a gravel bed. Anescape exit consists of any surfacing used in thecase of an emergency that will allow a runawayvehicle to exit the downgrade off the road anddecelerate to a lower speed. For example, escapeexits can be side streets, side tracks or accessesthat are not normally signed as a safety ramp. Anarrester bed is a particular kind of escape exit. Thesections following list broad guidelines as to thedesign of arrester beds and escape exits.

Arrester Beds

From field tests and other research studies,rounded particles such as an uncrushed rivergravel (single size) produce higher decelerationsthan the more angular crushed aggregate becausethe vehicles sink deeper into the river gravel,transferring more energy to the stones over ashorter length. The use of a material with lowshear strength is desirable in order to permit tyrepenetration. Sand is not ideal because it compactswith time and moisture ingress. Crushed stone hasbeen used but is not recommended as to beeffective it will require longer beds and will needregular fluffing or decompaction. Note thatregular maintenance of all arrester beds to�fluff up� or decompact the material in thebeds is essential.

Nominal 10mm river gravel has been usedsatisfactorily in testing. The gravel should bepredominately rounded of single size free fromfine fractions with a mean particle size rangingbetween 12mm and 20mm. In general gravelswith a smaller internal friction angle will performbetter than those with larger internal frictionangles.

An appropriate crush test such as the Los Angelesabrasion test (or equivalent) should be used toevaluate durability of the stone. Stones with ahigh crush test will not deteriorate and willtherefore not produce fines.

A variety of bed depths has been used andrecommended throughout the world and moreresearch and experimentation are required. TheNSW Roads and Traffic Authority (RTA) havetested and constructed arrester beds at 350mm and450mm deep. The 450mm deep beds providehigher decelerations. A typical section along thelength of the bed is shown in Figure 15.3.

A gradual or staged increase of the depth of thebed should be provided over the 50m of the entryramp. This is to ensure a gradual rate ofdeceleration when entering the ramp. The first100 metres of the bed after the entry ramp shouldbe 350mm deep. The bed depth should thenincrease over the next 25 metres to 450mm andremain at that depth for the rest of the bed. A bed

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April 2002 15-11


15

5.0

mw

ide

50m

mdeep

Skirting

wall

Concre

tetr

affic

barr

ier2

00m

Long

5m

50m

entr

yra

mp

100m

@350m

m

350m

mdeep 7m

mscre

ened

river

gra

vel

Serv

ice

Road

350m

mdeep

Skirting

wall

(ifre

quired)

Concre

tetr

affic

barr

ier

(ifre

quired)

CR

OS

SS

EC

TIO

NO

FB

ED

SE

CT

ION

AL

ON

GT

HE

BE

D

EM

ER

GE

NC

YA

RR

ES

TE

RB

ED

PL

AN

EM

ER

GE

NC

YE

SC

AP

ER

AM

P

450m

mdeep

25m

25m

Figure 15.3 Typical Emergency Escape Ramp and Arrester Bed Layout

constructed to this design would accommodatelow speed entries within the 350mm deep sectionof the bed. Vehicles entering at higher speeds willslow down significantly before reaching thedeeper section of the bed thus reducing thechances of the vehicle being damaged. Also seeFigure 15.4 for a typical arrester bed cross section.

Following evaluation of the field tests by RTA, ithas been determined that the following results canbe applied in developing designs for arrester beds.This method is the preferred design method to beused when designing sand or gravel arrester beds.

� The average deceleration achieved in sand orgravel beds is:

- Sand 350mm deep 2.8 m/sec

- Sand 450mm deep 3.4 m/sec

- Gravel 350mm deep 3.0m/sec

- Gravel 450mm deep 3.7m/sec

These decelerations may be used in the followingformula to calculate the length of an arrester bed:

L = V² / (26a + 2.55G)

Where

L = length of full depth bed excluding 50mtransition at start (m)

V = entry speed (km/h)

a = deceleration (m/sec)

G = grade (%) - (positive for upgrade, negativefor downgrade).

Note: The entry speed depends on:� weight of truck� rolling friction resistance� air resistance (frontal area of truck)� initial speed� length of grade� slope of grade.

This speed can be assessed using the Mx (Moss) add-in executable TRUCK.exe, which incorporates thesefactors.

� A 50m entry ramp provides a satisfactory andsafe means of entering the full depth of thearrester bed; this entry ramp is not included incalculations for bed length.

� Where insufficient length is available at aparticular site for stopping the vehicle at theanticipated entry speed, the bed depth shouldbe increased in stages from 350mm up to450mm. The increasing depth will providegreater deceleration toward the end of the bedallowing the vehicle to stop within the availablelength. Experience may indicate that a depthgreater than 450mm will be required � each

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15250mm base course

3.0m 2.5m 2.5m

3.0% 3.0%

3.0%1 on 3

1 on 3

Profilegrade

Washedsingle sizedriver gravel

150mm base courseFilter cloth coveredperforated plastic pipe

Flush seal surface

Gravel bed arrester

Serv

ice

road

Arr

este

rbed

1.5m

1on

2

Subsoil drain

1m

Figure 15.4 Typical Design for Gravel Arrester Bed System

case should be designed on its merits.

� From RTA�s experience in the initialapplication of test results in designing andconstructing arrester beds, 10mm diameterround uncrushed river gravel is the preferredmaterial to use in the beds. Sand has problemsof drainage, compaction and contamination andshould not be used unless alternative materialsare unavailable. Beds using sand will require astrict maintenance regime to ensure theircontinued effectiveness.

Escape Exits

Lengths will vary depending on the gradient ofthe facility and the surface material used (specificto the site). Wambold et al (1988) recommends thefollowing formula to determine the length of atruck escape exit.

L = 0.004V² / (R + G/100)

Where

L = Distance to stop (m)

V = Entering velocity (km/h)

G = Percent grade

R = Rolling resistance expressed as equivalentpercent grade divided by 100.

Values of R for several materials are given inTable 15.5.

Table 15.5 Rolling Resistance Values

Surfacing Material Rolling Resistance (R)Portland Cement Concrete 0.010Asphaltic Concrete 0.012Gravel Compacted 0.015Earth, sandy and loose 0.037Crushed Aggregate, loose 0.050Gravel, loose 0.100Sand 0.15

For escape exits, careful consideration of the landuse adjacent to the exit is required. Normal streetsshould only be used at the top of steep grades

where the truck has not generated any speed butthe driver realises that the truck has developed abraking problem and wants to stop. Existing roadsand streets used for property access should onlybe used where the traffic volume is very low andthere is a very low probability of an escapingtruck meeting another vehicle.

Spacing

For new projects Table 15.6 may be used as aguide when considering the need for safety rampsor exit routes on grades greater than 6% and withnumbers of trucks exceeding 150 per day.

Table 15.6 Approximate Distance from Summit toSafety Ramp

Grade (%) Approximate Distance from the Summit to Ramp** (km)

6 � 10 310 � 12 2.512 � 15* 2.015 � 17* 1.5

>17* 1.0* Grades > 12% require approval from the Regional

Executive Director when designing new roadways. ** Actual distances will depend on site topography,

horizontal curvature and costs.

The above distances are not absolute and greaterdistances could be acceptable, as site location isdependent on other factors. The need for a facilitywill be increased if the number of commercialvehicles is more than 250 per day.

Summary of Design Considerations

� The length of the escape ramp must besufficient to dissipate the kinetic energy of thevehicle.

� The alignment of the ramp should be straight orof very gentle curvature to relieve the driver ofundue vehicle control problems.

� The arrester bed material should be clean, noteasily compacted and have a high coefficient ofrolling resistance.

April 2002 15-13


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� The full depth of the arrester bed should beachieved in the first 50m of the entry to the bedusing a tapering depth from 50mm at the startto the full depth at 50m.

� The bed must be properly drained and apositive means of effecting the drainage mustbe used.

� The entrance to the ramp must be designed sothat a vehicle travelling at high speed can enterit safely. A 5° angle of departure or less isrequired, and as much sight distance as possibleshould be provided. The start of the arrester bedmust be normal to the direction of travel toensure that the two front wheels of the vehicleenter the bed simultaneously.

� Comprehensive signing is required to alert thedriver to the presence of the escape ramp.

� Vehicles that enter the ramp will have to beretrieved, as it is likely that they will not be ableto remove themselves from the arrester bed. Anappropriate service road adjacent to the ramp isrequired to effect retrieval.

� When the location of the ramp is such that thelength is inadequate to fully stop an out-of-control vehicle, a positive attenuation (or �lastchance�) device may be required. Care isrequired that the device does not cause moreproblems than it solves � sudden stopping ofthe truck can cause the load to shift, jackknifingor shearing of the fifth wheel, all withpotentially harmful consequences to the driverand the vehicle. Judgement will be required onwhether the consequences of failing to stop areworse than these effects. Crash cushions orpiles of sand or gravel have been used as �lastchance� devices.

15.7.5 Brake Check and BrakeRest Areas

A Brake Check Area is defined as an area set asidebefore the steep descent as distinct from a BrakeRest Area which is an area set aside forcommercial vehicles part way down or at thebottom of the descent.

These facilities should be provided at least to anunsealed gravel condition on routes that have longsteep downgrades and commercial vehiclenumbers of around 500 per day, especially onNational Highways and Principal State Roads.These areas, when used, will ensure that driversbegin the descent at zero velocity and in a lowgear that may make the difference betweencontrolled and out-of-control operation on thedowngrade. It also would provide an opportunityto display information about the grade ahead,escape ramp locations and maximum saferecommended descent speeds.

These areas will need to be large enough to holdseveral prime mover and semi-trailercombinations, the actual numbers depending onvolume and predicted arrival rate.

The location will need good visibility withacceleration and deceleration tapers provided.Adequate signage will be required to advisedrivers in advance of the facilities. Special signs,specific to the site, will need to be designed forthese areas.

15.8 Geometry of AuxiliaryLanes

15.8.1 Starting and TerminationPoints

The start and termination points of an auxiliarylane should be clearly visible to approachingdrivers from that direction. The start point shouldbe prior to the point at which the warrant is met toavoid potentially hazardous overtakingmanoeuvres at the start of the lane. Visibility tothe start of the taper should be sufficient for thedriver to assess the situation and make a decisionon the course of action to take. The desirablevisibility to this point is given in Table 15.7 andshould be measured from an eye height of 1.15mto an object height of 0.6m (tail light). The objectheight is based on the assumption that the courseof action will be determined by the action of thepreceding vehicle.

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Table 15.7 Sight Distance to Start of AuxiliaryLane

Design Braking Distance RoundedSpeed Distance for 5 sec. of Visibility(km/h) (m) Travel (m) Distance (m)

50 20 70 9060 30 83 11570 43 97 14080 59 111 17090 78 125 200

100 101 140 240110 129 153 280120 162 167 330130 202 181 390

The termination of the auxiliary lane shoulddesirably be at a point where there is sufficientsight distance for the driver in the faster lane todecide whether to complete or abandon theovertaking manoeuvre. Tables 15.8A and 15.8Bset out these values.

The sight distance should be measured from aneye height of 1.l5m to an object height of zero inthe middle of the through lane and 20m past thestart of the merge taper. The sight distances for thecompletion of overtaking of cars and prime moverand semi-trailer combinations match reasonablywell with the position from which the �left laneends, merge right� sign can be read (see Manual ofUniform Traffic Control Devices).

As an absolute minimum, stopping sight distanceshould be provided, measured from an eye heightof 1.15m to an object height of zero at the start ofthe merge. Completion of any overtaking thenrelies on the standard warning signs at the end ofthe auxiliary lane. The positioning of these signsas specified in the Manual of Uniform TrafficControl Devices supports the completion ofovertaking of cars and prime mover and semi-trailer combinations.

In all cases, a further check should be made thatstopping sight distance is provided, measuredfrom an eye height of 1.l5m to an object height ofzero in the middle of the traffic lane at the end ofthe merge.

Table l5.8A Sight Distance to End of ClimbingLane

Design Car & B-double Type 1 Type 2Speed PM & semi routes Rd Train Rd Train(km/h) operation routes routes

50 100 100 105 12060 130 130 135 15570 150 160 175 20580 185 200 220 26090 230 250 280 325

100 285 305 345 400110 350 350 350 400120 385 385 385 400130 400 400 400 400

Notes:1. Distances derived from analysis of the length

required to complete the overtaking of relevantvehicles using the Troutbeck model for overtakingbut with no approaching vehicle.

2. Heavy vehicle speeds assumed to be the lesser of15 km/h below the design speed or typicalmaximum vehicle speed.

3. Overtaken car speeds > mean free speed butslightly < design speed.

4. At the higher design speeds, the speed ofovertaken cars is more dominant than the speed ofthe overtaken trucks.

5. The sight distance should be measured from aneye height of 1.15m to an object height of zero inthe middle of the through lane and 20m past thestart of the merge taper. Distances include anextra 30m to provide clearance and perception ofthe merge taper. This is why the object point islocated 20m past the start of the merge taper.

Table 15.8B Sight Distance to End of OvertakingLane

Design Car & B-double Type 1 Type 2Speed PM & semi routes Rd Train Rd Train(km/h) operation routes routes

50 110 120 130 14560 135 145 160 18070 165 180 195 22580 200 220 245 28590 250 270 305 355

100 300 330 345 400110 375 410 410 435120 430 430 430 435130 450 450 450 450

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Notes:1. Distances derived from analysis of the length

required to complete the overtaking of relevantvehicles using the Troutbeck model for overtakingbut with no approaching vehicle.

2. Heavy vehicle speeds assumed to be the lesser oftypical truck speeds for the design speed (seeTable 6.3) or typical maximum vehicle speed.

3. Except for 130km/h design speed, overtaken carspeeds match the design speed.

4. At the higher design speeds, the speed ofovertaken cars is more dominant than the speed ofthe overtaken trucks.

5. The sight distance should be measured from aneye height of 1.15m to an object height of zero inthe middle of the through lane and 20m past thestart of the merge taper. Distances include anextra 30m to provide clearance and perception ofthe merge taper. This is why the object point islocated 20m past the start of the merge taper.

It is desirable for the termination point to be on astraight to give drivers a better visual appreciationof the approaching merge. Termination on a left-hand curve should be avoided because slowvehicles are seriously disadvantaged by lack ofrear sight. It is also desirable that the terminationpoint be on a down grade to minimise the speeddifferential between vehicles.

15.8.2 Tapers

Diverging Taper

The widening of the pavement at the start of theauxiliary lane is achieved with a taper. The lengthof the taper should be sufficient to permit easydiverging of traffic with the slower traffic movingto the left and the faster traffic going to the rightlane. This length depends on the speed of theapproaching traffic and the width of the throughlane. The rate of lateral movement is assumed tobe 1.0m/sec, giving the following formula fortaper length:

TD = V W / 3.6

whereTD = Diverge Taper Length (m)V = 85th Percentile approach speed (km/h)W = Amount of pavement widening (m)

If convenient, developing the widening around ahorizontal curve can improve appearance andcontribute to an easier divergence of the trafficinto the fast and slow streams.

Merging Taper

At the termination of the auxiliary lane, thepavement width should be reduced by a taper thatallows the two streams to merge into one. Sincethis situation is equivalent to the dropping of alane, drivers will be less prepared for the mergingaction than they would be if merging from anacceleration lane. It is therefore necessary toadopt a lesser rate of merging than for the taperson acceleration lanes and a rate of 0.6m/sec isused (as opposed to a rate of 1.0m/sec foracceleration lanes - see Chapter 13 � Intersectionsat Grade). The minimum length depends on thespeed of the approaching traffic and the width ofthe lane and is determined from the followingformula:

TM = V W / 2.16

where

TM = Merge Taper length (m)V = 85th Percentile approach speed (km/h)W = Amount of pavement widening (m)

(This formula has been derived on the basis of amerging rate of 0.6m/sec of lateral movement.)

A �run out� area should be provided through themerge area to accommodate those vehiclesprevented from merging as they approach thenarrowed section. This can be achieved bymaintaining a total pavement width in thedirection of travel equal to at least the sum of thefull lane width plus a shoulder width of 3.0m overthe full length of the taper plus 30m (see Figure15.6).

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Table 15.9 Tapers for Diverges and Merges

Design Speed Taper Lengths (m)(km/h) Diverge Merge

50 50 8060 60 10070 70 11580 80 13090 90 150

100 100 165110 110 180120 120 195

15.8.3 Cross Section

Pavement Width

The width of the auxiliary lane should not be lessthan the normal lane width for that section ofroad.

Shoulder Width

Because the total pavement has been widenedover the section with an auxiliary lane, a shoulderwidth of 1.0m is often satisfactory. This width willhave to be increased in areas of restrictedvisibility (e.g. around curves) and in the mergearea at the end of the lane (see above).

Crossfall

The crossfall of the auxiliary lane will usually bethe same as the adjacent lane. Because of theadditional width of pavement, the depth of waterflowing on the pavement should be checked toensure that aquaplaning does not occur. It may benecessary to change the crown line to overcomethis type of problem.

Lane Configurations

The specific circumstances of each design willdictate the preferred treatment for individuallocations but the following considerations shouldbe taken into account when deciding on the layoutof the design:

� If duplication is a longer term goal, providing asection of four lane divided road may be alogical first stage;

� Providing a four lane section of divided road isapplicable when the analysis of the road showsthat a spacing less than 5km is required and thetopography is suitable;

� The merge areas of opposite overtaking lanesshould be in accordance with Figure 15.5;

� Diverges may occur opposite each otherwithout any special requirements. At locationswhere additional definition is required, apainted median may be installed as shown inFigure 15.5;

� Overtaking lanes in opposite directions may beoverlapped provided the start and ends arearranged in accordance with Figure 15.6 and a1m painted median is included (see alsoSection 7.4.2).

The general development of overtaking lanes isshown in Figure 15.6. Details of the linemarkingand signing are discussed in Section 15.9.

15.9 Overtaking Lanes andAccesses or Turnoutson the Right

Overtaking lanes are constructed to provide roadusers with an opportunity for unhinderedovertaking of slower vehicles. There is anexpectation by drivers that slowing or stoppedvehicles will not obstruct this overtakingmanoeuvre. For this reason it is highlyundesirable to have right turning vehicles fromthe overtaking lane.

Slowing or turning vehicles should notcompromise vehicle use of overtaking lanes.

When siting overtaking lanes the presence andlocation of accesses and turnouts needs to beconsidered and appropriate facilities provided.When accesses or turnouts are located to the rightof the overtaking lane the following steps need tobe followed.

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*

**

**

*

*

*

UNACCEPTABLE*(Overlapping Merges)

UNDESIRABLE*(End of Merges Opposite)

ACCEPTABLE*(Starts of Merges Opposite)

DESIRABLE*(Starts or ends of merges ideally separated by 3 secs of travel time)

PAINTED MEDIAN DEFINITION FOR ADJACENT DIVERGES*(Continuity lines should not overlap))

Painted Median

Notes:

1. The actual location of the start or end of merges to be determined in conjunction with vertical alignment.2. Linemarking to be in accordance with Guide to Pavement Markings.3. Drawing Not to Scale.

Figure 15.5 Overtaking Lane Configurations

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1 1

1

1

1

W

30m min.

W W

W

Min Diverge Taper

Min Merge Taper

END OF OVERTAKING LANELocate on downgrade beyond crest. Desirably

locate on a straight alignment and straight gradewith greater than stopping sight distance to the

commencement of the merge.

30m min.*

START OF OVERTAKING LANEDesign to direct vehicle into left lane

1

1

1

1

1

2

1 Shoulder widths to be in accordance with Chapter 7 (Section 7.3) and Section 15.8.2.

Shoulder tapers should be 1:50.

Signposting and linemarking should be in accordance with the Guide to Pavement Markings.

2

3

Notes:

3

3m

3

* See Figure B3.9 ofGuide to PavementMarkings

3m

Figure 15.6 Development of Overtaking Lane

1. Assess the access/turnouts usage. (Volume andvehicle type, seasonal or slow).

2. Consult with owner or LGA on current usageand future potential.

3. Assess the risk of right turn to the access/turnout from the overtaking lane.

4. Can the overtaking lane be relocated to avoidthe access/turnout? If so determine a moreviable location.

5. Can the access or turnout be closed to use? If soclose the access/turnout following consultation.

6. Can right turn access be prevented bylinemarking or median? If so provideappropriate barrier and construct a u-turnfacility clear of the overtaking lane.Consideration needs to be given to the amountof extra travel to carry out this manoeuvre. 1.5 minutes of travel with a maximum distanceof 1.5km is considered to be acceptable ((a), (b)and (c) in Figure 15.7)

7. Can the access or turnout be relocated beyondthe overtaking lane? If so construct in newlocation.

8. Can the accesses or turnouts be rationalised andcollected by a service road with the egressrelocated beyond the overtaking lane?Construct service road and new entry beyondthe overtaking lane.

9. Even after all those considerations it may stillbe necessary for the overtaking lane to remainat this location. Should this be the case thenappropriate right turn protection (Figure15.7(e)) or pull over to the left (Jug HandleFigure 15.7(d)) to create a road crossing is to beprovided. The jug handle facility within theovertaking lanes is for passenger and lightcommercial vehicles only. It is not suited to thelarger heavy and slow moving vehicles due tothe time to cross the lanes. Note that sightdistance to this facility must be at least safeintersection sight distance. Locating these laneson left hand curves should be avoided becauseof the difficulty of providing adequatevisibility.

Service road collection, u-turn facilities and leftpull overs are shown in Figure 15.7. Figure 15.8provides a flow chart for this process.

Right turns that are downstream of an overtakinglane can also cause problems for the traffic flow.At worst, they can provide a higher degree ofhazard due to the combination of a vehicle that isstopped while waiting to turn and the increasedspeed of the through traffic. Right turningvehicles may also cause traffic bunching to reformprematurely.

The desirable minimum spacing is 15 seconds oftravel past the end of the merge taper. This isbased on:

� 5 seconds travel time until the driver is facedwith a new decision after the merge;

� 3 seconds travel time to see and comprehend aright turn sign;

� 7 seconds travel time to accommodate thestandard spacing of the right turn sign beforethe intersection.

At this spacing, even though traffic flow mayinitially be compromised when there is a turningvehicle, the right turn is still likely to be within thezone where headways are being re-establishedafter the overtaking lane. This means that it isunlikely that bunching will reform prematurely.

15.10 Linemarking andSigning

15.10.1 Overtaking Lanes

The general linemarking and signingrequirements are given in the Guide to PavementMarkings, Figures B3.9 and B3.10. Taperdimensions should be taken from Table 15.9above.

The provision of advance signs for auxiliary lanespromotes road safety and improves the quality ofservice as perceived by the driver. Having seensuch a sign, a driver wishing to overtake mayrelax his search for overtaking opportunities andis less likely to accept gaps with low safety

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April 2002 15-21


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Figure 15.7 Overtaking Lanes and Accesses or Turnouts on Right

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No

Isovertaking

lane locationstill desirable?

Yes Provide protectedright turn or left

pullover

Consultation requiredDetermine

Access/Turnout usage -current and future

IsAccess/Turnout

high risk?

No

Yes

No Protection required.Note: This would be

a rare case.

Canovertaking

lane berelocated?

Yes

No

Select new site

No

Canaccess/turnout

be closed?

YesConsult and close

Canaccess/turnout

entry beprevented?

Yes

No

Provide appropriatebarrier and U-turn

facility

No

Canaccess/turnout

entry berelocated?

Yes Construct access innew location

Is distanceto U-turn & back

acceptable?<1.5km or

minute?

Yes

No

Canaccess be

rationalised orcollected by

serviceroad?

Yes Construct service roadand egress/U-turn

No

Relocateovertaking

lane

Figure 15.8 Flow Chart - Overtaking Lanes and Accesses or Turnouts on Right

margins. Advance signs are particularlyappropriate when significant bunching occurs for3 minutes of driving time (at the slow vehicle�sspeed) before the commencement of an auxiliarylane.

General practice for marking overtaking barrierlines on rural roads is described in the Guide toPavement Markings. For auxiliary lanesconstructed as three-lane road sections, threeparticular aspects are of relevance:

� In the direction of the auxiliary lane (direction1) it is normal practice to provide a continuousbarrier line over the full auxiliary lane length -including tapers - to prohibit any use bydirection 1 vehicles of the third or opposingtraffic lane. This also serves to define thecentreline of the road and indicate that thecentre lane is primarily for direction1 traffic.

� For direction 2 traffic a barrier line is generallyprovided adjacent to the auxiliary lane divergeand merge tapers.

� For direction 2 traffic adjacent to an auxiliarylane in direction 1, the direction 2 laneseparation line marking should follow normalpractice for two lane roads. This means that, ifsight distance permits, direction 2 vehicles maybe permitted to use the centre lane as anopposing traffic lane provided no vehicles areencountered in that lane.

Some use of auxiliary lane sections by opposingtraffic is allowed, particularly when trafficvolumes are low. However there may be caseswhere more restrictive line marking isappropriate. These will generally arise when thereexists a combination of the following factors:

� short auxiliary lane length;

� moderate to heavy traffic volumes;

� sight distances only marginally adequate forovertaking; and

� perceived operational or safety problems on agiven road section.

The use of more restrictive line markings shouldnot be too widespread, however, since the

presence of apparently unnecessary barrier linescan lead to driver frustration and a reduced qualityof service on a road.

15.10.2 Climbing Lanes

On two lane roads, climbing lanes should betreated as an overtaking lane and markedaccordingly.

On multilane roads where a climbing lane isrequired, only the slow moving vehicles arerequired to enter the climbing lane. In thesecircumstances, the general requirements for thelayout of the climbing lane are shown in Figure15.9. Details of the signing and marking are givenin the Guide to Pavement Markings, Figure B3.8.Taper dimensions should be taken from Table15.9.

Figure 15.9 Development of Climbing Lane onDivided Road

Notes:

1. Refer Section 15.8 for design details of merge anddiverge areas.

April 2002 15-23


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2. Shoulder widths in accordance with Chapter 7.

3. Shoulder Tapers shoulder be 1.50.

4. Signposting and linemarking in accordance with theGuide to Pavement Markings and MUTCD

15.10.3 Descending Lanes

Linemarking should be provided in the same wayas for overtaking lanes with appropriatemodifications of the signing to suit thecircumstances.

15.10.4 Passing Bays

Signs and marking for passing bays is provided asshown in Figure B3.8 of the Guide to PavementMarkings.

References

AASHTO (1994): A Policy on Geometric Designof Highways and Streets.

Austroads: Rural Road Design (1989): Guide tothe Geometric Design of Rural Roads.

Cox, R.L. (1999): Captain Cook Highway - FinalReport on the Current Level of Service andOptions for Improving the Level of Service fromBuchans Point to Yule Point - QueenslandDepartment of Main Roads.

Cox, R.L. (2001): Determiningn the Level ofService on Roads with Multiple Overtaking Lanes- Road System and Engineering Forum,Queensland Department of Main Roads.

Queensland Department of Main Roads,Queensland (2000): Guide to PavementMarkings.

Queensland Department of Transport (1992):Development of Design Standards for SteepDowngrades, Transport Technology Division.

Roads and Traffic Authority, NSW � Road DesignGuide, Section 9.

Transportation Research Board (2000): HighwayCapacity Manual � Special Report 209.

Wambold J.C., L.A. Rivera-Ortiz and M.C. Wang(1988): A Field and Laboratory Study to EstablishTruck Escape Ramp Design Methodology -Pennsylvania Department of Transportation.

Relationship to OtherChapters

� Relies on Chapter 13 for details of accelerationand deceleration lanes;

� Sight distance requirements rely on Chapter 9to some extent;

� Cross section details are taken from Chapter 7;

� Chapter 5 has additional discussion onovertaking requirements;

� Overtaking opportunities are discussed in moredetail in Chapter 12.

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Overtaking Lanes

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