Geometric road design for heavy vehicles – advancing but

24th ARRB Conference – Building on 50 years of road and transport research, Melbourne, Australia 2010

© ARRB Group Ltd and Authors 2010 1

GEOMETRIC ROAD DESIGN FOR HEAVY VEHICLES – ADVANCING BUT STILL A WAY TO GO

Ricky Cox, Queensland Department of Transport & Main Roads, Australia

Dr Owen Arndt, Queensland Department of Transport & Main Roads, Australia

ABSTRACT Historically, geometric road design for heavy vehicles focussed on providing sufficient road space within the road cross-section and for vehicle swept paths at intersections. Design criteria related to heavy vehicle performance were limited to grades and climbing lanes.

The 2003 Austroads guides for the geometric design of roads incorporated truck stopping sight distance criteria for the first time and listed situations that should be checked for truck operation. In some of these situations, the provision for trucks simply reflected good design practice for car operation.

The 2009 Guide to Road Design now requires designs to be checked to ensure that they are safe for trucks and includes additional design criteria based on Australian research and experience over the last 10 years.

This paper covers:

How geometric road design for heavy vehicles has been advanced by the 2009 Guide

The research, experience and assumptions behind the development of some of the criteria

The relevance of design vehicles

The new challenges and compromises that will be faced by road designers

Important design criteria that still need to be developed.

INTRODUCTION Geometric road design criteria cover a wide range of issues such as choice of horizontal and vertical curve size, traffic lane width, intersection types and layout, sight distance, overtaking provision, etc. However, all geometric road design standards have some underlying basis of vehicle dynamics, vehicle size, driver characteristics or some combination of these.

Vehicle size has an obvious impact on traffic lane width and clearance to vertical obstructions. As far as maximum vehicle width and maximum vehicle height are concerned, these have been static in Australia for more than 35 years although recognition has been given to the need for some roads to cater for special vehicles with increased width and/or height. Increases in vehicle length lead to increases in the time needed for a heavy vehicle to clear an intersection and have an impact on the time needed to overtake a heavy vehicle. Less obvious is the link between vehicle length and vehicle swept path.

The swept path of a vehicle is the area of ground swept by all parts of the vehicle as it moves along the road. More importantly, swept path width increases when a vehicle is on a horizontal curve because the rear wheels offtrack with respect to the front wheels of the vehicle.

Geometric road design criteria that are directly related to swept path are:

Lane Width.

Curve widening.

Method of applying curve widening.



Turning roadway (left turn slip lane) widths.

Turning roadway / left turn geometry.

Roundabout geometry.

Median Openings.

Driveway entrances to properties.

Vehicle turning templates.

Geometric road design standards that are indirectly related to swept path are:

Application of superelevation because traditionally, the application of curve widening has been tied to this.

Aspects of roundabout geometry that control the path steered by the drivers of heavy vehicles.

Historically, geometric road design for heavy vehicles focussed on providing sufficient road space within the road cross-section and for vehicle swept paths at intersections. Design criteria related to heavy vehicle performance were limited to grades, climbing lanes and sight distance for some intersection manoeuvres.

In 2003, the Austroads Guide for the Geometric Design of Rural Roads incorporated extra design criteria to cover trucks. This was based on the Austroads report AP-R211/02 (2002) by McLean, Tziotis and Gunitallake. For the first time (possibly even in the world) a national design guide covered side friction factors for trucks, stopping distances for trucks and a design eye height for trucks. However, as shown in Table 1, and the following statement, Austroads (2003) tended to make designing for trucks an optional design check:

“The 19.0m prime mover and semi-trailer is to be used as the design vehicle for cross section elements and the car as the design vehicle for horizontal and vertical geometry.”

This situation was probably because Austroads (2002) took the position that only for major freight routes, could it be economically justified to design for both cars and trucks. Indicative volumes were included in Table 12 in Austroads 2002 and reproduced here as Table 2. Furthermore, Austroads 2003 did not include the side friction factors in Austroads 2002 (i.e McLean, Tziotis and Gunitallake (2002)) that were needed for the design of horizontal curves. However, Austroads 2003 did include criteria for Arrester Beds, Escape Ramps, Parking Bays and Rest Areas for heavy vehicles. Heavy vehicle requirements were also specifically covered in the design of railway level crossings.

The 2009 Guide to Road Design (Austroads 2009a) now requires all designs to be checked to ensure that they are safe for trucks and includes additional design criteria based on Australian research and experience over the last 10 years. This paper describes the extra criteria that were introduced and why they were introduced.



Table 1: Reproduction of Table 5.1 from Austroads 2003



Table 2. Indicative Truck Volumes at Which Truck-Based Standards are Justified for Horizontal Curve Radius and Horizontal Stopping Sight Distance.

Truck Volume (trucks/day) Design Speed Terrain

Single-Carriageway Dual Carriageway

90 km/h Level 100

Easy 100 – 150

Moderate 300 - 400

100 km/h Level 200 400

Easy 300 – 400 600 – 800

Moderate 700 - 1200 1,500 – 2,500

DESIGN VEHICLES Austroads 2006 specifies the current range of design vehicles for the geometric design of Australian roads – or more specifically, for ensuring adequate road-space. New Zealand has a different set of design vehicles that are generally slightly smaller than the corresponding Australian vehicle (see Table 5.2 in Austroads 2009b). Austroads 2006 also provides a range of Restricted Access vehicles which are essentially the design vehicles for routes to which these vehicles are restricted.

Austroads 2006 also provides:

Turning path (i.e. swept path) templates for a range of turning radii for each design vehicle

Guidance for the selection of the design vehicle and the appropriate checking vehicle. The design vehicle and the minimum turning radius for a particular case depends upon the road function, intersecting road types, adjoining land use (e.g. industrial or residential) and whether a road is a restricted access vehicle route (see also Table 5.1 in Austroads 2009b).

Guidance for the application of the templates including the provision of operating clearances.

Austroads 2006 now includes twelve vehicle types in order to cover all road-space cases (except possibly some special over-dimension routes that service ports or mining sites). With the advent of the Performance Based Standards scheme in Australia, road authorities regularly receive requests for “typical PBS vehicles” to supplement the standard design vehicles. However, this loses sight of the intent of the PBS scheme – namely, to improve freight efficiency in Australia by allowing vehicles to operate that meet performance based standards for a range of operating conditions rather than meeting the “traditional” prescriptive standards for dimensions, masses and turning paths, like for the standard design vehicles.

Since the PBS criteria have been developed to meet the needs of the existing road network, e.g. lane widths or the space generally available for turns within intersections, designers should note that specific turning templates for PBS vehicles are not necessary. Essentially, it is only necessary to use the normal prime mover and semi-trailer, or B-double or Type 1 or 2 Road Train as the design vehicle for road-space.

When it comes to speed dependent geometric design criteria, the 85th percentile truck speed at any point becomes the truck design speed for a given direction at that location. Because truck speeds vary considerably due to grade of the road, vehicle load and vehicle type, this means that one of the standard design vehicles does not necessarily represent the vehicles that correspond to the 85th percentile speed (see the section on Determining operating speeds below).

NEW CRITERIA New or revised design criteria or practices for trucks that were introduced in Austroads 2009a and 2009c are:

Determining operating speeds



Side friction factors for trucks (from Austroads 2002)

Use of transition curves

Application of superelevation

Reverse curves

Revised operating clearances for swept path

Revised curve widening criteria

Extended Design Domain (EDD) Stopping Sight Distance – including trucks

Sight distance on horizontal curves

Revised Intersection sight distance

Revised performance profiles on grades

Determining operating speeds Design speed is the fundamental parameter for the geometric design of roads. Design speed, or the range of design speeds where the geometry causes operating speeds to vary, effectively sets the standard of the road. Up until 1980, there was no clear definition of design speed. Design Guides typically defined it as a speed that would be exceeded by very few drivers in operation – which, at the time, automatically meant car speeds. And then, in the interests of affordability, most road authorities set a design speed to be used for different combinations of road function and terrain condition, with lower design speeds being uses for the less important roads (see Cox and Arndt, 2010 for further details).

In 1980, Austroads (then NAASRA) defined design speed as being greater than or equal to the 85th percentile speed under light traffic conditions. This followed on from research by the Australian Road Research Board into the operating characteristics of Australian roads (see McLean 1978, 1981). Design speeds for cars were set in conjunction with a speed prediction model (the Speed Environment Model) that was included NAASRA 1980. The Speed Environment Model predicted the 85th percentile speed for cars on the longer straights and for horizontal curves.

An enhanced form of the Speed Environment Model (namely, the Operating Speed Model) was introduced nationally in Austroads 2003.

Unlike car operating speeds, truck speeds are far more influenced by grades. Even a long grade of only 1% can cause large fully laden trucks to slow considerably below car operating speeds. With Australian design practice now explicitly covering truck operation as well as car operation (Austroads 2009a), a tool is needed for determining truck operating speeds. On existing roads, it is possible to get some idea of the 85th percentile speed for trucks by measuring speeds at key locations. For new roads, and to be certain of truck speeds at any location on existing roads, a speed prediction model for trucks is needed.

However, a model for predicting even the 85th percentile speed for trucks travelling around horizontal curves currently does not exist. Based on anecdotal evidence, truck speeds on horizontal curves tend to be 10km/h lower than car speeds for other than high speed roads, when car speeds are above 60km/h (Austroads 2003). This is also supported by the limited truck speed data that was given in McLean (1981).

Current preferred Queensland practice (RPDM 2001) is to use a vehicle performance simulation program called VEHSIM. This program models one of a number of predefined vehicles travelling along a road. The interaction with other traffic is not modelled. The program will slow vehicles for a curve if necessary but the speed slowed to is derived from the maximum side friction allowed for trucks for that speed and set out in RPDM 2001 and Austroads 2002, 2003 and 2009a. The program will also slow vehicles for descending long down-grades.

If VEHSIM is not available, Austroads 2009a includes graphs that allow speeds to be assessed on grades (both up and down) for a variety of situations. These graphs were updated for Austroads 2009a and generated by VEHSIM. Whereas Austroads 2003 (and 1980 & 1989) only provided graphs for what was for more than 20 years, an obsolete prime mover and semi-trailer



(for the purpose of fixing the start and end of climbing lanes), graphs are now provided for a range of modern vehicles and purposes, namely:

19m semi-trailer (33 t) carrying an average load (9.7 kW/t) for selecting truck design speeds where speeds are not controlled by horizontal curves (included as Figure 1). Note that the speed of this vehicle is used as a surrogate for the 85th percentile truck speed.

19 m semi-trailer (42.5 t), 12 litre diesel carrying a maximum load (7.5 kW/t) for determining start and end of climbing lanes.

B-double (62.4 t), 12 litre diesel carrying a maximum load (5.4 kW/t) for determining start and end of climbing lanes.

Type 1 road train (89.8 t), 12 litre diesel carrying a maximum load (3.8 kW/t) for determining start and end of climbing lanes.

Type 2 road train (140 t), 16.4 litre diesel carrying a maximum load (3.1 kW/t) for determining start and end of climbing lanes.

When using the abovementioned graphs however, it is necessary to be aware of when trucks are not able to make use of a downgrade (except towards the end of the grade) to increase speed because of limitations of the braking system on steep grades. The simplest way to make appropriate allowance is to use the guide for the treatment of steep descents in the Manual of Uniform Traffic Control Devices (i.e. Figure 4.25 in AS1742.2 or Figure 3.24 in the Queensland Manual of Uniform Traffic Control Devices). In such cases, the truck speed to be used for the downgrade may be the speed that may be maintained (or perhaps 5km/h higher) for an upgrade of the same magnitude, as shown on the relevant truck speed graph in Austroads 2009a. Also, it is not appropriate to infer for road design purposes, that trucks will travel at higher speeds on steep or long, steep downgrades because grade correction values are shown for these speeds in Table 5.5 of Austroads 2009a.

As stated above, VEHSIM will slow vehicles for descending long down-grades. Even so, speeds on downgrades should be checked against the guide for the treatment of steep descents in the Manual of Uniform Traffic Control Devices.



Figure 1: Determination of truck design speeds on grade in absence of measured speeds (19 m semi-trailer (33 t), 12 l diesel carrying an average load (9.7 kW/t))



Figure 2: Operating Speed Profiles generated by VEHSIM.

Side friction factors for trucks McLean, Tziotis and Gunitallake (Austroads 2002) introduced side friction factors for trucks (or more strictly, the 19m prime-mover and semi-trailer combination which the report recommended be the design heavy vehicle for geometric design). However, due mainly to timing, Austroads 2003 did not include these factors even though the longitudinal friction factors for determining truck stopping sight distance were included just prior to final publishing. This last minute change was largely due to McLean, Tziotis and Gunitallake in Austroads 2002 considering the factors that were in the then draft form of what became Austroads 2003, to be for an unrealistically conservative design condition – namely for an unladen articulated vehicle stopping in wet conditions.

Austroads 2003 (and 1980 and 1989) lists absolute maximum side friction factors for cars for a range of speeds to be used for horizontal curve design. The factors are the result of research by McLean (1983) and approximate an 85th percentile value that was observed for the respective speeds. Hence, they approximate the side friction demand that will be required (in terms of the point-mass curve design equation) for the tighter curves for a given 85th percentile speed. For speeds of 100km/h and less, the car factors are considerably higher than values listed in guides prior to 1980 (e.g, NAASRA 1967). However, the factors for speeds greater than 100km/h are close to the values that had traditionally been used.

Whereas the absolute maximum side friction factors for cars were based on observed driver behaviour, the side friction factors introduced in Austroads 2002 were based on what the authors called an equivalent risk approach. Specifically, with the equivalent risk approach, the ratio of the truck-based side friction factor design value for a given speed to the static roll threshold of the design truck was taken to be the same as the ratio of the car-based side friction factor for the same speed and the actual tyre-road friction which can be expected to be available for cars.

Direction of travel



In practice, the absolute maximum side friction factors for trucks yield a limiting curve speed (see Austroads 2009a for a definition) for trucks for a given radius curve that is about 7km/h below the car limiting curve speed (for curves with a radius less than about 600m). Given that 85th percentile car speed on curves is usually close to the limiting curve speed when vehicles slow for the curve, the truck limiting curve speed for the curve can be used as the truck design speed for the same curve (unless the grade causes the truck speed to be lower).

Revised curve widening criteria Up until Austroads 2003, curve widening to cater for the increased swept path width of heavy vehicles on horizontal curves was based on the single unit truck. This can be traced back through earlier editions of Austroads (1967, 1989) and to AASHO 1954. The use of the single unit truck instead of the semi-trailer combination was based on the much higher incidence of the former at the time, ease of calculating offtracking, and the fact that for curve radii above 120m, the difference in offtracking was insignificant. Now, the incidence of the prime mover and semi-trailer combination (and other articulated vehicle combinations) is much higher and the vehicles are longer. Cox (1998) showed that the percentage of articulated combinations exceeded the percentage of single unit trucks on most Queensland highways. Cox (1998) also provided curve widening values to suit the full range of design vehicles in 1998 (with these values still remaining current). These curve widening values were incorporated into the Queensland Road Planning and Design Manual of 2000 (RPDM) and have subsequently incorporated into Austroads 2009.

Draft versions of Austroads 2003 also planned to cover B-doubles but these were excluded in the final version. As it was, the curve widening was calculated by an approximation to a steady state turn formula which under-predicted the curve widening that was needed – especially for multi-lane roads (cf. Table 9.10 in Austroads 2003 and Table 7.11 in Austroads 2009a). Furthermore, the resulting lane widths in Table 9.10 in Austroads 2003 were applied to too wide a range of curve radii.

Reverse curves and Transitioned Curves A number of research projects have been undertaken around the world into the stability of trucks traversing reverse horizontal curves. Easa et al (2006) is an example of recent research on this matter, based on vehicle dynamics modelling software – specifically, VDM RoAD (Vehicle Dynamics Models Roadway Analysis and Design). Table 5.1 in Austroads 2003 did include a 0.6V spacing requirement for reverse horizontal curves for truck operation but this was similar to the requirement given in other guides (for example, RDM 1965, RDG 1988) without specific mention of trucks. In fact, the 0.6V spacing was predicated on the need to have room for superelevation development. In practice, the 0.6V spacing requirement has often been overlooked with the result that truck drivers experience greater difficulty in maintaining a path that enables them to stay within the traffic lane.

Austroads 2009a also provides the 0.6V spacing criteria (but it now explains how this also relates to the 2s of travel for making a transition path) and also includes an additional control to preclude unacceptably closer spacing. Moreover, Austroads 2009a has more specific and clearer requirements for when a curve should be transitioned, with the transition providing a more readily traversable path for heavy vehicles entering or leaving a curve. Reverse spiral transitions are considered acceptable.

However, the current criteria in Austroads 2009a do not cover the combination of the horizontal curve reversal with the vertical alignment. Easa et al (2006) recommend increasing the length of intervening tangent depending on:

Change in curvature between elements

Presence of spiral transition curves

Grade

Crest vertical curvature

Sag vertical curvature.

These additional spacing criteria should be considered for the next update of Austroads 2009a.



With respect to transition curves, there are revised length criteria. These criteria recognise the tracking and stability issues for heavy vehicles entering and leaving circular curves in situations where there is not the room for a large vehicle to make a suitable transition path within the traffic lane. The revised criteria took into account a number of modern studies, namely Blue and Kulakowski (1991), Bonneson (2000) and Glennon, Neumann and Leisch (1985) with respect to vehicle dynamics; Bonneson (2000) and Perco (2005) with respect to driver actions and vehicle tracking.

Even though the length criteria are not significantly different from the criteria in Austroads 2003 (if the latter is followed closely), the findings of the research listed above highlighted the need to more clearly state when transition curves were needed. Experience has shown that transition curves had tended to become seen as an optional (or somewhat less preferred) feature in some road authorities (see Cox 2010).

Application of Superelevation Linked to transition curves, reverse curves, amount of superelevation and side friction demand is the application of superelevation – that is, how and where the superelevation is introduced for a curve. Since this involves rotating the crossfall of the pavement (or in very rare cases, running vehicles over a diagonal crown) there is a fundamental element of vehicle stability linked to the rate of rotation and where that occurs with respect to the start and end of a curve.

Austroads 2009a has similar criteria to Austroads 2003 for the amount of superelevation needed on horizontal curves and where and how the superelevation should be applied. However, their description and explanation has been revised and linked to more recent research in order to overcome problems in interpretation that can be traced back to ambiguities in earlier guides. In turn, these ambiguities can be traced back to mistranslating some criteria from the American AASHO guides of 1954 and 1965. See Cox (2010) for further details.

For transitioned curves, it has been normal practice to tie the superelevation runoff length (i.e. the length of rotation from the zero crossfall point to the point where maximum superelevation is attained) to the transition length. This practice has a theoretical basis in that with the uniform attainment of superelevation in conjunction with the uniform increase in curvature (1/Radius) along the transition, there is a uniform increase in side friction demand.

With untransitioned curves, some road authorities (for example, Queensland Transport and Main Roads) have long used the practice of locating the superelevation runoff length equidistant about the curve tangent point. This location is intended (or assumed) to match the transition path that drivers still make when entering or leaving an untransitioned curve. Conversely, some road authorities (for example, VicRoads) for some time applied all of the superelevation on the tangent so that some part of the curve could not be deemed to have insufficient superelevation.

Austroads 2009a cites the research by Blue and Kulakowski (1991)that showed that transient conditions in roll and lateral movement are reduced for trucks when 50% of the superelevation runoff is located on the tangent (but with a maximum of 1s of travel into the curve). However, Austroads 2009a also covers situations when the superelevation runoff may need to be moved further onto the tangent (as much as 80% to 100%) and sets 60% to 70% as an acceptable compromise in the interests of uniformity.

Stopping distance When Austroads 2003 introduced stopping sight distance criteria for trucks, it made the point that when crest vertical curves provided sufficient sight distance for cars, sufficient sight distance was also provided for trucks travelling at the same speed as the cars – the greater stopping sight distance of the trucks was offset by the greater driver eye height.

On horizontal curves, the greater truck driver eye height offers little (sometimes no) advantage over the car driver eye height. Table 5.1 in Austroads 2003 only partly acknowledged this and recommended that it was preferred practice to avoid having features on a horizontal curve that would require a vehicle to have to slow or stop. In effect, this simply implied that it was not practical to achieve sight distance for trucks on horizontal curves. Achieving the necessary stopping sight distance for trucks on horizontal curves required clearing between 7m and 11m on the inside of a curve. Rarely was such clearing practical (or even possible), especially when there were hazards that warranted the need for a safety barrier. Furthermore, providing the



normal design sight distance for trucks on curved bridges or on curves under bridges or in road tunnels is impractical on other than very large radius curves unless truck speeds were markedly less than car speeds due to grades. The impractical widths usually resulted in no form of visibility widening being provided.

Austroads 2009a introduced a practical solution to the sight distance issue for trucks (and cars) with specific criteria for the provision of sight distance around concrete barriers and structures on freeways and interchanges. In these situations, a combination of being able to stop for stopped cars (being the most common hazard to have to stop for) and manoeuvring capability around small hazards is provided. The supplementary width that needs to be provided for manoeuvring varies between 2.5m and 4m, depending on the stopping capability provided. In practice, the manoeuvre width usually ensures reasonable capability for cars to stop for a small hazard and for trucks to stop for a stopped car.

The technical foundation for the provision of sight distance around concrete barriers and structures on freeways and interchanges is given in Arndt, Cox, Lennie and Whitehead (2010). Figure 3 shows an example of a shoulder on a bridge that provides stopping sight distance for cars and manoeuvre sight distance for trucks.

Figure 3: Sight Distance Widening on Southbound Ramp

Intersection sight distance In Australia, three sight distance models are used to provide sufficient sight distance on each of the legs at unsignalised intersections. These models are described in Austroads 2009c and their intents are:

Approach sight distance – to allow drivers to see sufficient details of the layout when approaching an intersection in order to decide what to do.

Safe Intersection Sight Distance (SISD) – to allow –

o a driver of a vehicle on the major road observe a vehicle on a minor road approach moving into a collision situation (e.g. in the worst case, stalling across the traffic lanes) and to decelerate to a stop before reaching the collision point

o sufficient distance for a vehicle to cross the non-terminating movement on two-lane two-way roads, or undertake two stage crossings of dual carriageways, including those with design speeds of 80 km/h or more



o approaching drivers to see an articulated vehicle, which has properly commenced a manoeuvre from a leg without priority, but its length creates an obstruction

Minimum Gap Sight Distance – to allow all car drivers to see, and be able to readily judge, gaps in the traffic stream (or see sufficiently far) to be able to undertake various manoeuvres. This model is based on observed driver behaviour in Australia in the early 1970’s. The gaps are what drivers are prepared to use when they have to. As such, most of the gaps in Table 3.4 in Austroads 2004c (and included here in Table 3) are less than those yielded by the SISD model (see Table 3). Research is currently under way to establish time gaps for equivalent truck manoeuvres with the aim of establishing a new time gap based model for intersection design (see Austroads/ARRB 2010). Preliminary results are included in Table 3.

Historically, these sight distance models have covered only car operation or have assumed truck operation has implicitly been covered due to greater truck driver eye height and lower operating speed. With Austroads 2009c, the Approach Sight Distance model and the Safe Intersection Sight Distance model have been further enhanced for truck operation. However, this is essentially a “stop gap” measure until a new intersection sight distance model is developed, based on the current research into gap acceptance times for heavy vehicles.

Table 3: Comparison of Time Gaps for Intersection Manoeuvres

Revised operating clearances for swept path Austroads 2006 sets the operating clearances that should be applied in order to ensure adequate road-space, namely:

The outside edge of the swept path remains within the paved area – although it goes on to say that in situations where space is restricted and turning speed is low, it may be necessary to allow the swept path of the design vehicle to encroach into a verge or traffic island with the wheel paths remaining on the pavement. This may occur when designing for a large bus to use a local street, or when checking that a design layout can accommodate an occasional vehicle larger than the design vehicle.

A minimum offset of 0.6 m is provided between the inside wheel paths and face of kerb or pavement edge.



It is on the last point that Austroads 2009a introduces a fundamental change: The clearance is now applied to the swept path (which applies usually to the body, including the front overhang) rather than the wheel path. This is consistent with the practice in the Queensland Department of Transport and Main Roads for more than 30 years and which shows that there is no appreciable cost impact. Given the significantly greater front overhang of some vehicles (mainly buses and some cab-forward prime-movers) compared to that on vehicles fifty years ago when the previous practice was established, it became necessary to change to a more defendable practice.

Applying the operating clearance to the swept path rather than the wheel path also allowed the clearance to be reduced from 0.6m to 0.5m. This then provided consistency with the clearances that exist for a 2.5m wide heavy vehicle in a “standard” 3.5m wide traffic lane.

Intersection Acceleration Lanes and Freeway entry ramps

McLean, Tziotis and Gunitallake in Austroads 2002 concluded that it was unrealistic to provide an acceleration lane of sufficient length for trucks to accelerate to truck design speeds for the through lanes unless the acceleration lane was combined with a down-grade. Note that this does not necessarily mean to the car design speed. Even for cars, Austroads 2009c (see note 1 to Table 5.4) recognises that it is usually more appropriate to design for cars to accelerate to the mean free speed – which is typically about 10km/h less than the 85th percentile car speed.

Whereas Austroads 2002 considered that it would be appropriate to design for trucks to merge at 10 to 20km/h less than the truck design speed of the through road on a route specifically designed for trucks, Austroads 2009c has had to face up to what may be tolerable for all road types. Whereas a preference is maintained for a maximum 20km/h speed differential (and more appropriately between entering trucks and the mean free speed of the through road), guidance on larger speed differentials of up to 40km/h or more is also provided. Furthermore, for intersections, the following alternatives are recognised as being appropriate in some cases:

a basic left-turn treatment comprising a give way or stop situation or a high entry angle channelsied left turn treatment

an acceleration lane length that is based on car acceleration only.

Even with freeway entry ramps, it is more common to allow for car acceleration only. However, on freeways, there is usually scope for vehicles on the through carriageway to change lanes under free flow conditions. Furthermore, with Austroads 2009e giving at least tacit preference for entrances with 100m to 200m of parallel lane, there is scope for heavy vehicles to reduce the speed differential by another 10 to 20km/h on the parallel lane section.

Deceleration Lanes and Freeway exit ramps

As with acceleration lanes, Austroads 2009c takes, on first appearances, a pragmatic stance with the design of deceleration lanes and freeway exit ramps to suit truck operation. Austroads 2009c states:

"Where a turning lane caters for a high percentage of heavy vehicles consideration may be given to increasing the length above that required for cars so that less interference to traffic flow occurs in the through lane as a result of trucks slowing. However, it is accepted that turning lanes should generally be designed for the deceleration of cars and that heavy vehicles may reduce speed in the through lane."

However, the comfortable deceleration rate of 2.5m/s2 that is recommended for deceleration lanes at intersections and which underpins the length of free exit ramps is suitable for truck operation. A truck driver who wants to decelerate at a more comfortable rate can then do so without unduly slowing traffic in the through lane. Nevertheless, on a truck route, it is not unreasonable to consider using a deceleration rate of 2.0m/s2 for design. The main challenge for designers is to make allowance for the different deceleration profiles that there will invariably be between cars and the trucks. Most car drivers, and certainly those at the 85th percentile speed, will not see the need to start decelerating from the point where trucks will start to decelerate.



NEW DESIGN CHALLENGES AND COMPROMISES Even though design criteria are included in Austroads design guides, it does not necessarily mean that they are adopted by all designers or even road authorities. It is quite commonly the case for new criteria to take some time to be adopted. Reasons are:

The new criteria are seen to be more onerous and result in unreasonable increase in cost. An example is the very limited use of the night time stopping sight distance criteria for cars – most road authorities have simply deemed it “policy” or not essential to design crests and sags to meet the night time criteria.

There is a sizeable overhead in learning the new criteria – especially if it is necessary to also develop or learn new design practices. A good example is the poor uptake of the Austroads Speed Environment Model that was introduced in NAASRA (1980). For further details see Cox (2003).

The criteria do not need to be used regularly – so they tend to be overlooked.

Many long term designers do not see the need for change simply because they have managed without them for so long and are not worth the extra design effort – again there are elements of this with the poor uptake of the Austroads Speed Environment Model as explained in Cox (2003).

The criteria are too readily dismissed as being impractical – possibly when the need for them was not obvious.

Some designers feeling that acceptance of the new criteria means accepting that their previous design efforts must be “wrong” – rather than being content with the previous designs being in accordance with the standards of the day.

Even though the road-space related design criteria for trucks have always been readily accepted (even when larger design vehicles have been introduced), the new sight distance and horizontal curve related criteria as described previously, do run the risk of slow uptake for the reasons that have just been outlined.

This is likely to be exacerbated by the extra effort to establish the truck operating speed profile (in each direction on a two-way road) which currently involves assessment due to grade with concurrent adjusting wherever the speed has to be reduced for a horizontal curve.

Restoration projects are projects on existing roads that are intended to keep the alignment and improve the cross section and riding quality. Whereas these roads would once have been reconstructed and realigned in order to bring them up to current standard, it is now not viable for road authorities to “throw away” what is still a very valuable asset. Restoration projects have become the most common form of road project in Australia and most developed countries.

Restoration projects typically require the use of the Extended Design Domain (EDD) for sight distance because the operating speeds will be (and usually already are) higher than the design speed that was used when the road was built. EDD for sight distance (and other criteria) has been included in Austroads 2009a and 2009c and has been used by the Queensland Department of Transport and Main Roads since 2003. Significantly, provision of sight distance capability for both cars and trucks (for their respective operating speeds) has been a mandatory requirement when using EDD. This even extends to the need to check for some basic night time sight distance capability. If the mandatory truck sight distance capability is not met, the guide is clear that a design exception exists (even if car sight distance capability is provided) with need then to formally document, justify and mitigate the design exception if it is to be retained.

Given the number of restoration projects making use of EDD and the consequent exposure to designing for trucks, hopefully designing for trucks for a greenfield site will quickly become normal practice.

However, restoration projects pose additional issues for designing for trucks besides sight distance. These are:

An existing curve radius may become deficient for trucks if the approach speed is increased. However, with truck speeds being highly dependent on grade, it is far less likely to be an issue



for trucks than it will be for cars. It is more likely to be a problem if a preceding radius has had to be increased because of a crash history on that curve.

The amount of superelevation on a curve and the pavement rotation rate may need to be changed to conform to current practice for the new operating speed. Likewise, the location of the pavement rotation in order to effect the superelevation, may need to revised.

Transition curves may need to be added to a currently untransitioned curve because of the changed operating speed and/or a revision to the application of the superelevation. However, if a curve is currently transitioned but with a shorter transition than current practice, it will usually be unnecessary to increase the transition curve length. Nevertheless, increasing the transition length is usually not difficult.

The spacing between reverse curves may need to be increased in order to meet the changed operating speed. It is most likely that the spacing will be deficient for current truck speeds with drivers having to make greater steering adjustments than normal for the curve radii. Sometimes, increasing the spacing between existing reverse curves will be a challenge. Most designers will think this can only be achieved by a realignment, which is likely to be outside the scope of a restoration project. And sometimes this will be so. However, and especially if widening of the formation is involved, it will often be possible to slightly reduce each radius by about 10m (maintaining the outer tangent point position) so that a longer (and very slightly realigned) intervening tangent can be accommodated. In addition to slightly reducing a radius, it may be necessary to also move the outer tangent point by about 2m to 5m (see Cox 2010).

CRITERIA THAT STILL NEED TO BE DEVELOPED Austroads 2009a, 2009b, 2009c are now the most comprehensive geometric design guides in providing criteria for trucks. However, there are still criteria and practices that need to be developed or at least refined in order to ensure that operational performance and truck safety are appropriately addressed as part of geometric road design. These are:

A speed prediction model for truck 85th percentile speeds - For restoration projects, the 85th percentile speed of trucks can be measured at critical locations with some allowance possibly being made for increased operating speed due to improved cross section and riding quality on sections where speeds are not restricted by grade. For new roads, a model for predicting 85th percentile truck speeds on curves can be readily developed (as has been done with the Operating Speed Model for cars). This is the subject of a current Austroads research project (TP1456 Expanded Operating Speed Model). For truck speeds on grades, models need to be developed for both up-grades and down-grades then combined with the model for speeds on curves. In addition, the work to develop the model for truck speeds on curves will yield/confirm the appropriate side friction factors to be used for trucks.

A time gap based intersection sight distance model as outlined earlier. This will also need a review of the car based time gaps in Austroads 2009b (see Table 3) since car and truck operation need to be separately checked due to:

o Different operating speeds in many cases

o Truck sight distance usually being the control on horizontal curves unless truck speeds are significantly lower than car speeds due to grade

o Car sight distance usually being the control on crest vertical curves because truck driver eye height affords greater sight distance to compensate for the greater time gap needed.

A time gap based roundabout sight distance model for trucks – currently, Austroads 2009d has time gap based sight distance criteria for cars at roundabouts. There are no special criteria for trucks. Development of appropriate time gaps may also provide better insight into the circumstances when a roundabout is, or is not, the most appropriate form of intersection control.

Reverse curve spacing – as explained earlier, the spacing criteria developed by Easa et al (2006) should be considered for the next update of Austroads 2009a. However, since these criteria have been developed primarily with the aid of vehicle dynamics modelling, it would be prudent to see if there is correspondence with the crash history of some existing reverse curve locations.



Confirmation of maximum superelevation - 6% to 7% is the maximum superelevation that is recommended for rural roads in Austroads 2009a, although it is also recognised that there are situations on the lower order roads that may warrant up to 10%. However, use of 10% (or even more than 7%) is discouraged where there are “vehicles with a high centre of gravity”. Certainly, most members of the Austroads review panel expressed concerns about such slow moving vehicles tipping over, even though there are many curves on the Australian road network (and on all types of road) that have 10% superelevation because of earlier design practices and there is not the operational experience to show that this is a problem. Therefore, research should be undertaken to confirm the maximum superelevation for truck stability (bearing in mind that insufficient superelevation is probably more of an issue) for different situations, including intersection turns. Hopefully, the current Austroads research project (TP1456 Expanded Operating Speed Model) will also provide data that will help with this issue.

Stepped/rolling gradelines – vehicle simulation work (using for example VEHSIM) and industry feedback have identified performance and fuel efficiency benefits at times from having such a gradeline instead of a longer but less steep single grade. Furthermore, on long down-grades, steps provide scope to regain control if a vehicle is in too high a gear to control vehicle speed. Therefore, guidelines should be established for when stepped gradelines provide a worthwhile advantage. In conjunction with this, guidance of heavy vehicle speeds on steep and long, steep downgrades needs to be incorporated in the next revision of Austroads 2009a,

Grades – as with acceleration lanes, McLean, Tziotis and Gunitallake in Austroads 2002 concluded that it was unrealistic to provide grades that would allow the design speed of trucks to be maintained on sustained upgrades. Specifically, it was unachievable for the higher range of design speeds and would be unachievable for the lower range of design speeds in other than flat terrain. Design criteria based on limiting the extent to which truck upgrade speeds fall below the design speed were proposed as being both more realistic and achievable. Speed decrements of 10 to 20 km/h were considered reasonable. Section 8.5 of Austroads 2009a does refer to AASHTO (2004) practice which recommends a speed decrement of 15km/h. It would be preferable if the next update of Austroads 2009a set out cases where 10, 15 and 20km/h decrements could be adopted.

CONCLUSION The Austroads guides for the geometric design of roads and intersection design have always provided design criteria for the necessary road space for heavy vehicles and for the provision of climbing lanes. However, Austroads 2009a and 2009c have advanced geometric design through making it necessary to always design for truck operation (in addition to car operation) in the areas of sight distance and horizontal curve design. In turn, this has required the need to predict the appropriate truck design speed at all points along the road.

Given that truck speeds vary along most roads due to grade (even on high standard roads), and at times, horizontal curvature, this means greater design effort in allowing for truck operation. The predicted truck speeds should also be backed up if possible by measured truck speeds on other parts of the road in question and other roads with similar geometry in order to better account for the range of trucks in operation and their loads.

Given the number of restoration projects these days with the need/ability to apply Extended Design Domain in order to assess the suitability of the road geometry, it is likely that the mandatory requirement to ensure both car and truck sight distance capability will overcome any resistance to designing for trucks in greenfield projects.

Given the large amount of attention now given by the media to heavy vehicle crashes, it is no longer appropriate or defendable (if it ever was) for a road authority to claim that it is unnecessary to design for trucks for other than road space requirements. This is why Austroads (2009a, 2009b, 2009c, 2009d) has taken the position it now has. Even so, there is still the need to refine some design criteria and develop additional criteria as previously outlined. In other words, geometric road design for heavy vehicles has advanced considerably but still has a way to go.



REFERENCES AASHO (1954). A Policy on Geometric Design of Rural Highways. American Association of State Highway Officials. Washington D.C.

AASHO (1965). A Policy on Geometric Design of Rural Highways. American Association of State Highway Officials. Washington D.C.

AASHTO (2004). A Policy on Geometric Design of Highways and Streets. American Association of State Highway and Transportation Officials. Washington D.C.

AS 1742. 2:2009, Manual of uniform traffic control devices part 2: Traffic control devices for general use.

Austroads 2002. Designing for Trucks - When, Where and How? AP-R211 McLean, J. Tziotis, M. and Gunitallake, T. Austroads, Sydney, Australia, 2002.

Austroads 1989. Rural Road Design: A Guide to the Geometric Design of Rural Roads. Austroads, Sydney, Australia, 1989.

Austroads 2003. Rural Road Design: A Guide to the Geometric Design of Rural Roads. Austroads, Sydney, Australia, 2003.

Austroads 2006, Design Vehicles and Turning Path Templates. AP-G34/06 Austroads, Sydney, Australia, 2006

Austroads 2009a, Guide to Road Design: Part 3: Geometric Design, Austroads, Sydney, NSW.

Austroads 2009b, Guide to road design: part 4: intersections and crossings – general, Austroads, Sydney, NSW.

Austroads 2009c, Guide to Road Design: Part 4A: Unsignalised and Signalised Intersections, Austroads, Sydney, NSW.

Austroads 2009d, Guide to Road Design: Part 4B: Roundabouts, Austroads, Sydney, NSW.

Austroads 2009e, Guide to Road Design: Part 4C: Interchanges, Austroads, Sydney, NSW.

Austroads 2010a, Heavy Vehicle Brake Safety on Long and Very Steep Roads: Final Report. Austroads Research Report AP-R362/10, Austroads, Sydney, NSW.

Austroads 2010b (ARRB 2010 Draft). Road Design for Heavy Vehicles, Progress Report: Gap Selection Criteria for Heavy Vehicles Austroads Project No. TS1607. Bucko, A. Ritzinger, A. Tziotis, M. and Eady, P. Austroads, Sydney, Australia, 2010.

Interim Guide to the Geometric Design of Rural Roads. National Association of State Road Authorities of Australia (now Austroads), 1980.

Arndt , O.K., Cox, R.L., Lennie, S.C. and Whitehead, M.T. Provision of Sight Distance around Concrete Barriers and Structures on Freeways and Interchanges. 4th International Symposium on Highway Geometric Design, Valencia, 2010.

Cox, R.L. and O.K. Arndt. Australian Experience with using Speed Prediction Models to determine accurate Design Speeds. 4th International Symposium on Highway Geometric Design, Valencia, 2010.

Easa, S.M., and Abd El Halim, A. Radius Requirements for Trucks on Three-Dimensional Reverse Horizontal Curves with Intermediate Tangents. TRB 85th Annual Meeting, Washington D.C., 2006.

Road Planning and Design Manual (RPDM) Queensland Department of Main Roads, 2001+. www.mainroads.qld.gov.au. Accessed 14 Oct. 2009.

McLean, J.R. Review of the Design Speed Concept, Report No AIR 1029-2, Australian Road Research Board, 1978.

McLean, J.R. Driver speed behaviour and rural road alignment design, Traffic Engineering and Control, 22(4), 1981.

McLean, J.R. Speeds on Curves: Side Friction Factor Considerations. Research Report ARR No. 126, Australian Road Research Board, 1983.



Queensland Department of Main Roads (2009): Manual of Uniform Traffic Control Devices (MUTCD).

Cox, R.L. A Review of the Effect of Increases in Vehicle Size on Australian Geometric Road Design Standards, Fifth International Symposium on Heavy Vehicle Weights and Dimensions, Australia 1998.

Cox, R.L. Application of the Austroads Speed Environment Model (Always? Often? Sometimes? Never?), Planners and Designers Symposium April 2003. Queensland Department of Main Roads, Brisbane, Australia, 2003.

Cox, R.L. Transition curves and application of superelevation – dispelling some myths and restoring good practice. Presentation at Transport and Main Roads Forum August 2010. Queensland Department of Transport and Main Roads, Brisbane, Australia, 2010.

Glennon, JC, TR Neumann & Leisch, JE 1985, Safety and operational characteristics for design of rural highway curves, report FHWA-RD-86, Federal Highway Administration, Washington, DC, USA.

Bonneson, RJ 2000, Superelevation distribution methods and transition designs, National Cooperative Highway Research Program, report 439, Transportation Research Board, Washington DC.

Perco, P 2005, ’Comparison between vehicle paths along transition sections with and without spiral curves’, International symposium on highway geometric design, 3rd, June 2005, Chicago, Illinois, Transportation Research Board, Washington, DC, USA, 23 pp.

Austroads 2008. Styles, T., Mabbott, N., Roberts, P., Tziotis, M. Safety Benefits of Improving Interaction between Heavy Vehicles and the Road System. Publication No. AP–T119/0 Austroads, Sydney, NSW.

Guide to the Geometric Design of Rural Roads. National Association of State Road Authorities of Australia (NAASRA now Austroads), 1967.

Interim Guide to the Geometric Design of Rural Roads. National Association of State Road Authorities of Australia (NAASRA now Austroads), 1980.

Road Design Manual Volume 1 (RDM) (1965). Main Roads Department, Queensland.

Roads and Traffic Authority (NSW): “Road Design Guide” (RDG). 1988

AUTHOR BIOGRAPHIES Mr Ricky Cox

Ricky Cox is the Principal Advisor (Design Innovation and Standards) in the Queensland Department of Main Roads. He has been involved in the planning and design of roads and with research and development of road design standards for more than 40 years. Ricky has also been responsible for the development of various computer systems used in road design and also has extensive experience in their application.

His particular interests are in vehicle swept path, vehicle dynamics and the relationship between heavy vehicle operation and geometric road design standards. He has been responsible for writing sections of the Queensland Department of Main Roads’ Road Planning and Design Manual. He has also had major input into new Austroads design guides.

Dr Owen Arndt

Dr Owen Arndt holds the position of Principal Advisor (Road Design Standards) within the Queensland Department of Main Roads. Owen has extensive experience in the planning and design of urban and rural roadways, motorways, intersections and interchanges. His experience also covers the management of road and noise barrier construction projects. Owen's current responsibilities include developing and updating road design standards, undertaking of research projects, providing specialist advice and reports on road design standards and projects. Owen is the Queensland representative on the Austroads Road Design Review Panel.

Documents

Geometric road design for heavy vehicles – advancing but