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The geometric design of roads refers to the design of the visible dimensions of the roadway. The aim of geometric design is to provide for the safe, efficient and economical movement of all types of traffic. The design process is aided by the use of geometric road design standards which have proven to provide acceptable design. The majority of material in this module will deal with alignment and cross-sectional design and is based on the Austroads publication Rural Road Design: Guide to the Geometric Design of Rural Roads (2003). This publication is recognised Australia-wide as the basic design guide, with other design guides tending to be variations on the Austroads guide. It is important to realise, however, that the Austroad’s publication, and the material in this module, are intended as a guide to road design and not as a mechanistic approach. The designer is required to interpret the material presented and to use engineering judgement in any design situation. The practice of good road design involves judgement as well as calculation. It involves comprises between conflicting goals. Experience assists the designer to arrive at an appropriate compromise that cannot be met by simply applying a set of mathematical rules. The designer’s aim should be to produce an appropriate design for the specific problem being addressed, while retaining a reasonable overall level of uniformity within the road network.
Three reasonably distinct stages may be identified in the development of a nation’s road system:
The majority of the Australian road network is currently a mixture of increasing the network’s capacity and improving the quality of service, i.e. stages 2 and 3.
Observation suggests that there are effectively three speed ranges that roads can be classified into:
Consideration of a nation’s stage of development and the role of driver expectancy in regard to speed therefore help in determining what is an appropriate road design standard for a specific situation. A number of other factors can be identified as influencing the choice of design standard, and these include:
One of the most crucial and important parts of the design process is the location of the road. The location procedure is an iterative process in which engineering, land use, economic, environmental and social factors are taken into account. The location of a large facility, such as a freeway, would probably involve a multi-disciplinary team of professionals. Several approximate locations are initially selected based on preliminary information and data. Possible choices are then narrowed down, usually with the help of additional information. The ultimate aim is to determine a ‘best’ route from a balance of cost and user benefit, taking into account socioeconomic and environmental impacts.
To produce a logical basis for the selection of speeds for geometric design it is necessary to define three speed parameters:
The horizontal alignment of a road is a plan view of the road projected onto a horizontal plane. It is the view of the road that would be obtained flying above it. The horizontal alignment is usually a series of straights (tangents) and circular curves joined by transition curves. Where long, large radius circular curves are used instead of straights the alignment is known as curvilinear alignment. This type of alignment may be used where the terrain is suitable (e.g. flat inland areas of Australia) to reduce driver headlight glare and to improve the driver’s perception of the speed of an approaching vehicle.
When a vehicle travels round a circular horizontal curve it is subjected to a radial force which tends to cause it to slide outwards. In order to resist this force it is usual to super-elevate the pavement.
The superelevation to be adopted should take into account factors such as safety, appearance, grade, speed, drainage, and the presence of intersections. Generally, the maximum superelevation should be about 0.06 m/m in flat country and about 0.12 m/m in mountainous terrain. For urban situations where intersections are more numerous and property access must be considered, maximum values of 0.04 or 0.05 m/m are desirable.
The coefficient of sideways friction at which skidding is imminent depends on:
By adopting desirable maximum values for superelevation and for coefficient of side friction, a set of values for the minimum radius of horizontal curves at various design speeds can be calculated. The following table shows the minimum radius for various design speeds.
| Vehicle Speed
(km/h) |
Minimum Radius (m) | ||
| Mountainous Terrain
e = 0.10 |
Undulating Terrain
e = 0.06 |
Flat Terrain
e = 0.03 |
|
| 50 | 49 | 56 | . |
| 60 | 83 | 95 | 105 |
| 70 | 133 | 154 | 175 |
| 80 | 194 | 229 | 265 |
| 90 | 277 | 335 | 398 |
| 100 | . | 437 | 525 |
| 110 | . | 529 | 635 |
| 120 | . | 667 | 810 |
A curve used to transition from a straight to a circular arc, or between adjacent circular curves, is known as a plan transition curve. The plan transition curve provides a length of road over which the radius can be changed gradually from the infinite radius of the straight to the radius of the circular curve, and this assists in vehicle handling on the road. A wide range of curve forms can be suitable for a plan transition but the curve most frequently used is the Clothoid.
Plan transition curves may be omitted when large radius curves are used or on relatively low speed alignments. In most high standard road design carried out today, large radius curves are used and plan transitions are not required.
On straights, the pavement has normal crossfall to shed water. A change from normal crossfall to superelevation occurs as the road changes from a straight to a curved alignment. On a two-lane, two-way road, the superelevation is normally developed by rotating each half of the cross section (including the shoulders) about the centreline of the road (axis of rotation). All curves, apart from those with large radii (larger than 4000m), should be superelevated.
The length required to develop superelevation should be sufficient to provide a good appearance and give satisfactory riding qualities. The criteria used to determine the superelevation length are:
For the case of a straight tangent to a circular curve, where no plan transition curve is used, the superelevation is placed in such a way that 50% to 70% of the length of development occurs prior to the tangent point, and 50% to 30% is within the curve. Where a plan transition curve is used the superelevation development commences in advance of the commencement of the plan transition, and the superelevation transition and plan transition end at a common point within the curve.
To ensure adequate clearance between opposing streams of vehicles, it may be necessary to widen the pavement on horizontal curves. Widening is required because vehicles tracking around a curve occupy a greater width than on a straight road section, and because vehicles tend to wander more on curves due to the extra control required in steering.
A traffic lane is that part of the roadway set aside for the normal movement of a single stream of vehicles.
Lane width is based on:
The desirable lane width on rural roads is about 3.5m. Lane widths as low as 3.0m may be used on low volume roads. Widths greater than 7.5m for two lane roads are not recommended because there may be a tendency for three lane operation to develop.
Shoulder width is measured from the outer edge of the traffic lane to the edge of usable carriageway. Wide shoulders have the following advantages:
The minimum width of a road shoulder on a two lane rural road should be 1.0m, unless volumes are below 150 vehicles/day.
A width of 1.5m to 2.0m ensures that the capacity of the road will not be affected by obstructions located adjacent to the shoulder. It will also mean that vehicles stopped on the shoulder will provide only minor obstruction to the traffic lane.
A width of 2.5m is needed to allow a passenger vehicle to stand clear of the traffic lanes. A width of 3.0m allows a passenger vehicle to stop clear of the traffic lanes with some clearance, and also allows a commercial vehicle to stop clear of traffic.
In road design the aim should be to provide shoulders of 1.5 to 2.0m wherever possible, and up to 2.5 to 3m on higher volume roads.
Crossfall is the slope of the surface of a carriageway measured normal to the centreline. The purpose of crossfall is to drain the carriageway on straights and curves, and to provide superelevation on horizontal curves.
| Type of Pavement | Crossfall % |
| Earth, Loam | 5 |
| Gravel | 4 |
| Bituminous Seal Coat | 3 |
| Bituminous Concrete | 2.5 - 3 |
| Portland Cement Concrete | 2 - 3 |
Table drains are located on the outside of shoulders in cuttings. The side slopes of table drains should be flat enough to minimise the likelihood of overturning of out of control vehicles. A maximum slope of 4 horizontal to 1 vertical is recommended, with a desirable maximum slope being 6 to 1.
Catch drains are located on the high side of cutting batters to intercept overland flow before it flows down the batter face.
Batter slopes should be as flat as economically feasible to improve maximise safety and to improve appearance. However flatter slopes usually cost more and where earthwork volumes are significant maximum batter slopes will usually be adopted. Materials in fills will generally be unstable at a batter slope greater than 1.5 to 1 unless angular rock facing is used. Cut slopes should be consistent with material stability and in material other than rock will generally vary between 1.5 to 1 and 2 to 1. In rock, slopes as great as 0.25 horizontal to 1 vertical may be feasible.
A major aim in road design is to ensure that the driver is able to sight any possible road hazard in time to take evasive action. This aim is related to the geometry of the road by the concept of sight distance. Sight distance, as used in road design, is based on a number of stylised assumptions regarding the nature of hazards and driver behaviour. The hazard is assumed to be an object of sufficient size to cause the driver to change driving behaviour. Specific values are assumed for driver’s reaction time and the dimensions determining the sight line, although all of these parameters would have a range of values in practice.
The values adopted in the Austroads publication Rural Road Design are:
A theoretical stopping distance can be derived by assuming the driver travels at the design speed during the reaction time and then decelerates from the design speed to a stop. Assumptions are required for the values of reaction time and the coefficient of longitudinal deceleration and these enable a notional stopping distance to be calculated for an initial design speed. This distance is considered to be the minimum sight distance that should be available to a driver.
| Design Speed (km/h) | Stopping Sight Distance (m) (Assumed reaction time 2.5 seconds) |
| 80 | 115 |
| 90 | 140 |
| 100 | 170 |
| 110 | 205 |
| 120 | 245 |
| 130 | 280 |
The overtaking action has a large number of variable components including the judgement of the overtaking driver and the risks he is prepared to take, the speed and size of the vehicles involved, the actions of the driver being overtaken, and the actions of the other drivers in the vicinity. It can be assumed that there are two basic considerations in regard to the provision of sight distance for overtaking manoeuvres:
In studying overtaking behaviour it is useful to consider time gaps between vehicles. However, to convert study data to design data a change from time gaps to road length is usually considered beneficial. The Australian Road Research Board has carried out a major study on overtaking on Australian rural roads. This research provides the basis for the overtaking sight distances recommended in the Austroads Rural Road Design book.
The values recommended are based on the following assumptions:
In checking a length of road for overtaking sight distance it will be found that the continuation sight distance (OSD) is fairly critical, and it will be on this figure that a percent allowable for overtaking is calculated for the road section. Sections of road assumed to provide for overtaking will commence at a point where establishment sight distance is available, and finish where the available sight distance drops below the continuation sight distance.
The overtaking distances recommended are not an indication of where barrier line marking should occur. If these figure were used for the purposes of line marking it would be found that they would be unduly restrictive for many safe manoeuvres e.g. overtaking very slow vehicles.
The longitudinal profile of a road consists of straight grades joined by vertical curves. In addition to smoothing the passage of a vehicle from one grade to another the vertical curve increases the sight distance, particularly over crests.
Convex vertical curves are known as summit or crest curves and concave vertical curves as sag curves. At crest curves the minimum length of curve is determined by the requirement to provide stopping sight distance, or by appearance requirements. Lengths above the minimum may increase the sight distance over the crest but may also reduce the sight distance available on the approaches. The minimum length of sag curves is generally determined by considerations for motorist comfort or discomfort due to vertical acceleration, or by appearance. In some instances the length may be governed by drainage, headlight performance or overhead restrictions to the line of sight.
Various curve forms can be used for vertical curves but traditionally the parabola has been used, mainly for ease of manual calculation.
It is usually not feasible to construct roads with grades sufficiently flat so that all vehicles can operate at the same speed. Therefore it is necessary to use a design standard which takes account of vehicle performance and provides travelling conditions suitable for individual vehicles and the traffic stream as a whole.
Design standards will usually recommend a general maximum grade for normal design, but with the facility to vary this value in particular circumstances. On high speed roads grades up to about 3% provide a very satisfactory level of service and minimise the adverse effects of having different size and mass vehicles in the traffic stream. On roads with more modest design speed, grades up to about 6% generally cause little problem. Gradients over 10% bring problems of very slow climbing speeds and potentially high downhill speeds for many vehicles.
The traditional methods of road design have been limited by drafting techniques where the road is considered as a series of two dimension views. Thus normal design presentations contain a plan view, a longitudinal profile along the road centreline, and a series of cross sectional views at regular chainages or at points of particular significance (e.g. tangent points). Such an approach can produce good results if carried out by an experienced designer. Equally, it can produce poor results if the designer considers each view independent of all others. Adherence to appropriate design rules and tabulations, as set out in previous sections of this module, gives no guarantee of a satisfactory result unless a broad design perspective is adopted which considers the road from the viewpoint of the road user.
The road user sees the road as a constantly changing three-dimensional continuum. It is the appearance of the road to the driver that determines the driver’s behaviour, and unless the road appears to the driver as the designer intended, then the designer has failed. The road must be considered at all stages of design as a three-dimensional structure which should be safe, functional, economical and aesthetically pleasing. While nothing can replace the attitude of continually viewing the road as a three dimensional structure, the following are some points which should be borne in mind by the designer to achieve a satisfactory design:
Computers were first used in the road design process in the early 1960's. Their initial use was to relieve the tedium of manual calculation. Although the computers were able to rapidly carry out the required calculations, the jobs were usually run in a batch mode and so the process suffered from slow turnaround times.
One of the strengths of modern computer road design systems is the ability to utilise survey information captured by electronic distance measuring equipment. The raw data can be electronically downloaded to a computer where it can be reduced to a form useful to the designer. The designer is then able to proceed with the road design without having to enter large amounts of survey data as was necessary in the early days of computer aided road design.
The major benefit of the computer is its ability to handle large quantities of data with precision. The modern computer aided road design process therefore enables the designer to seek for an optimum solution of high quality. Prior to the advent of computer aided road design systems the designer needed to manually perform numerous calculations for each design stage and time and money constraints limited the designer to perhaps two or three attempts at reaching an optimum solution. The use of a computer system now enables possibly twenty attempts or more, within a reasonable time period.
Road design software packages are commercial products. There are a variety of packages available and the selection of a 'best' package depends on a large range of factors including the anticipated type of use, available finance, hardware platform, etc. The packages mentioned in this section are given as examples only and no endorsement of any particular product is intended. There are probably descriptions of similar products at other Web sites that I am not aware of.
The computer aided road design system described in the Study Notes is the MX system, which was first developed in the United Kingdom in the 1970's as MOSS. It is now marketed by Bentley Systems in the USA and Australia.
The Trimble Terramodel system is marketed by Geocomp in Australia.
An Australian product is the 12d Model, marketed by 12D Solutions.
Another product description with reasonable graphics is found at the Softree Technical Systems Inc . site. Go through their 'Products' link to see sites for the 4 modules in their RoadEng package.
Page last modified 5 July 2006.