The Road Side Environment

In recent years there have been increasing efforts to examine ways in which the road environment can be changed to reduce the speed at which drivers travel. There are two ways in which this can be achieved. Firstly, the road environment can be physically changed with the use of speed control devices such as speed humps, roundabouts and other forms of road treatments. Secondly, the road environment can be changed to manipulate the driver’s perception of travel speed, which can, in turn, result in a lower travel speed.

This second approach is based upon research (Triggs, 1986; Bower, 1990) showing that driver's speed choice is often related to their perception of speed and not the vehicle speedometer.

Physical speed control devices

The notion of controlling vehicle speeds via the use of some form of engineering modification of the road environment was first initiated in The Netherlands during the 1960's (Fildes and Lee (1993). The objectives of these schemes were to reduce the conflict between pedestrians and traffic and accommodate the needs of local residents. These early Dutch traffic management schemes, based upon the 'Woonerf design', involved the development of local resident traffic areas where pedestrians and vehicles had equal priority. The objective was to reduce vehicle speeds, through the use of various road engineering treatments such as raised pavements, speed humps, staggerings and narrowings, and to improve local amenity through the use of benches, tables, gardens and different coloured pavement surfaces.

Engineering treatments based upon the original 'Woonerf’ concept are now commonly used in many Western countries. The collective term usually applied to such schemes is 'traffic calming', however, other terms often used include local area traffic management (LATM) and environmental adaptation (FORS, 1993). The primary objective of traffic calming measures is to reduce the impact of vehicles on the local environment by incorporating engineering treatments which restrain vehicle speeds and improve both traffic safety and local amenity.

Although 'traffic calming' treatments are used primarily for local traffic situations they can also be used on more major roads and incorporated in such a way as to create a larger area wide effect. In such situations careful consideration must be given to overall traffic impact and issues such as vehicle flow, network capacity and level of service need to be assessed (Brindle, 1992). Westerman (1990) indicated that the effects of speed engineering measures need to be viewed in terms of the total transportation system. While these speed management devices have the potential to stem the speed and flow of traffic in 'precincts' (local environments), there will be no overall gain to the system unless the main 'corridors' (arterial and collector roads) are capable of carrying the excess traffic. Cairney and Townsend (1991) also suggested that the overall effectiveness of traffic calming measures is, to a large degree, dependent upon good design and strategic placement of devices to reduce known accident problems.

There are numerous engineering treatments which can be incorporated to manage speed in a number of different situations. Armstrong et al. (1992) listed some 46 devices that have been used in various locations throughout Australia and still many more devices may be used in other countries. A description of each of these devices and their potential effect on speed is not possible in the context of this review. However, a brief overview of two of the most common treatments, speed humps and roundabouts, may serve to demonstrate how such devices can be used to manage vehicle speed.

Speed humps and raised road pavement sections are common measures adopted in many countries and have been reported (Engel, 1990) to have brought about the greatest change in speed behaviour, as compared to other speed control devices. Speed reductions in the order of 10 km/h to 45 km/h (dependent upon the pre-installation speeds) at such devices, and from 5 km/h to 25 km/h between such devices, have been reported (Stephans, 1986). Roundabouts are one additional speed control measure which can be used to break up

lengths of road that might otherwise encourage speeding behaviour (Fildes & Lee, 1993). Engel and Thompson (1992) reported speed reductions in the order of 30% to 65% at treated sites with the size of the reduction dependent upon the type of manoeuvre required to negotiate the roundabout.

The use of speed management devices to produce an area wide traffic calming effect have been shown to have major road safety and cost benefits. Ho and Fisher (1988) estimated the cost effectiveness of various traffic calming measures, reporting benefit cost ratios ranging from 1.5 to 1 for less speed restricting devices to as high as 6.8 to 1 for more severe speed treatments. Fisher and Van den Dool (1989), in an evaluation of over 65 treatment sites, reported that traffic calming devices were able to reduce traffic speeds, volume and accidents. They concluded that such devices were cost effective (benefit cost ratios above two to one) and resulted in accident and amenity reductions of approximately 50%. Similar results were reported by Chua and Fisher (1991) in an evaluation of an area- wide traffic calming scheme in Sydney, Australia. They reported significant reductions in accidents (50%), through traffic (35%) and speeding (25%). In the United Kingdom, Mackie et al. (1990) undertook an evaluation of the effects of area wide traffic calming treatments in five residential suburbs. They reported a 10% to 15% reduction in injury accidents and associated costs when compared to several matched comparison areas.

Perceptual speed control measures

Fildes and Lee (1993) have identified the use of perceptual countermeasures as a means of reducing the level of speeding behaviour. They suggested that environmental manipulations can effect a driver's perception of speed without requiring deliberate decisions to comply (such as manipulations that influence the pre-conscious perception of speed). Fildes et al. (1989) have argued that such speed control measures are likely to have long-term benefits because of the unobtrusive nature in which they influence a driver's perception of speed (they operate without the driver's awareness or need for action). Additional advantages are their low cost of application and the fact that they do not require the introduction of additional physical hazards on the road surface.

The most common of these perceptual treatments is the use of transverse lines marked on the roadway at the approach to roundabouts and intersections. Fildes and Lee (1993) stated that these treatments have been shown to have desirable long-term speed reduction benefits in both the United Kingdom and Australia. Maroney and Dewar (1987) reported on the use of a transverse line treatment across an exit ramp of a freeway in the city of Calgary, Canada. Over a three week period they reported a 2 km/h reduction in mean speed which was consistent across each hour of the day and each day of the week. The most impressive finding was a 25.5% reduction in drivers exceeding a pre-set 80 km/h speed reference point. They explained this significant increase in terms of increased attentiveness of drivers approaching the exit ramp intersection.

A range of other possible perceptual countermeasures that have been identified by Fildes et al. (1989) and include:

• lane width reductions - evidence suggests that reduced lane widths on particular types of roads can lead to speed and accident reductions.

• centre-line and edge-line treatments - the use of both centre-line and edge-line treatments (including transverse striping on the edges and shoulder region of the road) on the road may result in reduced speeds.

• curvature enhancement - in situations where curvature is tight and there is insufficient sight distance through the curve, the use of measures (such as chevron markers and novel guide-post arrangements) to enhance the curvature may lead to speed reductions. • intensive road treatments - restricting the number and size of travel lanes using wide white gravel medians with highlighted edge-line marking have been shown to reduce travel speed in some locations.