Intersection Design Considerations

Physical modification of the intersection environment has also been shown to be an effective means of reducing the number and severity of intersection accidents. The provision of special lanes for traffic, wishing to turn against the flow of opposing traffic, is one such design consideration which has been shown to be an effective means of reducing the number of intersection accidents as well as the overall level of traffic congestion. Hillier et al. (1993) have indicated that the introduction of special turning lanes, at a number of intersection in New South Wales, Australia, may, in certain situations be more effective than red light cameras; They reported that such treatments resulted in a 50% reduction in right angle and right-turn against accidents as well as a 15% to 25% reduction in rear end accidents.

Roundabouts

The use of roundabouts has been shown to be a particularly effective means of reducing the level and severity of intersection accidents. These devices are able to achieve high accident reductions by significantly reducing traffic approach speeds as well as the actual speed of traffic through the intersection. Engel and Thompson (1992) reported speed reductions in the order of 30% to 65% at roundabout locations. One additional benefit is that such devices provide an effective means of regulating traffic flow and reducing traffic congestion.

In comparison to traditional controlled or uncontrolled intersections, roundabouts may offer a considerable reduction in the overall number of traffic accidents (SWOV, 1993). In an evaluation of over 200 roundabout locations, undertaken in the Netherlands from 1984

to 1991, Schoon and Van Minnen (1993) reported a 47% reduction in the number of traffic accidents and a 71% reduction in the number of fatalities and injuries. They further reported that the greatest level of injury reduction occurred among passenger car occupants and pedestrians: 95% and 89% respectively. The lowest level of injury reduction was reported among cyclists (30%).

The cost of roundabouts can be relatively high in comparison to other types of engineering solutions. However, in a recent benefit cost analysis, undertaken by the Australian Bureau of Transport and Communications Economics (1993), it was reported that roundabouts were cost effective. Over a 20 year period a $300,000 roundabout could pay for itself two times over in terms of accident severity reductions and four times over in terms of reductions in particular intersection accident types.

6.8 SUMMARY

Intersections, by their very nature, provide potential points of conflict within the road traffic network. It has been estimated that traffic accidents which occur at intersections are one of the most common type of road accident events. Efforts to minimise intersection accidents have traditionally focused on education, engineering and enforcement measures. However, if commonsense does not prevail and road users cannot be educated to obey intersection traffic laws then reliance is typically placed on enforcement to ensure road user compliance.

Traffic signals are designed to maximise the capacity of traffic intersections whilst providing a safe operating environment for road users. However, signalised intersections still pose a significant road safety problem and it has been estimated that approximately 10% of all road fatalities and 18% of all accidents occur at these types of traffic locations. The large proportion of these traffic accidents occur when road users, for whatever reason, enter the intersection contrary to the red signal displayed.

This type of problem, termed "red light running" creates a high potential conflict situation which significantly increases the likelihood of an accident event. There are several types of red light runner. A large majority (70% to 80%) of red light running behaviour occurs as a result of road users being delayed or not being able to stop safely before the onset of the red signal. The remaining 20% to 30% of red light running behaviour is the direct result of road users who deliberately run the red light signal. It is this last type of road user behaviour which is the primary target of intersection enforcement activities.

The use of highly visible enforcement activities, at signalised intersection, has been shown to be an effective means of reducing red light running behaviour. The problem with this approach is that it is extremely resource intensive and the large number of signalised intersections makes effective enforcement impossible. Red light violation rates are also extremely low, especially when enforcement activities are highly visible, and police often believe that other traffic enforcement activities are more important and cost effective. Selective enforcement strategies provide one means of improving the effectiveness and efficiency of policing activities. The targeting of known high accident risk intersections at peak volume traffic times can result in maximum accident reduction benefits. The use of these strategies also increases enforcement visibility, due to the high traffic exposure, and therefore has the potential to significantly increase the overall level of deterrence.

The most effective means of enforcing signalised intersections is through the use of automated detection devices. These automated devices, commonly referred to as red light cameras, have been shown to reduce the level of red light running behaviour by up to 55%. Significant reductions in the number of accidents (between 10% and 25%) have also been reported at intersections where red light cameras have been installed. Red light cameras have also been shown to be cost effective with derived benefit-cost-ratios in the vicinity of two to one.

The overall effectiveness of red light cameras is directly related to their deterrence potential and ability to increase perceived apprehension risk and maintain it at a high level. There are a number of way of maximising the deterrence potential of red light camera operations. Firstly, red light cameras are usually used at high accident risk locations and the use of clearly posted warning signs can ensure a high accident reduction potential. Secondly, highly visible hardware installations can ensure that motorists are continually aware of red light camera operations. Thirdly, the rotation of a small number of cameras through a large number of treated intersections can increase the area wide deterrent effect. Fourthly, the widespread use of publicity can increase community awareness of enforcement operations and increase the level of perceived apprehension risk. Finally, when cameras are not installed at treated sites, the visible deployment of the flash unit may increase deterrence.

There are a number of problems associated with the use of red light cameras which have been identified in the research literature. These include generally low apprehension rates, the permanency of on-site enforcement treatments and the time taken to notify drivers that an offence has been committed. The use of new digital imaging detection technology may provide a possible means of solving these problems.

Digital imaging systems produce a better quality high resolution image than that of existing film based systems and have been reported as having the capacity to almost double existing apprehension rates (from 50% to 95%). One such digital imaging system has also been adapted for portable use, thus increasing the application and deterrence potential of red light enforcement operations. The portability of the system also reduces the significant costs associated with the installation of induction loop sensors in the intersection. The estimated cost of this system is approximately half that of existing red light camera systems. The last benefit of digital imaging systems is that they can significantly reduce offence notification delays because they do not require the time intensive tasks associated with the collection and development of photographic images. Red light violation information can be sent directly to a central processing unit (via telephone line) within minutes after the offence has been detected.

Enforcement is only one means of ensuring that road users adhere to the traffic laws which govern the use of intersections. Engineering considerations relating to traffic signal phases and the actual physical design of the intersection environment, have also been shown to be effective accident reduction measures. In particular, the use of appropriate intergreen timings and the introduction of special vehicle turning phases (for drivers wishing to turn against the opposing flow of traffic) have been shown to reduce the number of potential conflict situations. The use of active warning signs at high speed and/or low visibility intersections has also been shown to have considerable potential.

The physical design of intersections is one further means of reducing vehicular conflict at intersections. The provision of turning lanes has been shown to have a considerable

accident reduction potential. The use of roundabouts is considered to be one of the most effective means of managing intersection problems. These devices can significantly reduce the number of intersection accidents and have the added benefit of being able to improve vehicle flow, thus reducing traffic congestion.