The relationship between speeding and crash rate, and speeding and injury rate has been examined in many studies and reviewed by the American Transportation Research Board (TRB SR 254). The relationship between speeding and the likelihood of crashing should be logical: increasing speed increases the reaction distance (the distance travelled while the driver is reacting to a situation) and the braking distance. In reality, the relationship between speeding and crash rate is not simple, but it is consistent across studies. In two-car accidents, the greater the deviation in speed from the average, the higher the rate of crashes. This relationship is thought to be due to the increase in interactions between vehicles when travelling at different speeds. In single-vehicle accidents, the higher the speed, the greater the risk of crashing. The relationship between speeding and injury rate is straightforward: the faster the vehicle is travelling, the greater the energy absorbed by the occupants during the rapid change in velocity that occurs during a crash. Speed limits on roads are used to regulate traffic speeds and thus promote road safety by establishing an upper limit on speed, and by reducing the variance ('dispersion') of the speed of vehicles. As injury severity increases non-linearly in relationship to speed, curbing the 'top-end' speeders should also reduce the number of deaths and severe injuries in those accidents that do occur. Speed limits are usually assigned by category, type, and design of the road (Chin 1999). Many countries provide some type of enforcement to ensure that drivers obey the posted speed limits. In Australia, in 1999, there were 1474 fatal crashes, of which 437 were identified as involving excessive speed (defined as driving faster than the posted limit or too fast for the prevailing conditions), using data obtained from the Australian Transport Safety Bureau (ATSB). It is expected that, if the number of drivers who are speeding can be reduced, both the likelihood and severity of a crash will be lowered (Pilkington 2002). The enforcement of speed limits must be sufficient to ensure that drivers believe that if they speed, they will be caught. Police cannot be present on all roads at all times and, therefore, in many countries, there is increasing use of automatic speed enforcement, using detection devices (speed cameras) that may be manned or unmanned, conspicuous or inconspicuous. Automatic speed enforcement has the capability of being a substantial net revenue-raising activity. This blurs the line for the public as to whether governments use the device for safety or for fiscal reasons, and may harden attitudes towards their use. Some people view the introduction of speed cameras as a violation of their civil liberties. Legal issues, such as whether the owner or the driver of a vehicle is responsible for the speeding violation, also arise. The arguments for and against speed cameras have been highlighted in British Columbia, Canada. A trial (Chen 2000) concluded that the introduction of cameras had reduced speeding, with a corresponding decrease in collisions, injuries and fatalities. Nevertheless, in June 2001, the speed camera programme was discarded by the incoming government. Recently there have been requests from councilors in Vancouver for the programme to be reinstated (Vancouver Sun 2002). This review aims to determine whether the use of speed detection devices reduces the incidence of speeding, crashes, injuries and deaths. Questions that need to be answered centre upon the so-called 'halo' effects. For example, the length of time an effect of enforcement is present after detection devices have been removed is known as the time-halo. The distance-halo is the distance from the point of speed enforcement that the decrease in speed continues. Also, it is not yet known whether inconspicuous detection devices are more effective than those that can be seen. It is thought that while conspicuous cameras should reduce speeds at road crash 'black spots' (also called 'hotspots'), inconspicuous cameras should reduce speeds over a wider area (Keall 2001). Speed cameras may lower the mean (relative) speed on the road (e.g. the mean speed could fall from 110km/h to 107km/h in a 100km/h zone), or they may lower the top speeds (absolute) on the road (e.g. the top speed may be 150km/h pre camera and 130km/h post camera in a 100km/h zone). This review will consider both types of speed reduction. (Author/publisher)
Samenvatting