The effect of speed cameras on injuries from road accidents

The technology exists to ensure all road traffic is regulated to safe speeds

When motor cars first appeared on British roads they had to be preceded by a man waving a red flag to warn the public of the approaching danger. As the number of vehicles has grown, many ways of ensuring the orderly management of traffic and public safety have been tried. Despite improvements in vehicle and road design, traffic regulation, and driver behaviour and despite reductions in accident rates, road traffic accidents remain a major cause of premature death or serious injury.1 Can anything further be done to reduce this toll?

The epidemiology of road traffic accidents is well known. For prevention the most important measures are separation—of streams of traffic from each other and of vehicles from people—and control of speed. Limited separation has proved practicable only on motorways: the complete separation of heavy goods vehicles from cars, bicycles, and pedestrians is unrealistic. The role of speed in accidents and in seriousness of injury is incontrovertible, but the issue for prevention is how to apply and enforce speed limits. Abundant data have accumulated from observational studies on the effects of introducing speed limits (in urban areas, on motorways, for war time black out, for fuel economy in the petrol crisis) and also on the effects of relaxing speed limits.2 3

The Highways Agency recently reported the findings of a six year experiment in the use of speed cameras on west London trunk roads. The design was a before and after comparison of deaths and injuries in the three years before the installation of cameras and the three years afterwards. The numbers of deaths reduced threefold, from 68 to 20, and the numbers of serious injuries by over a quarter, from 813 to 596. Lesser injuries also fell significantly, from 4983 to 4375. The study considered the spillover effects of traffic seeking side roads to avoid cameras and found no compensating increases in neighbouring roads. Acknowledging the difficulties of ascribing causality from a before and after comparison, the study also included a control: comparable trunk roads in other areas of London with no cameras over the same period. A small reduction in accidents was observed on control roads, and all rate reductions in the study area were adjusted accordingly: even so, all reductions on the study roads remained highly significant.

American studies have also suggested an 8% reduction of fatal accidents for each reduction in average speed of 1 mile per hour (1.6 kilometre per hour).3 Another trial of area wide traffic calming schemes, based on speed bumps, road narrowings, and road markings, has shown a comparable reduction in overall accidents.5 The gradient in risk reduction by severity of injury in the speed camera project may be interpreted as a sort of dose response.

The next question for road safety and public health is whether the improvements in this study are sustainable and repeatable elsewhere. The risk compensation theory suggests that motorists will find other ways of injuring themselves and other road users. However, the technology to measure and record vehicle identification, time, place, and speed has been with us for many years and the automation of this technology makes enforcing speed limits more practicable. In the interest of safety we should expect all road traffic to be regulated to safe speeds, and in the near future to variably set safe speeds depending on prevailing conditions.