EXECUTIVE SUMMARY

  In London in 2002, over 41,000 people were injured in road accidents, more than 5,500 of whom were killed or seriously injured. Road traffic law is one of the main tools available to society to reduce the number and severity of road accidents, by defining behaviour which is held to be unduly risky—such as drink-driving—as illegal. These laws are only effective if they are obeyed. The likelihood of an offender being caught depends on the level of enforcement of traffic laws, by human policing and increasingly by automatic equipment such as speed cameras. Moreover, a significant level of enforcement is likely to have a deterrent effect and to persuade potential offenders to observe traffic laws. This report presents the results of a review of the relevant technical literature that was undertaken by TRL on behalf of Transport for London (TfL) to investigate "How Methods and Levels of Policing Affect Road Casualty Rates".

  The main aims of the review were:

—  to evaluate the findings from existing literature in order to determine whether increasing the level of traffic policing is likely to reduce the number of casualties in road accidents; and

—  to summarise the main pieces of work and draw conclusions, including any quantitative relationships between the level of enforcement and the numbers of accidents and casualties.

  In order to identify relevant material for inclusion in the review the TRL knowledge base (which comprises a number of databases, including the main catalogue of publications held both in the TRL library and elsewhere) was searched. In total 66 studies were included in the review: 30 were studies of speed or primarily speed enforcement campaigns, five were UK studies of speed camera enforcement, 13 were studies of drink-driving enforcement, 14 studies of red light camera enforcement, and four studies of seatbelt enforcement. In addition, three studies that have been conducted recently to investigate the mean effects of increased enforcement generally on accident rates were included in the review.

  The main findings were as follows:

—  The great majority of studies in the literature have found that increasing the level of traffic policing reduces the number of road accidents and traffic violations.

—  Theory suggests that the relationship between levels of policing and accident/casualty rates is non-linear. At zero enforcement level, accidents and casualties are expected to be at their highest levels. Increases in enforcement will have no noticeable effect at first but at a certain level, when drivers become aware of the increased police presence, accidents and casualties can be expected to begin to fall. Once a saturation point is reached, however, further increases in enforcement levels can be expected to have little or no effect. The challenges for road safety researchers are to establish the levels of policing that are required to bring about the initial decrease in accidents or casualties and to reach the saturation point, and to establish the accident and casualty reductions that can be achieved with these levels of policing.

—  Unfortunately, it is difficult in practice to establish the relationship between levels of policing and accident or casualty rates. It has not proved possible to establish the relationship by generalising from studies in the literature because appropriate information about enforcement levels is not given consistently by the different studies. The difficulty in establishing the relationship in London is compounded by the fact that the majority of studies in the literature were conducted outside the UK and that few studies assessed the effects of traffic policing in busy urban areas such as London. Those studies that were conducted in the UK were either small scale, having investigated the effects of policing on a limited number of roads, or were conducted many years ago. Therefore, new research appears to be required in order to establish how the level of policing in London affects the number of accidents and casualties. The results of the research would allow the likely implications for accidents and casualties to be taken into account when adjusting the level of traffic policing in any part of London.

—  Despite the difficulty of establishing the precise relation between policing levels and accident or casualty rates, some studies have provided limited information about the levels of enforcement required to improve safety. It seems as though stopping one in every six speeding offenders, for example, should have a noticeable effect.

—  On the basis of the literature it is also possible to discriminate between stationary and mobile methods of traffic policing. Each method can involve visible policing in either marked or unmarked police vehicles. Stationary and highly visible policing appears to be the most effective method for reducing violations and accidents, although stationary enforcement in unmarked vehicles has also been found to be effective. Mobile policing methods appear less effective, especially when unmarked police vehicles are used.

—  The effects of increased stationary enforcement seem to last for a limited amount of time after the police presence has been removed. The largest time "halo" appears to be eight weeks, although sustained police presence is required to produce such large effects. The distance halo of stationary policing appears to be in the range of 1.5 miles to five miles from the enforcement site.

—  There is evidence in favour of deploying traffic police largely at random over the whole road network. Theoretically it is likely to increase deterrence. In practice, the random allocation of stationary policing methods to different locations on the road network has been found to be effective, producing substantial impacts on accident rates and reductions in mean speeds and large distance halo effects. The main advantage of this method of traffic policing is that it requires relatively low levels of police manpower.

—  Speed cameras have been found to be particularly effective enforcement tools. They appear to be more effective than physical policing methods in reducing mean speeds and accidents. However, physical policing methods have still been found to be effective and the effects of speed cameras appear to be mainly limited to the speed camera site. On the basis of the literature reviewed, the minimum distance halo associated with physical policing is about five times greater than the minimum associated with speed cameras.

—  Studies of the enforcement of the drink-drive law have also shown that increased policing tends to reduce accidents and casualties.

—  Red light running cameras have been found to be very effective in reducing accidents and casualties. The best estimate for the effects of red light cameras is a reduction of between 25-30% in injury accidents.

—  Although few studies have investigated the effects of seat belt enforcement on accidents, a number have found that increased enforcement of seatbelt laws can increase wearing rates, which is likely to reduce casualties.