The Effect of Police Presence on Urban Driving Speeds

T he speed of vehicles on urban streets has been a subject of concern for some time. Australia has one of the highest urban speed limits in the world (60 km/h), and a Victorian study found that many residential streets operated with a mean speed well above the speed limit; the normal 85th percentile speed being between 70 and 75 km/h.l Surveys carried out by the Traffic Accident Research Unit (TARU) on urban streets in Sydney have indicated that similar high speeds exist in New South Wales. There are a variety of ways of attempting to control traffic speeds. These include such measures as street design, publicity campaigns, and police enforcement. Armour showed that speeds could be reduced by limiting street length and street width.’ Marconi investigated measures such as mini-roundabouts and increased numbers of stop signs and found that this type of measure could be effective in residential areas, particularly on approaches to intersections.’ Enforcement is naturally a major consideration in any attempt to control traffic speeds. Unfortunately, there is little evidence available about the effectiveness of police traffic law enforcement. The little research that has been undertaken has concentrated mainly on highway situations while the problem of enforcement in urban areas has been virtually ignored.’ The literature on police traffic law enforcement was reviewed by Armour,’ The review concluded that there was good evidence that the presence of an enforcement symbol (e.g., a police car) will reduce vehicle speeds and that this reduction can be maintained for some time after the vehicles have passed the enforcement symbol. The review also discussed the possibility of producing a “memory effect” of enforcement. A memory effect of enforcement is produced if drivers reduce their speed in areas where they have previously seen enforcement symbols, even when no enforcement symbols are present.” It has been shown that a memory effect of enforcement can be produced in highway situations using high levels of enforcemtibut it was not known if such an effect could be demonstrated on urban roads using the low level of enforcement resources available in New South Wales. TARU undertook a field study to examine the effects of enforcement on traffic speeds on urban roads. The study was designed to investigate both the immediate and longer-term effects of the presence of an enforcement symbol. It consisted of an enforcement symbol being present on chosen sites for a number of days in February and March 1982. During the first week the police were present on four days and their presence was gradually reduced over the following weeks. Speed surveys were carried out before, during and after the enforcement period. The aim of this article is to describe this field study and to discuss its implications for the deployment of enforcement resources in New South Wales.