Speed, speed limits and road traffic accidents under free flow conditions

Abstract

The relationship between various measures of traffic speed, under free flow conditions, and accident rate is investigated for two groups of sites, one in the Tyne and Wear county of the UK and the other in Bahrain. The effect of speed limits on traffic speed is estimated for both groups of sites. In Bahrain, there is statistically significant evidence of an association between mean speed and accident rate. In Tyne and Wear the statistical evidence is weaker, and points to a stronger relationship between accidents and the variability of traffic speeds. In both areas, there is an apparent decrease in accident rate if the percentage of heavy vehicles increases, with the speed distribution held constant. In both areas the effect of speed limits is to reduce the mean speed of traffic by at least one quarter. Higher speeds are associated with longer trips.

Introduction

The main benefit of high speed traffic flow is that the cost attributed to travel time is reduced, despite higher fuel and other running costs together with the associated higher environmental costs (Aljanahi, 1995). However, this advantage may be offset by a possible increase in the number of accidents and the fact that injuries sustained are likely to be more severe if accidents occur at higher speeds.

Road traffic accidents are a result of many factors, although excessive speed is often thought to be a major cause. Speed limit signs were introduced as a safety measure; and the relationships between the speed characteristics of traffic, speed limits, and accidents have been debated each time limits have been changed (e.g. Solomon, 1964, Munden, 1967, Brenac, 1990, Finch et al., 1994). Other contributory factors include: road design; traffic volume; weather; reasons for travelling; drivers under the influence of alcohol and drugs; economic pressures on drivers to save time; training of drivers; and road safety publicity campaigns. Fortunately, the frequency of road traffic accidents is decreasing, in the UK at least, despite an increasing number of vehicle registrations (Department of Transport, 1995). There are many explanations for this and any longitudinal investigation of changes in traffic speed characteristics and accident frequencies is likely to give misleading results. Instead, different road links (sites) were compared over the same period. The sites were selected from the County of Tyne and Wear in England over a five year period (1988–1992), and State of Bahrain over a four year period (1987–1990). It was thought that the two areas were sufficiently diverse to allow generalisation of any common trends.