Estimating the number of accidents at intersections from a knowledge of the traffic flows on the approaches

Abstract

The increase in the number of accidents at intersections, relative to non-junction accidents, may be explained by the basic fact that an increase in the number of vehicles on the road is generally accompanied by an increase in the number of collisions, which rise at a faster rate than single-vehicle accidents. Generally, more than 50% of the collisions occur at intersections. Data from a number of countries studied support the above statement and show that over the years, the number of intersection accidents has increased at a faster rate than other accidents. The study includes a statistical analysis of the general trends in the number of intersection accidents, their severity, and an analysis of types of accidents at intersections. A model was developed which enables the estimation of the expected number of accidents at individual intersections. It was found that vehicle exposure can be used as the basis for estimation. The exposure, denned as the number of occasions for accidents, was calculated through the sum of the products of flow at the 24 points where vehicle paths cross or merge. This measure of exposure, expressed as a traffic flow index, showed good correlation with the number of accidents. On the assumption that the number of accidents at an intersection, in a given time interval, is Poisson distributed, significance tests were made comparing the actual number of accidents with the expected according to the vehicle exposure index. Such comparisons are useful in the determination of accident “black spots”.