The scientific investigation of road accidents

Abstract

The casualty rate in road accidents, at 6000 per year, far exceeds that for other modes of transport, despite which there is little public concern. Their investigation by many police forces is now carried out by specialised units, using scientific techniques first used by the West Riding Constabulary in 1947, and brought into more general use by the Metropolitan Police in the mid-sixties. A primary technique is the estimation of vehicle speed from the length of skid mark. Many workers have carried out tests indicative of the relationship between these quantities. We have extended this work considerably, comparing the effects of differing tyre construction, conditon and composition, and of vehicle weight and speed. Our results show that on a given dry road surface a simple sliding coefficient of friction can be used which is substantially independent of tyre type, weight and speed. However, the tyre/road friction does vary with the nature of the road surface and its temperature, and a simple technique for measuring it is described. The energy absorbed in deforming a vehicle's structure in an accident will often account for a significant fraction of its original speed. Controlled crash tests performed by various workers show that the relationship between impact damage and impact velocity is a remarkably simple function, and we describe the techniques for using their data in the more varied conditions of real accidents. Finally the use of these techniques to reconstruct events leading up to two road accidents is described.