Further aspects of the British index of secondary car safety.

This paper develops earlier research into statistical methods for comparing the secondary safety of car models. Two papers (Broughton 1996a,b, Accident Analysis and Prevention, Vol. 28, pp. 89-99, and pp. 101-109, respectively) had concluded that the most satisfactory index of secondary safety is the one first used in publications of the U.K. Department of Transport, referred to as the British or DoT index. This paper shows that the distribution of the risk of injury when two cars collide depends principally on the difference in mass; as this rises, the driver of the lighter car is more likely to be injured and the driver of the heavier car is less likely to be injured, while the likelihood of both being injured reduces slightly. It also shows that the level of protection in fatal and serious accidents varies between models to a significantly greater extent than the level in all injury accidents. Car models of similar mass can provide significantly different levels of protection to their occupants, so there would be fewer casualties if all models were to provide the same level of protection as the most successful current designs. It is estimated that if the safety of all models were improved to the level achieved or exceeded by the safest twentieth of models then the number of drivers injured in two-car accidents would fall by 12% and the number killed or seriously injured by 22%.