Driver Behavior Revealed in Relations Involving Car Mass

Abstract

This paper shows that answers to three related questions on the relationship between car size and safety increase our basic knowledge about driver behavior. The three related questions are: 1) Given a car crash, how does the likelihood that the driver is killed depend on the size of the car? 2) How does the probability of a car being involved in a crash depend on its size? 3) How does the number of driver fatalities per car per year depend on car size? These questions are answered by reviewing a number of published studies performed by scientists at General Motors Research Laboratories. In all studies car size is characterized by the physical variable car mass. In examining the first question it is found that, given a crash, a driver in a 900 kg car is 2.6 times as likely to be killed as is a driver in a 1800 kg car. For the second question, it is found that the accident involvement rate for 900 kg cars is 0.72 times that for 1800 kg cars. By multiplying these two ratios, we obtain an estimate for the answer to the third question, namely, that we expect 2.6 × 0.72 = 1.9 times as many driver fatalities per car per year in 900 kg cars as in 1800 kg cars. This same question is examined directly using fatality and registration data to give a ratio of 1.7. The closeness of this to the previously cited value of 1.9 indicates a fairly consistent overall picture. The 28% lower accident involvement rate for the smaller cars is interpreted in terms of human behavior feedback. The drivers of smaller cars are reducing their driving risk taking, presumably, in part, because of their intuitive understanding that they are more likely to be seriously hurt in a crash.