Causal influence of car mass and size on driver fatality risk.

OBJECTIVES This study estimated how adding mass, in the form of a passenger, to a car crashing head-on into another car affects fatality risks to both drivers. The study distinguished the causal roles of mass and size. METHODS Head-on crashes between 2 cars, one with a right-front passenger and the other with only a driver, were examined with Fatality Analysis Reporting System data. RESULTS Adding a passenger to a car led to a 14.5% reduction in driver risk ratio (risk to one driver divided by risk to the other). To divide this effect between the individual drivers, the author developed equations that express each driver's risk as a function of causal contributions from the mass and size of both involved cars. Adding a passenger reduced a driver's frontal crash fatality risk by 7.5% but increased the risk to the other driver by 8.1%. CONCLUSIONS The presence of a passenger reduces a driver's frontal crash fatality risk but increases the risk to the driver of the other car. The findings are applicable to some single-car crashes, in which the driver risk decrease is not offset by any increase in harm to others. When all cars carry the same additional cargo, total population risk is reduced.