The Distribution of Vehicle Mass in the On-Road Fleet of Passenger Vehicles

Crashes involving two passenger vehicles account for 25% of all traffic fatalities in the United States. Large differences in the weight of the crash-involved vehicles can have a strong influence on injury and fatality risks in both vehicles. A simple, spreadsheet-based model is developed to examine empirically the distribution of mass in the on-road fleet of passenger vehicles from 1975 to 2001. The model relies on publicly available data to compute an index of mass variance. The results indicate that mass variance decreased substantially beginning in 1975, during the start of a period of significant downweighting of new cars. The variance reached a minimum in 1996 and has been increasing since then. Simulations were conducted to examine the frequency of hypothetical crashes involving large disparities in vehicle mass. The fraction of crashes in which the mass ratio (heavier vehicle to lighter vehicle) was 1.75 or greater was 6% in 1975, 1% in 1996, and 2% in 2001. This paper also illustrates how the model can be used to project the future impacts of hypothetical scenarios in which consumers select different sized vehicles.