In the last few years a number of additions to the technical literature on relationships between car size or mass and occupant risk of fatality or injury have appeared. This new information is reviewed, synthesized and used as the basis for additional calculations aimed at better identifying causal factors. Five studies from two countries consistently support that when cars of similar mass crash head-on into each other, driver risk is inversely related to the common car mass. Size is the dominant causative factor in this relationship, and in the higher rollover risk in lighter cars. Mass and size are causal factors in single-car nonrollover crashes. Mass exercises a dominant causal effect on car driver risk in crashes between effect on car driver risk in crashes between vehicles whose masses differ by more than about 10%. As 70% of car occupant deaths occur in crashes involving only one car, and lighter/smaller cars increase driver risk in all of these, a smaller/lighter fleet causes increased casualties. Because mass is a dominant causal factor in crashes that account for over 50% of car occupant fatalities, mass reductions (even if size remained unchanged) would cause casualty increases. Any measure that reduces the mass of cars, even if car size remains unchanged, will increase car occupant fatalities. (A)
Abstract