The injury of occupants restrained by seat belt is dominantly affected by the vehicle crash characteristics and seat belt effectiveness. Matching both performances is essential in attaining occupant's safety with most reasonable weight and cost.

Hitherto the efficiency of occupant's energy disposal has been evaluated in terms of a concept of Ride-Down, a ratio of energy absorbed vicariously by the vehicle to the occupant's initial kinetic energy.

This long established idea, however, has a certain limit for further analysis. In this paper, a new method of assessing relative effectiveness of vehicle structure and seat belt system is proposed to enable to attain a balanced approach as a whole.

The Ride-Down efficiency is decomposed to one that concerns vehicle structure and the other that relates to restraint system. The former is determined as relative Ride-Down efficiency when the simulated system of the vehicle is subjected to collision with a standard restraint system. The latter is obtained in the relation of conventional and the newly obtained method.

The two-way Ride-Down approach paves the way to indicating straightforwardly the most effective direction in which occupant protection is harmoniously attained in each vehicle design.