Safety analysts have exhaustively studied accident statistics to gain a better understanding of the relative roles of various vehicle characteristics in passenger safety. It is clear from these studies and from physics, as noted above, that occupants of lighter vehicles are at a basic disadvantage to those of heavier vehicles in two-vehicle collisions.
However, if most vehicles in the fleet are made lighter, the relative weights of vehicles in most collisions will not change. Consequently, a key issue here is whether reducing the weight of most vehicles in the fleet while maintaining basic structural integrity will adversely impact vehicle safety–beyond the adverse impact caused by those remaining vehicles that retain higher weight (older vehicles and freight trucks).
Some analysts have argued that weight reductions will have strongly negative impacts on fleet safety even in accidents where the role of weight is ambiguous–for example, in accidents where two (lighter) vehicles collide with each other. In the current fleet, in accidents where two cars of identical weight collide with each other.
The occupants have an injury risk roughly proportional to the weights of the vehicle pairs; occupants of 2,000-pound vehicles colliding with each other would have roughly one and one half times the risk of occupants of 3,000-pound vehicles in a 199 similar collision.
This seems to imply, at face value, that weight reductions will increase injuries. The basic problem with all such interpretations, however, is that they are derived from data on a vehicle fleet in which car size and car mass are strongly related to one another.
In other words, in today’s fleet, if a car is lighter, it is also smaller–and has a smaller front end with which to absorb the energy of a crash. Consequently, some portion of the greater risk of lighter cars will be associated with their size (and perhaps structural strength) rather than their weight. The dilemma for analysts is figuring out the relevant importance of each.