must absorb all of the initial kinetic energy of the vehicles. Since kinetic energy is proportional to
mass, heavier vehicle(s) must absorb more impact energy than lighter vehicles in the same types of
crash. This has both positive and negative implications. First, assume that the differences between
the heavy and light vehicles are differences in materials and design, and that their bodies are
equally stiff and strong. Given the higher forces, the heavier vehicles will experience a greater
depth of crush and greater crash duration, yielding reduced deceleration forces on their
passengers--a substantial benefit. The heavier vehicles, however, may run a somewhat greater risk
of intrusion into the passenger compartment, if the accidents are unusually severe. Although
making the front end of lighter vehicles less stiff would address part of this problem, this would
leave these vehicles more vulnerable in accidents involving heavier vehicles and higher speeds, and
might adversely affect handling characteristics.
Finally, in (5), the design of passenger restraint systems and the interior space itself is the
critical factor, although an unrestrained passenger will crash against the interior with a velocity
that is dependent on the velocity change of the vehicle--which is weight-related in a multiple-
vehicle collision.What Accident Statistics Tell Us
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