similar collision.^87 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.
Some analyses have identified vehicle mass as the more important factor than size.^88 A recent
study concludes, however, that virtually all of the variation in injury risk for accidents such as
“collisions between cars of equal weight” can be explained by the differences in car length among
different pairs of equal weight vehicles.^89 In other words, the study found that, in today’s fleet: 1)
lighter cars generally are smaller cars with smaller crush zones, 2) small cars generally are scaled
down versions of large cars, that is, cars’ overall design do not vary much with size, and their
overall energy absorption characteristics do not vary either, so that 3) for the same accident
severity, the deceleration imposed on the occupant compartment is inversely proportional to car
length.
Even if the second study is correct, there still are important categories of accidents, as
discussed above, where weight will play a protective role--by reducing the velocity change and
deceleration of the vehicle in a collision. Consequently, at best, a reduction in the weight of light-
duty vehicles will have some adverse impacts on the safety of the light-duty fleet, even if crush
space and structural integrity are maintained--especially during the time when heavier light-duty
vehicles remain in the fleet, but perhaps permanently in collisions with freight vehicles and off-
road obstacles. Also, the net impact of weight reduction on barrier crashes and crashes into
vehicles of similar weight remains unclear. Quantifying this impact will require substantial analysis
of available accident statistics, and perhaps the collection of additional data, to determine the
relative importance of each accident type and the impact of vehicle weight on that type.
Design Solutions
Various design solutions have been proposed to compensate for the automatic momentum
disadvantage experienced by lightweight cars in collisions. Because crush space is a critical factor
in passenger safety, designs that increase crush space can compensate somewhat for the increased
velocity change experienced by lightweight cars in collisions. Although increased crush space can
be achieved by structural design, an interesting possibility is to deploy an external air bag
immediately before a crash.^90 Such a bag, deployed by a radar warning of the impending crash,
would create a substantial temporary addition to crush space. The availability of low-cost radar
systems and strong, flexible materials for the bag make this system an interesting one that may
(^87) Evans and Frick, see footnote 83.
(^88) Ibid
(^89) D.P. Wood et al., "The Influence of Car Crush Behaviour on Frontal Collision Safety and on the Car Size Effect," SAE paper 930893, 1993.
(^90) C. Clark, "The Crash Anticipating Extended Air Bag Bumper Systems," paper presented at the 14th International Technical Conference on the
EnhancedSafety of Vehicles, Munich, Germany, May 23-26, 1994, cited in B. O’NeiIl, Insurance Institute for Highway Safety, memo to Policy
Dialog Advisory Committee on Greenhouse Gas Emissions from Personal Motor Vehicles, Jan. 12, 1995.