Crash Compatibility and Crash Incompatibility

Crash compatibility, crash incompatibility, vehicle compatibility, and vehicle incompatibility are terms in the automobile crash testing industry. They refer to the tendency of some vehicles to inflict more damage on another vehicle (the “crash partner vehicle”) in two-car crashes. Vehicle incompatibility is said to lead to more dangerous, fatal crashes, while compatibility can prevent injury in otherwise comparable crashes. The effect can be summed up in the fact that 80% of the fatalities in light truck and car collisions occur in the car. However structural compatibility would help the survival of the occupants of the heavier or less flexible vehicle also.

The most obvious source of crash incompatibility is mass; a high mass vehicle such as a van or SUV will tend to cause much more serious damage in a crash with a lighter vehicle such as a typical sedan or compact car. Another incompatibility is in the specific shape, stiffness, or other design aspects of the impacting vehicles. For example, SUVs and pickup trucks ride higher than cars and lack crumple zones, which leads to greater crash partner damage. Body on rail frame design tends to defeat the crumple zone of the other car by concentrating the force.

The National Highway Traffic Safety Administration has done studies of the aggressiveness of vehicle designs. Aggressiveness corresponds to the risk for the driver of the struck vehicle. A 2003 NHTSA study that eliminates weight as a factor found that car design is the least aggressive, minivans are 1.16 times as deadly, pickups are 1.39 times as deadly, and SUVs are 1.71 times as deadly. When weight is included in the analysis, light trucks (including SUVs) are 20.8 times as deadly in side impact crashes and 3.3 times as deadly in head on crashes. In 1999 there were 12,242 people in the US killed in vehicle on vehicle collisions, so improving vehicle compatibility would prevent several thousand vehicular homicides each year.

These studies have been controversial as they affect public perception and policy decisions on CAFE standards and light truck regulatory loopholes. American motor companies have tended to emphasize increased safety to the occupants of heavier vehicles, while Japanese motor companies have paid some attention to vehicle compatibility. In the deregulatory environment of recent years, no governmental steps have been taken to improve vehicle compatibility. Individual car makers such as Honda and Ford Motor Company have claimed improvements to vehicle compatibility, but the lack of an objective voice on the matter makes evaluation difficult.

Although much of the crash incompatibility debate in recent years has centered around SUVs, the concept has been around far longer. When subcompact cars were introduced in the 1970s, there was a fear that incompatibilities of mass and design could lead to more serious injuries for drivers of these smaller, lighter vehicles. Crash incompatibility is an area of active study, although to date only a small fraction of crash tests focus on two-car crashes, and an even smaller proportion are properly designed to address incompatibility issues.