Crash Incompatibility

Crash incompatibility, crash compatibility, vehicle incompatibility, and vehicle compatibility 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 most obvious source of crash incompatibility is mass; a high-mass vehicle such as a large MPV 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. In particular, research by Michael Anderson and Maximilian Auffhammer suggests that "controlling for own-vehicle weight, being hit by a vehicle that is 1,000 pounds heavier generates a 40-50% increase in fatality risk." Incompatibility may also result from the specific shape, stiffness, or other design aspects of the impacting vehicles. For example, some SUVs and pickup trucks ride higher than cars and lack crumple zones to absorb impact energy. Another source of incompatibility is that heavier vehicles are required to have stronger front ends because of today's test requirements like the NCAP test.

The National Highway Traffic Safety Administration has done studies of the "aggressiveness" of vehicle designs. The term "aggressiveness" is used to denote the average injury risk a vehicle imposes on occupants of other vehicles during collisions. A 2003 NHTSA study estimated that in vehicle to vehicle crashes, the design of minivans was 1.16 times as aggressive as cars, pickups were 1.39 times more aggressive, and SUVs were 1.71 times more aggressive than cars. When weight was included in the analysis, light trucks (including SUVs) were estimated to be 3.3 times more aggressive than cars in head-on crashes and perhaps more so in side impact crashes.

These studies have been controversial as they affect public perception and policy decisions on CAFE standards and light truck safety test standards as they exist today. Besides, the numbers above are difficult to translate into any meaningful steps because the NHTSA does not define a car or a light truck very well (e.g., the Chrysler PT Cruiser is classified as a light truck whereas a Lexus LS 600h L, a vehicle that weighs 66% more per published specifications, is classified as a car). So, it would not make sense to say that eliminating all light trucks (which includes minivans, SUVs and pickups) would eliminate incompatibility because there would still be smaller vehicles crashing into larger vehicles. This is the case in Japan, which has few light trucks but crash incompatibility is considered to be a major issue.

There has been extensive research and testing done by NHTSA, other governments, research organizations as well as automobile manufacturers to find solutions that improve safety in the small cars when colliding with larger vehicles. In the United States, a group of experts proposed major steps to improve compatibility and these have been accepted as a voluntary regulation by American automotive manufacturers as well as by most other companies selling vehicles in the U.S. The Canadian government has also accepted these recommendations. The recommendations require all manufacturers to either (a) lower the height of the primary structure (also called frame rail) of all SUV and pickup trucks so that they overlap the primary structure of the cars; or (b) add another structure (called Secondary Energy Absorbing Structure) to the SUVs and pickup truck that cannot meet the first option. Latest studies have shown that these have improved the safety in small cars when struck by SUVs. However, no such benefit has been observed in pickup truck to car crashes.

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 remains an area of active study.