Collision incompatibility, collision compatibility, vehicle incompatibility, and vehicle compatibility are terms in the automobile crash test industry. They refer to the tendency of some vehicles to cause more damage to another vehicle ("crash partner vehicle") in two-car crashes. While vehicle incompatibility is said to lead to more dangerous, fatal accidents, compliance can prevent injury in otherwise similar collisions.

Following a compatibility test involving a 2001 Dodge Ram 1500 and a 1997 Honda Accord.
The most obvious source of collision mismatch is mass; A high mass vehicle such as a large MPV or SUV will tend to cause much more serious damage in a collision with a lighter vehicle such as a typical sedan or compact car. Specifically, a study by Michael Anderson and Maximilian Auffhammer suggests that "controlling the weight of your own vehicle creates a 40-50% increase in the risk of death from being hit by a 1,000 pound heavier vehicle". [1] Incompatibility can also be caused. the specific shape, stiffness or other design aspect of the impacting vehicles. For example, some SUVs and pickup trucks run higher than cars and do not have crash zones to absorb impact energy. Another source of incompatibility is that heavier vehicles must have stronger front ends due to today's testing requirements such as NCAP testing. [2nd]

The National Highway Traffic Safety Administration has worked on the "aggressiveness" of vehicle designs. The term "aggression" is used to indicate the average risk of injury at which a vehicle is exposed to occupants of other vehicles in a collision. The 2003 NHTSA study estimated that in vehicle-to-vehicle crashes, the design of minivans was 1.16 times more aggressive than 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, it was estimated that light trucks (including SUVs) were 3.3 times more aggressive than cars in frontal collisions and perhaps in side-impact crashes. [3]

These studies have been controversial as they influence public perception and policy decisions about CAFE standards and light truck safety testing standards that exist today. NHTSA does not define a car or light truck by weight (for example, the Chrysler PT Cruiser is classified as a light truck, while a Lexus LS 600h L, 66% heavier according to published specifications, is classified as a car). So although there are no proposals to eliminate light trucks (including minivans, SUVs, and pickups), doing so will not eliminate the incompatibility because there will be lighter vehicles hitting heavier vehicles.