The term "compatibility" refers to how the structures of the different vehicles involved in a collision interact. Particularly when the vehicles involved in a crash are very different, there is often insufficient compatibility between them. The vehicles involved in a collision might be cars that differ in terms of their weight, rigidity or geometry; in a broader sense, however, the term can also be applied to collisions between vehicles and unprotected road users such as pedestrians or cyclists.

Incompatibility is very evident when cars collide with commercial vehicles. These vehicles have very different weights. In addition, the height of a truck's side members, fenders and other structures may mean there are big differences between the geometries of the vehicles involved in a collision.

If no technical countermeasures have been taken, a car will often underrun a truck, which means that the energy-absorbing structures of the car (side members etc.) undershoot those of the truck or its trailer and are thus largely ineffective. Consequently, the energy involved in the crash has to be absorbed by parts of the car's body, such as the A- and B-pillars, which are not suited to this purpose. There is often considerable penetration into the passenger compartment, resulting in serious and often fatal injuries.

• Underride guards