Vehicle crashes consist of a series of collisions; the most common type is a frontal impact. The initial impact is between the vehicle and another object, while the occupants continue to move forward at the precrash speed as the vehicle slows down. Unrestrained occupants will continue at the precrash speed until they abruptly stop against the decelerating vehicle interior or surfaces outside the vehicle, experiencing high accelerations and loading. Restrained occupants collide with their restraints, giving them a longer time and distance to come to a stop so that they experience lower loads and acceleration levels.
The front structures of vehicles are designed to crush during frontal crashes, thereby absorbing a portion of the crash energy and allowing the passenger compartment to stop over a greater distance and longer time than does the front bumper. By coupling the occupants to the passenger compartment structure with snug-fitting belts, they will “ride down” the crash with the vehicle frame for a longer time period. For small children to “ride down” the crash, a snug harness couples them to the child restraint, and a tight installation of the child restraint (using seatbelt or LATCH) couples them to the vehicle.
In other crash directions, the occupant motion is primarily toward the point of vehicle impact. Although side impacts usually have a lower change in velocity than frontal impacts, there is much less vehicle structure available to absorb energy between the occupant and striking object. Rear impacts are generally the least severe among all crashes, with restraint provided by the vehicle seat back and head restraint. Rollovers involving more than one-quarter turn of the vehicle project the occupant towards the roof, making restraint use critical to achieving good injury outcomes.