Crashes: What actually happens to the body in a crash?
In a car crash there are actually three collisions:
1. The car
At 30 mph, when a car hits an object that is not moving, the vehicle will crumple in about two feet. As the car crushes, it absorbs some of the force of the collision.
2. The body
The second collision is the "human collision." At the moment of impact, passengers in the car are still traveling at the vehicle’s original speed. When the car comes to a complete stop the passengers continue to be hurled forward until they come in contact with some part of the automobile, for example, the steering wheel, the dashboard, the front window or back of the front seat. People who are unbelted can also cause serious injuries to others when they collide with each other. People in the front seat of a car are often hit by unbelted rear-seat passengers as they fly forward with incredible force.
3. Internal organs
In a crash, even after a human body comes to a complete stop, internal organs are still moving. Suddenly, these internal organs slam into other organs or the skeletal system. This "internal collision" is what often causes serious injury or death.
During a crash, safety belts distribute the forces of rapid deceleration over larger and stronger parts of the body such as the chest, hips and shoulders. The head, face and chest are less likely to strike the steering wheel, windshield, dashboard or the car's interior frame. People wearing safety belts are not thrown into another person or ejected from the vehicle. Also, the safety belt helps belted drivers maintain control of the car by keeping them in the driver's seat. This increases the chance of preventing a second crash.
Source: National Highway Traffic Safety Administration, U.S. Department of Transportation