Seat belts are worn when travelling in a car. When the car slows down suddenly (decelerates), each person in the car will experience a force exerted by their seat belt. If the deceleration is large, the seat belts may cause compression injuries.
Car safety features such as seat belts, airbags and crumple zones all work to change the shape of the car, which increases the time taken for the collision. This decreases the rate of change of momentum, which decreases the force of the collision on any people within the car.