Safety features such as seatbelts, airbags and crumple zones are now used in modern cars. Crumple zones change the shape of the car, which increases the time taken for the collision.
These crumple zones are areas of a car that are designed to deform or crumple on impact. There are two ways to explain their action.
Work done = braking force × distance
During a collision there is work done (or energy transferred). When a car crashes into a wall. When a car crashes into a wall for example, the car and driver are brought to rest in a very short distance. The car's safety features increase the distance travelled, because the work done would be the same, the force of the collision on any people within the car must be smaller.
During a collision there is a change in momentum. The force of the collision is equal to the rate of change of momentum. The safety features decrease the rate of change of momentum by increasing the time of the collision, which again decreases the force of the collision on any people within the car.