Sit bag and kinetic engery

The main purpose of seat-belts (NOT seat bells) and air bags is to lengthen the impact time during a collision, thus reducing the impact force acting on the passesnger.
Since [ impluse = change of momentum]
impulse = force x duration of impact
hence, force x duration of impact = change of momentum
or, force = change of momentum/duration of impact
Lengthening the duration of impact can therefore reducing the force to such a magnitude that could be withstood by a normal person without causing injury.