When a vehicle crashes, it slows down very quickly. This is a large deceleration, which is a negative acceleration. Newton's second law says that force = mass × acceleration (F = ma). A large deceleration therefore needs a large force on the vehicle and on every person inside it. A large force can break bones, injure the neck and damage organs.
Deceleration = change in velocity ÷ time taken. If the change in velocity stays the same, a shorter stopping time gives a larger deceleration and a larger force. A longer stopping time gives a smaller deceleration and a smaller force. Crumple zones, seat belts, air bags and cycle helmets all work by making the stopping time longer, so the force on the body is smaller.
We can estimate the force in a typical crash. A 70 kg passenger travels at 15 m/s, about 30 mph. If the car hits a wall and the passenger stops in 0.1 s, the deceleration is 15 ÷ 0.1 = 150 m/s2. The force is 70 × 150 = 10500 N. If a seat belt and air bag make the stopping time 0.3 s, the deceleration is 15 ÷ 0.3 = 50 m/s2 and the force is 70 × 50 = 3500 N, about a third as big.