How do crumple zones and seatbelts save lives?

Crumple zones and seatbelts save lives by doing one thing: they make you stop more slowly. Everything in a moving car, passengers included, has inertia and keeps going forward at the same speed unless something stops it. In a sudden stop, an unrestrained person carries on and hits the interior with a force equal to many times their normal weight. A crumple zone slows the collision so that the occupants decelerate over a longer time, which lowers the peak force on them. A seat belt stretches during the impact, absorbing the passenger's energy and lengthening the time they take to stop.

The trick is to weaken the car in some places and strengthen it in others. Crumple zones are sacrificial outer parts, at the front and rear, that are weakened on purpose, while the passenger cabin becomes a safety cell with reinforcing beams and higher-strength steels. People sometimes think crumpling makes occupants less safe, because the body collapses. That is a misconception: the crumpling happens outside the rigid cabin, and it is what slows the collision.

The three-point belt has a famous author. Nils Bohlin, a Swedish engineer who had worked on ejection seats at Saab, developed it to its modern form for Volvo, which introduced it in 1959 as standard equipment. Bohlin then studied 28,000 accidents in Sweden. Unbelted occupants died at all speeds, but no belted occupant was fatally injured at speeds below 60 mph.

Volvo made the patent open in the interest of safety and gave the design to other carmakers for free.