What is a crumple zone, and why is a stiffer car less safe?

A crumple zone is a section of a vehicle designed to deform progressively in a collision, absorbing energy and extending the time over which the occupants decelerate.

The physics, which is the whole point. In a crash, the occupants' bodies must go from travelling speed to zero. What injures people is not speed but deceleration — the rate of that change — and the forces it produces.

Deceleration depends on the distance and time over which it happens. Stopping in 10 centimetres produces far higher forces than stopping over 60 centimetres. A crumple zone buys distance and time by collapsing in a controlled way, and the energy consumed deforming metal is energy not transmitted to the people inside.

Why a rigid car is worse. An old car with a stiff structure stops almost instantly on impact — and so do the occupants, who experience the entire velocity change over a very short distance. The car looks less damaged afterwards; the people inside fare far worse.

This is the origin of the persistent belief that older cars were "built better". They were built stiffer, which looks like strength and is the opposite of what protects you.

The design is not uniformly soft. Modern vehicles combine:

Deformable crumple zones front and rear.

A rigid safety cell around the occupants, designed not to deform, preserving survival space. Intrusion into that space is what kills people.

Load paths directing forces around the cabin.

Structures designed to manage impacts with other vehicles, including compatibility with different ride heights.

Béla Barényi at Mercedes-Benz patented the principle in the 1950s, and it took decades to become universal.

Everything else works with it. Seatbelts with pretensioners and load limiters, and airbags, all extend deceleration further — they exist to spread the same velocity change over more time.

A visibly destroyed car frequently indicates the structure worked, which is worth knowing when looking at crash photographs.