[KMag]

But the crumple zones don't help the objects (now projectiles) flying about your car. The crumple zones will finishing crumpling around the same time that something sitting on the rear deck of your car will hit you in the back of the head at 50 kph.

The original point was about unsecured objects in the car becoming deadly.

[barrkel]

Of course they do. They decrease the rate of deceleration.

Whether it's enough to make a difference, we should make calculations. I'd expect 50kph is where marginal increases in mortality start to become larger, so it wouldn't take much to reduce harm, I believe.

[KMag]

Crumple zones decrease the acceleration of your body, but don't do anything to the acceleration af objects placed on the rear shelf of your car, assuming negligible friction on the rear shelf of your car. Plenty of people get injured in accidents by kleenex boxes placed on that rear deck of their car.

You're travelling at 50 kpm (let's call it 14 m/s). You strike a brick wall. The cell phone on the rear deck of your car flies off the rear deck with minimally deceleration before it leaves the rear deck, travelling 14 m/s through mid-air. Let's make a linearizing approximation and say your body is stopped in 0.8m with constant acceleration. Let's assume the phone started out 1.5m behind your head. By the time your head is stopped by the crumple zone, the phone has travelled 1.6 m, meaning the phone is still 0.7 m behind your head, still travelling 14 m/s (50 kph).

Granted, there are a bunch of simplifying assumptions here (linear deceleration via crumpling, frictionless rear shelf, zero air resistance), but it shows there are plenty of realistic scenarios where something placed on the rear shelf of your car strikes you in the head only minimally slower than the speed at which you were driving.