A couple of comments on the "Center punch and blade to cut seatbelt?" thread made me think of this.

I was at a brain trauma unit for a while.

While there I was surprised by how many people had been hurt by stuff flying around their vehicles in an accident instead of bumping their heads on windshields or door posts.

Things as seemingly trivial as a clipboard in the back seat.

Everything that is loose in a vehicle becomes a flying missile aimed at you if you are in an accident.

If it is on the dash board, rear window deck or sun-visor it is likely aimed straight at your head.

If you are in an accident at 30 mph that is how fast it will be moving.
You you are driving at 60 mph when the accident happens the missile will be going 60 mph.
At 60 it will have 4 times the momentum it did at thirty and will do 4x the damage instead of 2x.
(force = mass X velocity^2)
Some professional pitchers can throw a fastball a bit faster than 90 mph.

Even a cheap pulp fiction novel will do serious damage to your brain if it hits you at any of those speeds
So think about if it was a water bottle or a wrench.

It would be far safer if loose stuff was in the trunk.
Next best would be in a compartment.
On the floor is a distant third.

So, how many of you have missiles aimed at your head inside of your car when you are driving?