The argument about the cars is spread over lots of places on the net and the conclusion is that when two identical cars, both travelling at 50MPH which collide head on, the damage and deceleration is exactly the same as one car colliding with an immovable object at 50MPH.

When one car identical to the first two travels at 100MPH and collides with the immovable object, the damage and deceleration are far greater.

Shame Newton can't come and put his oar in to confirm it? ]:)