[GTP owner]

Maggot got it right. Two cars of exactly the same type and mass hitting each other at 100km is NOT equal to a 200km/h collision. If your car weighed substantially less than the opposing vehicle (say you are a barina and they are a truck) and you go backwards as a result of the collision at 100km/h, then, and only then is the force equivalent to a 200km/h collision.
So two equal cars colliding will have exactly the same crumple zones, so will be exactly the same as hitting a brick wall. There will be no reduction in force as a result of their crumple zone giving way unless you weigh more than they do.
This is why it is seen as safer for you to have a heavier car (as in a 4x4) because more of the force is transferred to the smaller car - ie. they go backwards, while you do not completely stop. As an example - you have a landcruiser and they have a barina. They will go backwards at 20km/h, meaning their impact is the equivalent of a 120km/h collision, which is unsurvivable. The cruiser decelerates to 20km/h, meaning the force is of a 80km/h collision, which will probably be fatal.