[ScottBurson]

The elasticity doesn't matter for the equivalence of the two scenarios (head-on collision at 50 vs. brick wall at 50). We've assumed, albeit implicitly, that in the head-on case, the cars have equal and opposite momentum. Whether the collision is perfectly elastic, perfectly inelastic, or somewhere in between, a car will experience the same forces in the two scenarios (assuming, of course, that the elasticity is equal in the two cases).