Crumple zones remove energy making the rest of the accident less dangerous.  Air bags operate on a similar principle 1 second post accident most of the speed is gone even if the car is still in motion.  In other words survive the 80MPH crash and the secondary 40MPH crash is vastly less dangerous.<p>PS: Sure, there are a tiny fraction of edge cases such as falling off a cliff where it&#x27;s a longer term problem, but it&#x27;s reasonable to ignore them.