Quote from Gimpster :

Maybe the solution is to make them not solid and give them a mass that puts them in the right weight category as a concrete barrier and then when you hit one it should act more like hitting another car. There is still the potential for adverse physics reaction as is seen some time in car to car collisions but it would happen less. More like a really heave tire stack.

This is probably the best solution to this, I hate when I glance at a barrier and because it doesn't like me, decides to reverse the gravitational force on my car.
Going 40MP/h into one isn't a good idea, nor is it at 40MP/h Braking speed.
Even the crappy little UF1 likes to go into Orbit on first gear.