several years ago I saw a television show that highlighted rear safety guard design on European semi tractor trucks versus rear safety guard design on American semi tractor trucks. The European version has a special break away feature which will prevent injury in all but the most catastrophic underride collisions. Conservatively, I'd say you have around a 100% better chance of survival when rear ending a European semi tractor truck than when rear ending an American one.

http://www-nrd.nhtsa.dot.gov/pdf/Esv/esv16/98S4O07.PDF