Originally (back in the 1960s), frontal impact tests were done at 30mph. As you increase the speed the energy increases as the
square of the speed, so doubling the speed
quadruples the force of the crash!
The USA now test to 35mph I believe.
Here is an American coach crash that was done at 35mph:-
http://www.youtube.com/watch?v=2gvuGeRNHMU
I'm no longer involved with vehicle testing so I don't know what standards (if any) apply to coaches (or, indeed, trucks).
ECAP standard car frontal impact tests are now performed at 40 mph I believe, but with a deformable honeycomb structure on the block face (so the effect is lessened slightly).
At this speed, survival is not guaranteed, despite airbags (and seatbelts of course).
Coach passengers would stand no chance without preventative measures. In this case, the collapsing seat mountings would ameliorate the effects on some passengers whilst compounding the injuries of those crushed during the crash.