Why don't we all just drive around with mattresses on the hood:

A Europe-wide collaboration led by Roger Hardy of the Cranfield Impact Centre at Cranfield University near Bedford in the UK has developed an experimental system for cars that aims to cut this death toll and reduce the severity of injuries. When the system detects that the car is about to hit a pedestrian, it automatically raises the rear of the bonnet (hood), releasing a giant airbag in front of the windscreen.

Legislation to protect pedestrians is already responsible for the "high forehead" look of new European cars... they have to meet a certain set of crush guidelines to ensure someone doesn't, well, get crushed. This'd make, what, eight or nine airbags in a well-equipped car? These are not cheap devices!