Now that cars have as many internal airbags as can be squeezed in, the inevitable next step is to fit an external one.

Volvo (who else?) has fitted the world's first pedestrian airbag, which inflates to cover the windscreen in the event of a pedestrian collision. If that sounds a bit random, the idea is as follows. If a pedestrian is hit by the car, a sensor triggers the bonnet to be raised and the windscreen airbag to go off.



Perhaps surprisingly, hitting the bonnet is not as dangerous as you might expect, because it is a relatively thin metal panel which deforms under impact. What is really dangerous is the bonnet deforming until it hits the top of the engine, when it stops suddenly – that is the potentially fatal point. Raising the bonnet means it has more space to deform.

The point of the windscreen airbag is that if the pedestrian flies over the bonnet (so a very big impact indeed), they may hit the windscreen airbag and be saved. Certainly, in a crash bad enough for a pedestrian to hit the windscreen of an approaching car, their chances would not be good without an airbag to cushion the impact.

As safety experts point out, the idea that an impact by itself kills people is a slight misconception. The critical factor is the rate of deceleration. The whole idea of modern cars is that they deform progressively in a crash – basically the front and back are deliberately weaker than the centre of the car, so as to absorb the impact before the occupant does. Anything that reduces the rate of deceleration, like an airbag or a deforming bonnet, increases the chances of survival.