Airbags are inflated by a small explosive charge and bike helmets do not protect against explosions...

The idea is to create additional cushioning through controlled inflation, so that any whiplash is met with an opposite force thus neutralising the bounce. The airbag should inflate in such a way as to soften the impact by absorbing the force, something like three-fourth inflated football hitting the earth.

I think the timing would be difficult. A car senses the impact, and then has some time before the passengers hit the steering wheel or dashboard. By the time a bike helmet senses impact, the shock would have already been trasmitted to the rider's head.

The cushion whould have to immediately extend beyond the person's helmet to be effective, would it not? I'm wondering, what would the effect be on the wearer's neck when his head is held up while the rest of his body is violently slamming down? I'm guessing not pretty.

The current design of helmets is light-weight and close to the head and tends to protect the wearer from skull fractures while minimizing neck problems.

Coming to think of it, I'm reminded of an airbag for helicopters designed in Israel, which prevents the helicopter from direct impact on crashing. Any ideas, here ?