[mynameishere]

Fiberglass won't give you any protection in a crush

Damn, just put the tank of air in front. Any wreck will cause a mighty explosion balancing out the various vectors involved. I haven't done (and couldn't do, really) the math, but this seems perfectly reasonable.

[olefoo]

Or use the compressed air to fill a whole vehicle airbag that triggers on items on a collision path.

This would mean that your safety systems are highly dependent on software and effective sensors, but hey, cheap car.

[elad]

It would also mean that your level of protection depends on the amount of air in your tank.

As someone who chronically forgets to fill-up, I wouldn't like that idea...