Used all the time in racing cars and aircraft, they are called fuel cells. Added with the current technology on the road for collision avoidance, and we have what you desire.
Those are intended to contain liquid fuels, not gaseous ones under pressure. If you want an impact-proof gas cylinder, you will find they weigh more than the engine of the car does, empty.
To give you some idea of how long this has been promoted, watch this very early 80s video:
http://www.youtube.com/watch?v=irvktfQvu4M
Those lightweight tanks? Yeah, turned out they delaminated and ruptured after a while, which is why they never went anywhere in the marketplace.
For a given volume tank, you will get far less range on natural gas, as well.