As do CNG tank valves. It's actually pretty hard to get a ruptured CNG tank to ignite or to even rupture. Most cylinders are carbon fiber wrapped aluminum and are resistant to most non-AP small arms fire.

Other things to consider is that likely the concentration of NG adjacent to a tank rupture will be too rich to ignite. NG is lighter than air so it will tend to float up rather than settle or accumulate near the ground.

I would think that an accident with enough force to rupture a composite CNG bottle would kill the car occupants long before you'd have to worry about an explosion.

I've seen a demonstration where they dropped a car equipped with a CNG tank from 100 feet, and it did not rupture the tank. The one case I've heard of a ruptured tank was on a bus where the battery box was located above the tank and leaking acid from a battery ate away at the bottle resulting in a rupture.

All in all, it's a pretty safe technology. We have passenger ferries which go between Norfolk and Porsmouth, VA which are CNG powered.

Regards,

Tom