[brianl703]

Natural gas is only flammable within a certain concentration range...I believe it's something like 10% or 15% in air -- above or below that, it will not ignite (I looked this up a while back).

Natural gas is also lighter than air and won't puddle under the car like gasoline will.

The evening news pictures of houses blown to smithereens due to a natural gas leak are certainly dramatic, but that's a very rare occurence. I've certainly never known anyone that's happened to.

[discostu]

The flammability is secondary to the gas under pressure to me. Even non-flammable gas can explode if the tank ruptures wrong (I got to see a CO2 tank put a nice hole in a wall once, cool stuff but only because it only hurt a wall). The fact that there could be fire just adds to the drama

How much more often would it happen in vehicles than with houses? Houses don't smack into each other nearly as often as cars. Like I aluded to, think of how many times you've seen a gas tank ruptured in a car accident, now think about the difference with that being a bucket of liquid and a pressure tank.