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.
Thanks for everyone's replies, ehem, including Chris' interesting recollections...
Tom, I think that maybe you've hit the nail on the head with these comments. I was Googling a bit and ran across the website,
The Properties and Benefits of Natural Gas . Two things you said resonate with stuff from this webpage--that a lighter-than-air gas will tend to dissipate rather quickly (unless, I suppose, it is leaking into an enclosed space, like the passenger compartment), and that natural gas actually has a rather narrow combustion range. According to the website, concentrations below 5% and above 15% will not ignite. Relatively speaking, I think a CNG leak would be less dangerous than a gasoline spill at an accident scene, like the train/bus collision I was mentioning in my OP.
I don't think I would ever worry about BLEVE. Hanging around near/inside a CNG-powered bus (CNG-powered cars are exceedingly rare around my neck of the woods) that was completely engulfed in flames would be the last place I would want to be.
Speaking of BLEVE, for some reason I caught two examples of it on TV this weekend on those TV shows that compile old news footage. One was a tanker rail car full of propane that had derailed and caught on fire. The explosion looked like a small atomic bomb, with the mushroom cloud rising from the site. Or maybe what a "daisy cutter" fuel-air bomb detonation looks like. The other one was a fire at a welding supply company in Dallas. The storage yard had
dozens of acetylene tanks. When they started to "cook off," they literally turned into little rockets and could see the flaming trail of the tanks as they rocketed hundreds of yards away, narrowly missing vehicles on a nearby freeway. Scary stuff.