According to the Propane Education and Research Council, whoever they may be, there are more than 270,000 on-road propane vehicles in the United States. Not a huge number, but they're out there.
If I were involved in an accident where leaking fuel and fire were a concern, I'd like the leaking fuel to be propane rather than gasoline. Being a compressed gas, I would think a propane fire would be more localized, like a blow torch, while a gasoline fire being mostly liquid, would spread. I believe this is why arsonists and crazed revolutionaries choose gasoline over propane as the main ingredient in their Molotov penistails. This is all a Moot theory and has no scientific basis.