Mr. TCU, can't say I know for sure, but here's a couple guesses:
- Propane and CNG are under pressure; so, any leak has the potential for the entire tank of gas to end up in a confined space,
- As the vapor travels up it can contact spark sources such as electric motors in the elevators or anybody in the parking area who happens to be smoking.
A gasoline leak can be just as serious, but that's the tyranny of the majority for you.
In an accident with a fuel system rupture, you may be less likely to be barbecued with fuel that dissipates into the air rather than stuff that flows under the passenger compartment
