Why are propane powered vehicles not allowed in underground parking?

Propane is a heavier-than-air gas, so any leakage would accumulate in the already underground structure. From there it would most easily continue to drain into floor drains before it could be dispersed enough to be rendered harmless. In the floor drainage piping system it would represent a massive, and building, continuous threat until something sparked it.

In the outside, it’s far less hazardous, since the far larger and always-circulating air currents would easily disperse it before it found any natural or structural low points where it could accumulate.

@CWOTUS Interesting!

Are propane accessories allowed?

@CWOTUS Ah, that makes sense, didn’t even occur to me. Should have though since I used to fill propane tanks at a gas station.

Probably because a large hot gasious explosion is a lot worse in a confined space.
Throw a full airosol on a bonfire and watch it go pop and have a laugh.
Put that same airosol in a house let it go pop, and see how much of your house is left afterwards. Theres also the prospect of structural damage to the support of the underground structure which would make it very hazardouse for any emergency services that had to go in and rescue any potential victims, who would most probably have servere percusive and burn injuries.

And if there was a propane propelled car a few spaces away which didn’t also go pop, but had a leak, youre back in asphyxiation territory as stated above.

https://www.youtube.com/watch?v=KAyuq8sDCeg
OOOOOOoooo….. ouch!!!!..... and thats with the roof off the building!