Why do you see signs saying, “No propane vehicles allowed”?

Cecil replies:

There’s an off chance you could cause a fire or explosion — a prospect that would definitely enliven a dull trip to the mall but makes your petit-bourgeois garage owner break out in a cold sweat. Propane, whether used in cars or otherwise, is stored in liquid form in high pressure tanks. Occasionally some leaks out via a pressure relief valve or what have you, not a problem on the open road but potentially a big problem in a confined area, since propane is heavier than air and collects at low spots, e.g., the bottom floor of an underground parking garage or the low point in a tunnel (the entrances to which often sport anti-propane warnings). Some propane buffs, who regard the stuff as nature’s perfect fuel, say the danger is exaggerated, and my none-too-thorough search has failed to turn up any documented cases of propane disasters in garages or tunnels (although wasn’t there some sort of bang at the World Trade Center a while back?). Be that as it may, it’s only polite to humor the worry warts and leave your propane powered Pontiac parked on the street.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.