"I think that hydrogen has a much lower flashpoint than gasoline. But the bigger difference in that hydrogen, being a gas, must be stored under pressure in the vehicle. Propane conversion has been available for many years, but we only see it on large vehicles like municipal buses and such, since they have the necessary mass to protect the tack from being crushed in an accident and causing explosive decompression." Hydrogen can be a liquid if kept very cold. It is not "a gas" at all conditions. See: Space Shuttle.
Propane is a gas at normal room temperature and pressure. If kept pressurized, it can be a liquid, as in the tank in your barbecue.
There are propane-powered vehicles of all sizes on the roads. Also CNG vehicles (compressed natural gas), not only busses.
Please get the science right if you are going to make statements of "fact".
--Boris
Boris, Did not my first sentence say "I think"? I made no claim that this is a statement of "fact". You are correct that hydrogen is a liquid if super cooled. The liquid hydrogen is pumped into the shuttle just before take-off. There is no provision to keep it cold once on board. If liquid hydrogen is allowed to warm up, it returns to its gaseous form. This is not a system that could work in automobiles with current technology. I suggest you read more carefully before you post a nasty reply.