Well if you are that worried about the dangers of a flammable compressed gas being on the road, I suggest you don't drive. Ever notice a little sticker on a vehicle that says CNG or LPG? There are thousands of vehicles driving around with propane and compressed natural gas tanks that could be possible bombs if hit hard enough in just the right direction to cause a leak and then find a source of ignition. It may even be more dangerous than gasoline, but have you heard of any explosions in real life from propane or CNG in a vehicle on the road (not tests)? I haven't.
The idea of installing propane injection in your car is to make more power and make your car faster for everyday driving. That in itself will probably prove to be much more dangerous than the chance of the propane exploding. If you feel that propane is so terribly dangerous, don't install it or anything else that could be flammable and then trade your car in for a Volvo in case you get hit by somebody with propane in their car. Also make sure you don't own any handguns, because we all know that guns kill people and handguns especially have no other purpose.
OK, back to the subject, HC's are being used as refridgerants in vehicles in europe with no problems. That placed a condenser full of pressurized flammable gas in front of the radiator and would likely have a much greater chance of being damaged in a vehicle accident than a propane bottle in the trunk, but there haven't been any reported explosions or deaths attributed to the HC refrigerants in millions of miles driven. Does the danger exist, yes. How dangerous is it in real life, aparently not very.
OK, Off the soap box.