Just copied my earlier post in the 'natural gas' thread. More appropriate post for here!!
LPG is very commonly used in Australia. Most (if not all) of out taxis use it, as well as quite a few family cars, vans, 4WDs, etc. Some manufacturers here, like Ford, sell cars that only have an LPG tank. Most cars have used a separate LPG and petrol tank.
The disadvantages: if you run both fuels then you need room for both tanks. The LPG tank is usually fitted under the vehicle, or inside the boot (trunk) or cargo area. LPG vehicles here cary a small red 'LPG' sticker on the front and rear number plate.
Fortunately, safety hasn't been a real cause of concern. I have seen one video clip of a car that caught fire and the tank later exploded and blew the car to bits (just like in US movies!! ). This wasn't due to an accident, was away from suburbia, and the occupants had moved far enough away from the car not to be injured. After saying that then I must say that LPG cars have proven to be extremely safe. In Australia they have to be correctly fiited, inspected and maintained.
A car uses more LPG per mile than petrol, but it is still more economical to run on LPG here.
LPG is $0.49/litre or less & petrol is hitting $1.30/litre at the moment.
LPG = $1.85/US gallon
Petrol = $4.92/US gallon
When we go to a service station here there are extra pumps for LPG. They plug into a separate filler that is usually near the petrol filler. The problem that you might have in the states is whether you have LPG pumps available at many/any servo's. The cost of LPG would be determined by how much LPG the US has available to them in comparison to petrol.
Most bugs have been worked out of LPG systems, here, and vehicles run very well off LPG. There is a small decrease of power.
Rodd
http://www.advancedfuels.com.au/
Another point that concerns me a bit with the US; does the US have all it's eggs in one basket by relying too much on petrol? I am in no way criticising the US and I don't know if that is the case. If the US has good gas reserves then it might be a safer option if the government also ensured that there were plenty of LPG service stations around at least for emergency services and supply services.
Also, here is an interesting link for running LPG in a diesel engine!!
http://www.dieselgas.com.au/home.htm