Just to clarify, Transport Canada regulates safety much like the US DOT. It is Environment Canada that regulates emissions like the US EPA. Although the Canadian and American economies are tightly integrated, Environment Canada does not require the use of EPA or CARB certified alternative fuel systems. However, it does specifically prohibit tampering with emission controls and requires that vehicle emissions meet regulated standards regardless of which fuel is used. In this regard, they are exactly the same.

As for geography, ArtsBest did not specify his location. It could be CA or it could be OK although his report of $1.93/GGE suggests another state. Many states only have OBD and tailpipe testing and do not look for EPA certificates at annual emission testing.

It is true that the retail price of propane is often higher than the price of gasoline, even in non-GGE terms. However, it is possible to get a good price for propane with a little planning. Since I live just across the river from Buffalo, NY, I called up the local manager of Ferrell Gas. He told me that that the current (winter) price of propane motor fuel is $2.35/gallon with an account. For a motor fuel account, he would expect you to buy their minimum quantity break of propane, which is 1000-5000 gallons per year. 900 gallons/year would also likely qualify as well. If you drive 50 weeks out of a year and had a 20 mpg propane vehicle that racked up 400 miles/week, you would go through 1000 gallons/year. Similarly, you would only have to drive 320 miles/week with a 16 mpg vehicle to also reach 1000 gallons/year. I expect other propane suppliers (like Amerigas, etc) would offer similar deals.

However, it makes more sense to lock-in your propane cost during the summer when the propane market is low. If you did that last summer, you would be paying $1.95/gallon right now in Buffalo. If you add in the sales and road taxes and deduct the $0.50/gallon federal propane tax credit, your net price for fuel would have been $1.7635/gallon. In terms of gasoline energy, that net price of propane works out to about $2.38/GGE. In any case, if you're not buying a lot of fuel, the economics of a conversion will generally not work for you.

As for EPA certification, that has nothing to do with safety as it is purely the ability of the alternative fuel system to meet emission requirements. All certified systems start off as uncertified, universal injection systems. Safety is regulated by NFPA 52 (natural gas) and NFPA 58 (propane) or similar state regulations. Mainstream aftermarket systems (Impco, Prins, Technocarb, etc) meet NFPA safety requirements.

With modern injection conversions, systems are configured so that the fuel trims on propane or natural gas are the same as they would be on gasoline. When the system is correctly set up, injection conversions typically have better emissions on the alternative fuel. It is entirely possible for uncertified system to be as clean as (and as durable as) a certified system.

If you have a public CNG station (run by a gas utility) along your commute, CNG will generally be a lot cheaper than propane in terms of energy and, even though the conversion cost is higher, will generally have a faster payback. I was just checking the price of residential natural gas in Chicago and it's going for $0.4720/therm (monthly basis, HHV), which translates to ~$0.60/GGE. If you had to put it in your own CNG VRA (not cheap) and added in $1.00 for O&M and deducted $0.317/GGE tax credit, your price for motor fuel would work out to $1.28/GGE. The NFGC CNG station in Buffalo is selling CNG at $1.28/GGE today.