It's no joke. The gubmint requires motor vehicles to pay tax, no matter how they get down the road. When people switched cars and trucks to Propane in the 70's, they had to pay a $75 fuel tax and display the sticker on their windshield in Texas. When you drove your car to the propane dealer, he had to check that you paid the tax or no fill up. Many people took 20 gal tanks in their trunks to fill and then transfered them to their other vehicle to avoid the tax. Propane eventually was MORE expensive than gasoline to take the benefit away.
If you look back at some of my posts from years ago about switching to ethanol vehicles, the main reason we hadn't done it years ago was not that it wasn't possible, but that you might make a few gallons behind the shed and have a sip without paying the tax. E-85 became the answer. They had to poison it somehow to keep the people from drinking without paying tax. You don't need to mix it with anything for it to work fine. But a shot with Coke is pretty good. I guarantee you that if enough vehicles start to show up with nothing but batteries, or H2, they will invent a road tax sticker for you to buy to get you plates and safety sticker.
Just as a hypothetical, if all vehicles started to get 100mpg or more, the gas tax would go to $1 a gallon instead of 39 cents. The government likes vehicles to get 10mpg for the tax revenue. They have no reason to push for higher mpg or alternative fuels. It would be like a home builder saying he thinks we should be forced to live in trailers. What's in it for him? The government is not our friend. It is in direct competition with business for our money. If private enterprise comes up with a way to take money from the government share, they tax it more.
I have a 72 International truck that runs on propane. Many companies did not want to sell me propane because they had to fill out a special form on the sale to pay the road tax. If I only used the propane to "heat" the vehicle, I didn't have to pay the road tax.
The engine is still as clean as the day it came out of the factory (on the inside).
I wish more people understood this subject as well as you do.