Doesn't have to be. I would imagine that most, if not all states have annual emissions inspections these days. No reason mileage couldn't be recorded when you're in for that, and be billed by the state for the mileage you've driven since the last inspection.
your right it doesnt have to be, but there are some serious issues that basicly mean that plan will not work.
ill give you 2 examples.
1) you live in some city. you spend 100% of your time driveing in and around that city. you always purchase gas in/around that same city. That city then collects local sales tax, and that state collects state tax to repair those roads you drive on. City uses tax $ to repair city roads, state uses some of the tax money to repair major state highways that run through/around the city. This plan seems to have you pay for what you use.
2) you have a sales job and you cover a 3-4-5 (some large) state region. You spend 90% of your time driving from site to site to do you job. You put 60K miles a year on your car but they are spread out over (say) the midwest. You buy gas twice a day every day. So as you travel through one area, you stop and buy gas there. 300 miles down the road, rinse and repeat. Those local jurisdictions get tax $ to repair roads you drove on for some short period of time. Those midwest states you traveld through all got a little piece of the gas tax as you drove through them and stopped and got gas. Point is, on avg you distributed the amount of tax you payed around to the various municipalities that should have gotten it.
now enter your simple "i drove 20K miles this year I pay $ 680 tax bill." but to who! the state? which state? how much goes to each? What if the states you drove in have different tax rates (which they do)... Are we going to make the user keep a log of how many miles they traveled in each state? How about local municipalities?
Some suggest that dropping the state and fed taxes in favor of a millage based system would work, but keeping municipality taxes at the pump. you still have to overcome the how much of your tax pie each state gets if you dont specifically track were you go.
the point is, its not as simple as saying my odometer rolled over another 15K this year, ill include an extra $800 on my fed tax return this year. Now COULD you make it that simple, shure, but thats not going to happen.
My short term solution (for the next 10 years say) is to move away from per gal tax and to a % sale tax. If the price doubles again, so does the tax money.
While we are at it, All prices for EVERYTHING should be after tax. If i was president, consumer protection rule would be all prices for any good/service is always final price paid. Taxes, fees, surcharges included.
