In the next 25 years, all the gas stations are going to close and all that gas tax revenue will need to be replaced, as that's how the roads are maintained. Gas tax is a usage tax, so to replace it with something equivalent, we'll need to be able to track our vehicle's road usage. Maybe cars will just self report, as new cars will just be a consumer electronics device with wheels and a broadband connection anyways.
But more than likely the solution will be backwards compatible one at first, like all the license plates becoming e-License plates, with eInk screens and a Fast-Trak style responder for tracking tolls.
Extrapolating on this, the idea that you could get into a vehicle and drive along public roads without those roads making a record of your passage is one that will slowly go away. Get used to it. Seriously.