Where I live, they are looking into ways to charge people by mile driven. Seems like that would be easy to abuse and perhaps a bit Orwellian, but it's probably the fairest system. What happens with the gas tax is that people with old cars tend to subsidize people with newer cars that get better gas mileage (based on gas taxes paid). A lot of hybrid SUVs will beat the fuel economy of economy cars from the early 90s, but do significantly more damage to the roads. So it becomes a bit of a "wealth inequality" issue since people with newer cars tend to have more money (or reckless spend) than people who have older cars.
