Some of us (myself, Rubber Toe, Ohmman) volunteered for the California Road Charge Pilot project. This is a trial to charge a fee per road mile instead of the usual road tax that is part of the retail cost of fuel. No money will actually change hands in this trial, but the trial is underway to determine if the State of California will abandon the gasoline fuel tax and replace it with a mileage tax.
This pilot program has a number of reporting options. One is manual reporting by submitting odometer readings. Another is to have a GPS device installed in your car to track your "on-road, in-state" driving. The third option is to have the car's built in telemetry send mileage data. There is a method to adjust for out-of-state driving for the latter option. Other options under consideration will be a weekly/monthly/annual pass for unlimited mileage, so no reporting is required. This test project is due to end in March 2017.
Oregon already has a mileage option that is live. I recall that only a couple thousand individuals have signed up for it, however. They get billed monthly for their mileage driven, and the receive credits for the gasoline tax paid at the pump.
Personally, I think that this is the future. Despite individuals who fear Big Brother and all, it seems the most equitable. It is hard to justify a fixed rate per gallon of fuel when cars can obtain anywhere from 12 to 40+ miles per gallon for driving the same distances.