There is a less expensive way: the odometer. (Score 1)

I accept that we need to pay for roads. For the sake of argument lets accept that if cars use less gas the current system to pay for roads may run into a problem.

But why use such an invasive and expensive technology such as GPS.

What about the odometer: when you re-up for license-tabs, the law would require you to submit mileage. You get taxed on that.

There will be fraud, but it wont be too bad if the inspection/emission tests are used as an opportunity to verify odometer readings.

What about miles driven out of state? Add a simple claim form that lets you take miles off. (This may not be such a big deal for most, as the other state driven in will eventually want their cut).

The odometer, less expensive and you keep most of your privacy.