I really hope they do this because now the fuel tax is hidden and once people get a monthly bill, they may actually do something to vote out the people trying to overtax them:

The year is 2020 and the gasoline tax is history. In its place you get a monthly tax bill based on each mile you drove — tracked by a Global Positioning System device in your car and uploaded to a billing center.
What once was science fiction is being field-tested by the University of Iowa to iron out the wrinkles should a by-the-mile road tax ever be enacted.

(via)