Consider that they guy buying a Toyota Prius gets a big tax credit, then runs down the road using far less tax-revenue-generating fuel. Instead of a fine he gets a tax credit, is allowed to legally avoid the additional road taxes, and is probably invited to drive alone in the HOV lanes to boot.

The problem is that the structure for collecting road taxes as a use tax - by taxing the fuel - no longer works. It would now make more sense to charge road taxes based on annual mileage and weight of vehicle.

No matter which way you go, someone's going to cry foul! Should be interesting to watch this unfold.