The government has a problem with your new, high-mileage, low-pollution car: you aren’t paying enough in gas taxes to keep up the federal highway system. Recently, at Congress’s behest, the Congressional Budget Office studied another way to raise money; charge drivers per mile driven, rather than (or in addition to) taxing your gas.
The Vehicle Mileage Tax (VMT) is meant to directly reflect some of the costs of maintaining the highways that are not a direct consequence of the gas you burn; road damage (mostly thanks to semis), noise, accidents and road congestion.
Sound absurd? Actually, a form of VMT is in place on the nation’s toll roads already, and several countries are charging trucks by the mileage traveled. The technology to track where you drive and how far is now available, as well, combining things like the E-ZPass with GPS and wireless networking.