There's some news going on currently about U.S. States looking to impose new taxes on electric car owners ...
Washington State electric car tax going into effect in 2013 .. they already passed a law requiring a $100/year additional registration fee for electric cars. Later they plan to shift to a per-mile road usage fee.
Oregon considering per-mile road use tax on electric car owners .. Here the Legislature is considering a per-mile fee, and apparently they've already studied it some and are confident for their system.
Virginia Governor proposes eliminating gasoline tax, raising taxes on electric cars ... That Governor proposed yesterday to shift how the highway fund gets money, to eliminate the gasoline tax and add other taxes including an increase to sales tax and a flat $100/year tax on electric cars.
The core issue is that there's a long term shift towards high efficiency electrified vehicles. This means a reduction of gasoline use per mile of driving. It means that because the U.S. highway system is funded by gasoline taxes, this long term shift will reduce the tax income per mile driven. Obviously over the long term it's going to bleed the highway system dry of money, and the government has to change how the system is funded.