It's the VMT tax —vehicle miles traveled tax — and how would the government know how many miles you've driven? There "could be tracked by installing electronic equipment on each car to determine how many miles were driven; payment could take place electronically at filling stations."
The great thing about this tax is that it would help cancel out whatever advantages you got from buying a car that gets excellent mileage per gallon of gas. After you spent all that money on a damned Volt because you thought you'd save so much on gas, they've got to get that money back.
And one of the things the government needs money for is a "public awareness" program to prepare you to accept the new ways in which it will extract money from you.