AP:

Transportation Secretary Ray LaHood says he wants to consider taxing motorists based on how many miles they drive rather than how much gasoline they burn—an idea that has angered drivers in some states where it has been proposed.

The reason, says LaHood, is that gas taxes don't cover the cost of the federal and state highway and road infrastructure.

Okay, fine.  But a mileage tax isn't as good as a gas tax.  A mileage tax will motivate people to drive less, but it won't incentivize you to by a fuel-efficient car.  A gas tax, on the other hand, will do both.  Right?