And while Congress can’t agree on whether to proceed, several states are not waiting. They are exploring how, over the next decade, they can move to a system in which drivers pay per mile of road they roll over. Thousands of motorists have already taken the black boxes, some of which have GPS monitoring, for a test drive.

“This really is a must for our nation. It is not a matter of something we might choose to do,” said Hasan Ikhrata, executive director of the Southern California Assn. of Governments, which is planning for the state to start tracking miles driven by every California motorist by 2025. “There is going to be a change in how we pay these taxes. The technology is there to do it.”

The push comes as the country’s Highway Trust Fund, financed with taxes Americans pay at the gas pump, is broke. Americans don’t buy as much gas as they used to. Cars get many more miles to the gallon. The federal tax itself, 18.4 cents per gallon, hasn’t gone up in 20 years. Politicians are loath to raise the tax even one penny when gas prices are high.

“The gas tax is just not sustainable,” said Lee Munnich, a transportation policy expert at the University of Minnesota. His state recently put tracking devices on 500 cars to test out a pay-by-mile system. “This works out as the most logical alternative over the long term,” he said.