PAYING for roads in the United States was once straightforward. Petrol (gasoline) taxes, easy to administer and collect, were directed into the federal Highway Trust Fund. Those revenues were in turn distributed to the states for maintenance and investment. State petrol taxes provided further funds. And because petrol consumption was a rough proxy for road use, the system satisfied the principle that consumers should pay for a product in proportion to their use of it.

That old model no longer works. First, the federal petrol tax has failed to keep pace with prices: it was last raised, to 18.4 cents per gallon (3.8 litres), 20 years ago. Most states also do not index their petrol taxes. Second, petrol taxes no longer serve as an adequate stand-in for road use. Cars are becoming increasingly fuel-efficient (see chart 1): by 2025 new vehicles must, by law, run at an average 54.5 miles per gallon (mpg). The more efficient a vehicle, the less tax is paid to go a certain distance; drivers of electric cars pay no petrol taxes at all. So not only are revenues falling, but drivers of gas-guzzlers are being asked to carry more of the burden. Petrol taxes may be green, but they are gobbling up the tax base. And third, Americans are simply driving less than they once did (see chart 2). All this helps explain why the Treasury has had to bail out the Highway Trust Fund to the tune of $41 billion since 2008. A recent report from the Tax Foundation, a think-tank, found that almost half of road spending in America came from general revenues rather than user payments like petrol taxes.

Congressional committees have considered alternative funding models over the years but, as observers of Washington over recent weeks may have noticed, the capital’s politicians sometimes struggle to get things done. So states and municipalities are seeking to plug the gap. Various ideas have been thrown around, from sales-tax rises to tolls to bond issues. But the most radical comes from Oregon. The state invented the petrol tax in 1919, but officials now want to replace it with a vehicle-miles-travelled (VMT) fee. Under it, drivers would be charged for the distance they travel, rather than taxed on the petrol they consume. GPS-equipped gizmos would track journeys, ensuring that motorists were not charged for driving on private roads or out of state. Those who preferred to keep their whereabouts secret could opt to be charged on odometer readings, or simply to pay a flat fee. A bill that would have applied a VMT fee to all new vehicles doing 55mpg and above died in the last legislative session; instead, 5,000 volunteers will join a new VMT scheme in July 2015. They will be charged at 1.5 cents per mile rather than paying the state petrol tax (30 cents per gallon).

Oregon has been mulling a replacement for the petrol tax since 2001. The VMT policy has been fine-tuned in pilot programmes; the once-sceptical American Civil Liberties Union is now satisfied with the privacy protections (personal data is destroyed after 30 days), and local environmentalists cautiously back the idea, though they would like lower fees for more efficient vehicles. Some economists would prefer a more sophisticated system, with charges that vary by time and place (to reduce congestion) and weight of vehicle (SUVs inflict more wear and tear than Minis). The review of Oregon’s first pilot suggested that an element of congestion charging could be included. But Jim Whitty, the mastermind of the scheme, says that issue is probably best handled at local rather than state level.

Still, Oregon’s long slog is telling. The design of its scheme shrewdly addresses the most acute public concerns, says Trey Baker at the Texas A&M Transportation Institute. Yet its backers could not persuade legislators to pass it in mandatory form (the watered-down bill passed with big bipartisan majorities). In many states, including Oregon, new taxes require legislative supermajorities. And even with Oregon-style safeguards, many drivers will heartily dislike handing over their personal data to governments. “It’s as much a political challenge as a technical challenge,” says Dan Sperling, a transport expert at the University of California, Davis.

But something will have to be done. Earl Blumenauer, an Oregon congressman, backs a national VMT fee but admits he is “swimming upstream”; the Obama administration has rejected such proposals before. Mr Blumenauer wants the Treasury to sponsor Oregon-style pilot programmes in other states, in the hope of building momentum for a national scheme. (Some other states are already considering VMT programmes.) In the meantime, to keep the Highway Trust Fund solvent, he wants to raise the federal petrol tax. Given the goings-on in Congress lately, some may find that ambitious.