THERE IS an upside to President Trump’s un­or­tho­dox style of communication: Sometimes he comes out with a good idea that a less mercurial national figure might avoid out of conventional political caution. So it was with his remark during an interview with Bloomberg News, to the effect that he “would certainly consider” increasing the federal excise tax on motor fuels to help pay for an increase in federal infrastructure spending.

Mr. Trump’s one condition should present no obstacle: He said he could support a hike only if the money went to pay for highways, but the law already requires that it go into a trust fund dedicated to the purpose. Beyond that, his statement was just right. The two main revenue sources of that trust fund — an 18.4 cents -per-gallon federal gasoline tax and a 24.4 cents-per-gallon tax on diesel — have not been raised since 1993. That is to say, they have been cut, when you take inflation into account, by 40 percent over the past 24 years. Consequently, the Highway Trust Fund (which also subsidizes mass transit) is chronically underfunded and transportation needs are going unmet. Frightened of being labeled tax-raisers, presidents and members of Congress from both parties have shied away from increasing the levy or even letting it keep pace with inflation. Politicians chose instead to adopt a bill in December 2015 that purported to replenish the trust fund through such unsustainable budgetary gimmicks as a raid on the Federal Reserve’s cash flow.

Mr. Trump said he has been influenced by a friend in the trucking industry, which both benefits from a well-maintained road system and suffers from an insufficient one, in the form of congestion and vehicle damage. Accordingly, American Trucking Associations has long favored higher fuel taxes. The user-fee approach to paying for the roads makes sense for ordinary motorists as well: It’s penny-wise and pound-foolish to resist slightly higher prices at the pump, because automobile drivers, too, must pay for pothole-caused damage and time lost to traffic.

Of course no one likes to pay more for fuel; policy should be adjusted to help mitigate the impact of this inevitably regressive levy on those who can least afford it. Still, at $ 2.38 per gallon, Tuesday’s nationwide average price of regular gasoline was less than what Americans paid 70 years ago, adjusted for inflation. The tax increase needed to cover currently planned Highway Trust Fund spending would be small — roughly a dime per gallon, according to a 2015 Congressional Budget Office report . Ideally, Congress and the Trump administration could agree to a significantly larger amount, then index it to inflation permanently to assure the trust fund’s long-term stability.

By the way, a higher gas tax would help reduce fuel consumption and thus would be extremely effective in combating climate change as well. Admittedly, that’s not exactly one of Mr. Trump’s favorite causes. But he wouldn’t have to include it in his talking points.