WASHINGTON — IF you think your commute is getting worse, it’s probably not your imagination. And no, it’s not because there are more cars on the road. The potholes, the stalled construction projects, the congestion — it’s because the highway trust fund is almost empty and, without a fix, could run out of money this summer.

Federal transportation funding relies heavily on user-based fees, in the form of gas taxes. While that worked for decades, it began to break down after Congress stopped raising the tax, which has been stuck at 18.4 cents a gallon for over 20 years. Since then, people have begun driving less and using more fuel-efficient cars, which means less tax is paid. Even worse, the tax is not indexed to inflation.

In the past Congress has adopted a series of stopgaps to shore up the fund. But unless we reform the way we pay for transportation improvements, we will keep lurching from funding crisis to funding crisis, with our roads getting worse by the year. The only solution is to supplement the tax with dedicated federal funding — which would not only solve the money problem, but open the door to long-dreamed-of innovations in our transportation system.

The obvious solution, raising the gas tax, is a political nonstarter. And even if it could pass, Congress would be tempted to direct some or all of that revenue to other purposes, like deficit reduction — it did just that in 1990 and 1993.