That's because it's funded largely by a federal fuel tax that's been parked at 18.3 cents per gallon since 1993, even as cars use less gas than ever. Things are so tight that Congress made an emergency transfer of $18 billion to keep the fund afloat.

Oh, and the current funding bill expires May 31.

Meanwhile, electric vehicles are slowly gaining in popularity, in no small part because the federal government and many states offer tax breaks and other incentives. This is good—anything that weans us from gas has long-term benefits—but comes with a downside: a Tesla Model S, for example, exacts as much wear and tear on our roads as a BMW 5-Series, but the guy driving it isn't kicking in any gas taxes to help with upkeep.

A lot of people use that to knock EVs, but it's not that big a deal. Americans bought just 123,000 electric and plug-in hybrid vehicles last year, nothing compared to the 16.5 million new vehicles sold nationwide. And the amount these drivers would otherwise pay into the Highway Trust Fund is minuscule—reaching perhaps $200 million a year by 2024, even assuming aggressive EV adoption—according to a recent study from Carnegie Mellon's Center for Climate and Energy Decision Making.