As regular Source readers know, the ungainly phrase “federal Highway Trust Fund” (or HTF) burps forth often on this blog. Why?

Short answer: It’s a big federal pot of money that Metro and other transit agencies rely on to help fund a variety of transit and road projects. This chart helps show that:

Problem is, the Highway Trust Fund is heavily funded by federal gas taxes, which haven’t increased since 1993. In the meantime, the cost of everything else keeps rising, the reason the Trust Fund is — in the words of Metro’s government relations team — “continuing its downward spiral into insolvency.”

Check out this chart from the Congressional Budget Office:

The key numbers are at the bottom under the header “Memorandum.” As you can see, beginning in 2016 the shortfall for both the highway and transit accounts in the HTF are in the negative. And those numbers represent billions of dollars — meaning the Highway Account will be shy $10 billion in 2016 and the Transit Account will be $3 billion in the hole.

Ultimately, it’s up to Congress to decide what to do about it. Some activists and elected officials have indicated that they think it’s time for Congress to raise the federal gas tax from 18.3 cents a gallon. If not that, then Congress needs to find another revenue source.

The hope is that whatever Congress decides to do, they do it fast. The last long-term federal transportation bill was signed into law by President Bush in 2005. Since then, Congress has either adopted short-term patches to the HTF or short-term bills. The latest patch expires in May.

The topic will almost certainly rear its head at a hearing by the U.S. Senate Committee on Environment and Public Works this Wednesday in Washington. U.S. Department of Transportation Secretary Anthony Foxx will be testifying — and many transit agencies, Metro included, will be watching as a variety of other House and Senate committees begin discussing aspects of a future bill.