Above is a report prepared by Metro CEO Art Leahy and Metro staff for the agency’s Board of Directors. The short of it: Metro would eventually have to cut service if the federal Highway Trust Fund continues to wither.

The bigger issue is this: Congress is due this year to approve a long-term transportation funding bill to replace the one that expires in 2014. The new bill needs to tackle an ongoing issue: the federal Highway Trust Fund (HTF) is funded by the federal gas tax, which hasn’t been raised since 1993 (it’s 18.3 cents per gallon). Vehicles are more fuel efficient and people aren’t driving as much or using as much gas — thus, the HTF is struggling to keep up with expenses that have risen over time.

One of those expenses is helping supply funds to transit agencies across the country for items such as maintenance and construction projects. Local agencies tend to spend most of their funds on providing daily service and, thus, need the help of state and federal government for big expenses.

How to save the Highway Trust Fund from eventually going broke — it’s nowhere near that, yet — has been a source of considerable discussion in Washington D.C. for many years now. One idea that is constantly batted around is imposing a vehicle mileage tax that would tax motorists based on how many miles they drive instead of by the gallon.

But nothing has happened. Politics in D.C. are tough and the two-year election cycle in the House of Representatives makes things tougher. Hopefully this is just another threat to transportation funding that never comes to pass. Time will tell.