Ben Smith of Politico mentioned a little tidbit that has eluded some of us: The federal gas tax expires September 30, at the same time as the current reauthorization extension. Most of it, anyway — 4.3 cents of the current 18.4-cent-per-gallon tax will stay.

The gas tax isn’t part of the reauthorization; it’s just a coincidence that the dates are the same. Regardless, it’s another thing to worry about on that day.

Smith cites an accountant’s memo which details exactly what’s expiring:

— All but 4.3 cents-per-gallon of the taxes on highway gasoline, diesel fuel, kerosene, and alternative fuels (Secs. 4041(a) and 4081(d)(1))
— Reduced rate of tax on partially exempt methanol or ethanol fuel (Sec. 4041(m))
— Tax on retail sale of heavy highway vehicles (Sec. 4051(c))
— Tax on heavy truck tires (Sec. 4071(d))
— Annual use tax on heavy highway vehicles (Sec. 4481(f))

“What could possibly go wrong?” tweets kclightrail.

No one on Capitol Hill is seriously suggesting an increase in the tax, and transportation advocates are just hoping no one targets it for a cut. So, while the expiration of the reauthorization is the subject of much fanfare, the impending expiration of the gas tax has mostly flown under the radar, and that’s just the way many advocates want it.

But the date is circled on the staff calendar over at Transportation for America. “In this political climate, who knows if Congress will suddenly decide to cut the gas tax?” said T4America’s Steve Davis.

Indeed, as advocates have pushed (to no avail) for a higher gas tax to fund the nation’s transportation needs, they may now find themselves having to defend the too-low tax we now have.