That's one reason the tax hasn't increased in nearly 20 years, even as labor, steel and asphalt costs have risen sharply. Plus, as fuel efficiency increases, Americans can put more miles on roads while at the same time buying less gas, worsening the shortfall.

Other ways to plug the funding gap include instituting a mileage-based driving tax, shifting more of the burden to the states or taping private funding sources that would then charge the public user fees.

Calls to congressional staffers revealed little in the way of an effort to raise the tax at this time.

Still, as one Democrat staffer put it, with Republicans softening to the idea of raising taxes in general, "possibilities for this sort of talk has at least opened a bit."