This post originally appeared on Orphan Road.

Here’s something ironic: the increase in gas prices this year is causing a projected shortfall in gas tax revenue. Why? People are using less gas.

Actually, that’s not ironic at all — it’s exactly what you’d expect: supply and demand. It is, however, instructive in the over-reliance on a single source of revenue to fund transportation. It also highlights just how divergent gasoline consumption and road use are becoming.

Back, say, 50 years ago, when all the jobs were in the cities, every household had one car, and all those cars were basically interchangeable Fords and Chevys that all got roughly the same gas mileage, the gas tax would have been a fairly equitable and reliable way to fund road construction and maintenance.

Today, however, the picture is different, and only getting more so. With hydrogen, biofuels, and ultra-efficient electric cars on the horizon, consumption of gasoline will soon cease to be a useful predictor of road use.

Which leads us to other types of “use” taxes, such as static (i.e. toolbooth-based) or dynamic (radio transponder) tolling. Sen. Ed Murray elaborates:

“Almost worldwide after the end of World War II, gas tax was a way that transportation was funded. We are now moving into a period of time where we have to explore what is going to … replace the gas tax,” he said.

Murray, D-Seattle, said the state needs to look at tolls to fill the gap but added, “That’s going to be a long debate.”

It’s a debate well worth having.