Every five years, the Washington State Transportation Commission updates its 20-year plan, a document meant to inform state and local governments’ decisions on transportation spending and related issues. The commission is introducing its 2035 plan at a series of public forums around the region. The draft plan and an opportunity to comment online are also available at www.wtp2035.com.

The most significant issue the plan raises doesn’t involve concrete, steel rails or bridge trusses, but how the state pays for maintenance, operation and new construction in the coming years. The state’s gas tax, 37.5 cents per gallon, isn’t automatically adjusted for inflation and doesn’t account for the increase in the number of vehicles with improved fuel efficiency. With each passing year, the revenue generated by the gas tax pays for less and less.

The transportation commission, in the draft plan, calls for a transition from the gas tax to a road usage charge, basically treating transportation in general and roads in particular as a utility to be paid for based on how much of the service each of us use. (Actually, the commission couches its guidance in squishier language: “The commission recommends that the state continue to evaluate and plan for a possible transition from the gas tax to a road usage charge.”) But you get the picture.

A subcommittee was more concrete, recommending that such a usage charge be assessed either by charging a flat fee for an unlimited number of miles each month or year, a per-mile fee based on a vehicle’s odometer reading, a per-mile fee with mileage determined by a GPS-recording device, or a combination of two or more of those options.

Charley Royer, the mayor of Seattle from 1978 to 1989 and current Transportation Commission member, knows that such a transition in funding will require a change in thinking for motorists and legislators. Oregon and California, he noted in a teleconference with The Herald editorial board, are ahead of Washington state in terms of raising the issue and beginning work to implement a workable system.

Any system that uses GPS to determine how many miles are driven on state roadways will likely raise privacy concerns among many motorists, Royer acknowledged. And the gas tax, at least for the time being, remains the closet thing we have to a carbon tax, levying a higher tax on those who pump more carbon dioxide into the atmosphere.

“The gas tax is no longer a sustainable way to preserve and maintain our transportation system,” Royer said.

As a funding source, the gas tax is running on empty. Now’s your chance to weigh in on what replaces it.