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Washington state is just a few months away from testing a new way of paying for its roads by charging for every mile driven.

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It’s called a  and many believe it’s the future of transportation funding. The state is preparing a pilot program.

But there are plenty of unanswered questions.

About 2,800 Washington drivers will hit the roads in January, testing whether it makes more sense to charge them for every mile instead of the current gas tax. Some drivers will have GPS monitors attached to their cars. Others will simply use their odometer readings to track mileage. They will pay the gas tax at the pump but get reimbursed so that they are taxed for the number of miles driven.

Reema Griffith, executive director of the Washington State Transportation Commission, said the plan is to replace the gas tax if this road usage charge works.

“The full intent is to replace the gas tax,” she said. “How that happens, in terms of the time-frame of transition, is up in the air. Those kinds of things will ultimately be settled by the Legislature.”

That transition could last a decade. Griffith said the state can’t just turn the gas tax off.

“Our gas tax is heavily leveraged,” she said. “We have a lot of debt issued against it to generate all the dollars we have over the years to build all of these projects. We have about a 25-year debt that we have to deal with.”

Oregon is already using the on a limited basis, but it has no plans to get rid of the gas tax.

“The gas tax would have to stick around because given the technology for road usage charging there are older vehicles that just aren’t compatible with that, and we’d need some way of having them pay for the roads they are using,” said Michelle Godfrey with the Oregon Department of Transportation.

And it’s not just Oregon and Washington state looking at this. There are investigating whether there should be a regional or even a national system set up so that you would pay for every mile you drive, some with a gas tax too, others not.

“A visitor from, say, Washington to Oregon fills up in Oregon and drives in Oregon they’d be paying the Oregon fuel tax,” Godfrey said. “It’s not a perfect system because you could fill up in Washington and then drive in Oregon, and Washington is getting the benefit of that tax dollar.”

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That’s why Washington’s pilot program will include drivers from Oregon. Griffith said they will use drivers who frequently travel back and forth between the two states to try and find out how a regional system might work.

“How do our two states reconcile that?” Griffith asked. “We’re going to work it through, not just the collection of it, but dealing with our treasuries and our laws.”

So why the big push to change the funding mechanism for roads? Griffith said the gas tax just isn’t a stable funding source going into the future. Cars are getting more fuel efficient which means they are filling up less often. That translates into less gas tax money going forward. She said something needs to replace that dwindling revenue.

“It’s not if, it’s when,” she said, referring to when the gas tax no longer fills the bill. “We’re going to have to make a switch to something, and if it’s not this we just say, then what?” she asked. “The revenue has to come from somewhere for our infrastructure, and we’re talking billion and billions of dollars. It’s not a small amount of money.”

Washington has about 2,800 volunteers signed up for the year-long pilot project, but it is still looking for . Griffith said they would like to get about 8,000 signed up so the state can make sure it gets a good cross-section of drivers. The pilot needs people from all over the state, with vehicles of all kinds.