Finding money for state road projects has for decades followed a relatively simple formula. Washington, like other states, relies almost entirely on fuel taxes.
Finding an eventual replacement for that model, however, is anything but simple.
State leaders are now exploring perhaps the most likely possible successor: A "road usage charge." Some know it better as a mileage tax — essentially, making people pay based on how much they drive.
The Washington State Transportation Commission in January released its latest report as part of a "rolling assessment" of the concept that began in 2012, said Reema Griffith, the commission's executive director. A supplemental budget recently introduced by House leaders includes $450,000 to continue that study for another year.
So far, the assessment has raised as many questions as answers. And planners have a long way to go before the concept becomes reality in Washington.
"This would be a big change," Griffith said. "It's not going to happen overnight."
Leaders hope to rethink road funding as fuel tax revenues falter. Collections have flattened in recent years, and are projected to slide downward in the future. Vehicles with ever-improving fuel efficiency, along with changes in people's driving habits, figure to solidify that trend.
"The writing is on the wall in terms of what we're going to be facing, and we understand that," Griffith said. "But it does create a challenge for the long-term sustainability of how we're going to fund the transportation system."
The most recent report laid out three possible versions of what a road usage charge could look like in Washington. One model would allow people to buy flat permits to drive an unlimited number of miles in a given time period. Another would use an "odometer charge" based solely on miles driven. A third model would install devices on people's cars to track distance traveled and transmit data back to the state to determine the bill.
Planners haven't tackled the specifics just yet. Among the questions that remain unanswered: How does the state handle drivers who regularly travel outside Washington? What about out-of-state residents who use Washington roads? Would the tax apply the same to all vehicle types? And how much would people pay?
The idea has also raised questions about privacy, particularly under the model that uses mileage-tracking devices. Many see the notion of transmitting one's travel habits as too invasive. Recent data breaches underscore the need for security and privacy, said Rep. Ed Orcutt, R-Kalama.
"There are a lot of people concerned about it," said Orcutt, a member of the steering committee studying the issue. "If something like that happened, there would have to be a lot of protections."
The state will also have to consider the needs of urban and rural residents, who have very different driving habits, Orcutt said. The cost of building and maintaining roads in those settings also varies, he said.
It's too early to tell whether average motorists would pay more, less, or the same as they do in fuel taxes now. Developing a new system gives the state an opportunity to craft something more equitable to all residents, Griffith said.
That means adapting the concept to circumstances the current gas tax model ignores. Leaders could weigh things like vehicle type, emissions or even income level in deciding how to charge people, she said.
"This would open the door for a lot more consideration of a whole lot of factors," Griffith said. "It opens up a lot of flexibility."
There's widespread recognition that fuel tax revenues alone won't be able to keep up with a growing backlog of transportation needs, said Rep. Jim Moeller, D-Vancouver. Increases to the state fuel tax in 2003 and 2005 paid for a slew of projects in the last decade, including several in Clark County. But efforts to fund another large wave of construction through fuel taxes has remained stalled in the Legislature since last year.
If Washington adopts a road usage charge — a big if — there would likely be a transition period during which both systems would exist in some form at the same time. People would pay one or the other, not both, Griffith said. The goal is to create something that eventually replaces the fuel tax. But officials haven't ruled out keeping it, she said.
"We certainly haven't shut the door," Griffith said. "There may be value in having both."
Implementation of a road usage charge in Washington is likely still years away. The state could launch testing as early as 2016, Griffith said. A pilot program already launched in Oregon could give Washington an example.
Moeller said he believes the Vancouver area could be a good setting for such a testing phase. But technology will need to be refined before the system can be implemented broadly, he said.
Before the idea of a mileage tax is widely accepted, Moeller said there needs to be a "cultural shift" away from fuel taxes as the primary driver for road funds.
Griffith suggested the new model may not be as big a shift as people think.
"When you're paying a per-gallon charge, you are paying for your mileage," she said. "It's just a different metric."