The long-term problem facing the state and its funding of roads is that the gas tax just doesn’t go as far as it used to, even as the Legislature votes to increase it.

For starters, vehicles are becoming more fuel efficient and going further on each fill up.

In addition to that, some say the gas tax is unfair. A Toyota Prius driver, for example, doesn’t pay as much in gas tax as someone driving a gas-guzzling SUV. The state is looking to change the formula.

“There is no question that as cars consume less gas that means less gas tax revenue for the state for our highways and bridges,” Executive Director of the Washington State Transportation Commission Reema Griffith said.

A 25-member panel has been studying the idea of having people pay for the miles they drive. It could be fair and more reliable.

“This is intended to be a deliberative look at a transition at some point in the future when we see our gas tax revenue starting to dip and we need to start shoring those up,” Griffith said.

The plan is to make pay-by-mile an option for people who would rather do that than pay the gas tax. Griffith said the gas tax wouldn’t go away, but people wouldn’t be taxed twice.

“No one is talking about flipping the switch one day and unplugging the gas tax,” she said. “I think the intent now is the gas tax has been and continues to be a good workhorse for transportation and would stay in place until it withers away.”

The average Washington car gets 20 miles per gallon; it’s expected that number will jump to 35 miles per gallon in 20 years, according to the transportation commission. If that happens, gas tax revenues would drop by about 45 percent. Griffith said the state needs a funding mechanism during the transition.

The commission is envisioning a system where some people pay the gas tax and some people pay per mile. The price pencils out to about 2.5 cents per mile. That’s about the same amount you pay right now with the gas tax.

But collecting and tabulating those taxes is where you can run into privacy issues. Many drivers don’t want a GPS in their car that the state can track. Griffith said a GPS tracking system is only one of the potential options, and it wouldn’t keep data on where you go, other than to make sure the miles you’re being taxed for are in Washington.

Drivers could go GPS-free and pay an estimated amount up-front each year. You can simply use your odometer when you get your tabs to see how many miles you’ve driven. Of course, that dings you for miles driven outside of Washington. There might even be an option down the road that uses your smart phone.

It will be up to the Legislature to decide how it all works, or if it even goes forward.

Oregon is already doing it, with about 5,000 drivers opting in to the system.