BOSTON -- Paying a toll once meant stopping and rolling down the window. Then came E-ZPass, the system that skims from credit card or bank accounts when a car passes a toll booth, and cameras that detect license plate numbers and send bills to drivers.

Now the state is weighing a system to use GPS to track how many miles cars travel and charge their drivers based on use of roads and highways. The controversial plan to tax motorists for the miles they drive is designed to drum up money to fix crumbling roads and bridges.

Transportation planners want the money to supplement -- or even replace -- the state's 24 cent-per-gallon gas tax. They're backing a bill that allows the state to test the system with 1,000 volunteer motorists.

The measure doesn't specify how much motorists will be charged or how data will be collected. Details are left to the state Department of Transportation as part of the pilot program.

"We face a tremendous funding gap for transportation needs," said state Sen. Jason Lewis, D-Winchester, the bill's sponsor. "As we add more fuel-efficient vehicles to the roads, the gas tax is going to be declining, and that will exacerbate our revenue shortfalls. We need to come up with a more reliable funding source."

The idea is supported by Transportation for Massachusetts, a nonprofit group that is prodding the state to find new sources of money to fix potholed roads and structurally deficient bridges.

But the concept raises practical and political questions, from privacy issues related to how the state tracks mileage to fairness concerns over whether out-of-state drivers are taxed.

Massachusetts has an estimated backlog of $10 billion worth of basic maintenance work on roads and bridges - a number compounded by the steady decline in federal highway dollars.

The state collected more than $658 million in its last budget year from the gas tax, according to the Department of Revenue. The majority of that money goes into a fund for transportation work, which is also used to pay back money borrowed for major projects.

In addition to the gas tax, Massachusetts charges motorists a 2.5 cents per gallon tax to support the cleanup of underground fuel tanks. They pay a total of 44.95 cents per gallon, which includes a 18.4 cents-per-gallon federal tax that hasn't increased since 1994.

Mary Maguire, a spokeswoman for the Northeast chapter of the American Automobile Association, said the state should explore "all options" to find more money for roads and bridges.

"Revenues from gas taxes are down, we have more electric and fuel-efficient cars on the road these days, so we need to look at alternatives, in addition to considering increases in the state gas tax," she said.

Other states are already testing taxes based on how many miles vehicles are driven.

Since July, Oregon has been testing a program in which volunteers pay 1.5 cents per mile. Each month the state refunds those motorists what they paid under its 30 cents-per-gallon gas tax.

Oregon has signed a contract with a San Jose-based company, Azuga, that makes GPS devices that plug in below a vehicle's dashboard. The devices use wireless technology to report monthly mileage counts to a state account manager, who figures the charge for each motorist.

Lawmakers there authorized a project for up to 5,000 motorists, and about 900 have signed up to date, said Michelle Godfrey, a spokeswoman for the Oregon Department of Transportation.

Oregon officials initially faced a backlash from privacy groups concerned about tracking motorists when the proposed was proposed.

The state changed its plan to allow motorists to report mileage using other methods, such as handwritten diaries and smartphone apps, Godfrey said. In addition, data collected from GPS devices must be deleted within 30 days and cannot be used for other purposes.

"One of the things we need to prove, besides ensuring that the technology actually works, is whether or not people will accept the concept," Godfrey said. "We want people to buy into it, otherwise it just won't work."

Under a gas tax system, owners of fuel-efficient hybrid cars and gas guzzling pickup trucks may drive the same roads but won't pay equally for repair and maintenance.

A mileage tax is fairer, she said.

"You know exactly how much you drive and how much you're paying for the use of the road," she said.

Godfrey said Oregon officials will decide in the next year if they will expand the program statewide.

Connecticut, California, Colorado and Washington are studying similar test programs.

In Massachusetts, lawmakers raised the gas tax from 21 cents in 2013 as part of a $500 million transportation funding bill. They approved a separate measure that pegged future increases to inflation, but voters overwhelmingly repealed that law last November.

The repeal has state and local leaders worried about how to fill the funding gap for transportation projects.

Andrew Bagley, research director of the Massachusetts Taxpayers Foundation, argues that the state's gas tax remains a viable source of funds for transportation, but the levy needs to be increased to keep up with the state's needs.

"But that's going to be a real challenge for the state's leaders," he said. "Because as we saw from the ballot question last year, voters are resistant to raising the gas tax."

Christian Wade covers the Massachusetts Statehouse for CNHI's newspapers and websites. Reach him at cwade@cnhi.com

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