MONTPELIER, Vt. (AP) -- Carpooling, higher fuel economy, hybrids and electric cars may be good for the environment, but they're bad for government transportation funding, which relies on gasoline and diesel taxes to help pay for the building and upkeep of roads and bridges.

Now some states, including Vermont, are mapping out a possible alternative: taxing drivers not based on how much fuel is burned but how far each vehicle travels.

"I think there's general agreement that we eventually have to get to that if we're going to replace lost gasoline tax revenue," said Costa Pappis, a senior planner with the Vermont Agency of Transportation.

It's easy to see why officials are worried: Vermont expected to raise about $232 million for its transportation fund in the current fiscal year, with about a quarter of it coming from the gasoline tax of about 20 cents a gallon.

But those revenue collections have been shy of their target so far this year, just as they have been for the six years before. Vermont gas stations sold slightly more than 361 million gallons in 2006, their historic peak. By 2011, that figure was down to just shy of 330 million.

It's the same story in many other states.

Oregon is the farthest along in trying to address the problem with a "vehicle miles traveled" tax. Legislation there would impose such a tax on cars of 2015 model year or later that get 55 mpg or better.

If that sounds like a high number, it may not for long. New fuel economy standards agreed to by the government and automakers last year say that by 2025, cars will average 54.5 mpg.

Vermont Transportation Secretary Brian Searles said calculating how much of a VMT tax is owed would be done through the global positioning system devices that are expected to be standard equipment in cars later this decade.

"It's a GPS device that is capable of tracking location, time," he said, adding that he was aware that might raise privacy concerns.

It did with Allen Gilbert, executive director of the Vermont chapter of the American Civil Liberties Union.

"I'm sure there's going to be a big public outcry when people hear about this," he said.

But Adrian Moore, who has studied VMT tax implementation as vice president with the California-based Reason Foundation, said fears of Big Brother are overblown. He said a range of technologies are being tested in pilot studies, including one device that records miles traveled since a fill-up and then communicates with the gas pump on the next fill-up how much tax is owed.

If a GPS device can tell a driver when to take a left, it can tell what road the car is traveling on, and eventually could determine how much of the tax is owed to a city, a county or a state, depending on whether the motorist was using a city street, a county road or a state highway, Moore said.

James Whitty, a manager in the Oregon Department of Transportation's Office of Innovations and Alternative Funding, said the legislation there would let motorists choose between several technologies, as well as a range of public and private sector vendors that would calculate and collect the tax and relay it to the government.