President Obama's transportation department slapped down a suggestion by its own secretary Friday that the government tax motorists based on how many miles they drive rather than how much gasoline they burn. 

Secretary Ray LaHood floated the idea in an interview with The Associated Press. 

Gasoline taxes for nearly half a century have paid for the federal share of highway and bridge construction, but LaHood said they can no longer be counted on to raise enough money to keep the nation's transportation system moving. 

"We should look at the vehicular miles program where people are actually clocked on the number of miles that they traveled," the former Illinois Republican lawmaker said. 

Asked about the claim, transportation department spokeswoman Lori Irving immediately shot it down. 

"The policy of taxing motorists based on how many miles they have traveled is not and will not be Obama administration policy," she said. 

The mileage tax idea has already angered drivers in some states where it has been proposed. 

However, most transportation experts see a vehicle miles traveled tax as a long-term solution, though Congress is being urged to move in that direction now by funding pilot projects. 

The idea also is gaining ground in several states. Governors in Idaho and Rhode Island are talking about such programs, and a North Carolina panel suggested in December the state start charging motorists a quarter-cent for every mile as a substitute for the gas tax. 

A tentative plan in Massachusetts to use GPS chips in vehicles to charge motorists by the mile has drawn complaints from drivers who say it's an Orwellian intrusion by government into the lives of citizens. Other motorists say it eliminates an incentive to drive more fuel-efficient cars since gas guzzlers will be taxed at the same rate as fuel sippers. 

Besides a VMT tax, more tolls for highways and bridges and more government partnerships with business to finance transportation projects are other funding options, "What I see this administration doing is this -- thinking outside the box on how we fund our infrastructure in America," LaHood said. 

LaHood said he firmly opposes raising the federal gasoline tax in the current recession. 

The program that funds the federal share of highway projects is part of a surface transportation law that expires Sept. 30. Last fall, Congress made an emergency infusion of $8 billion to make up for a shortfall between gas tax revenues and the amount of money promised to states for their projects. The gap between money raised by the gas tax and the cost of maintaining the nation's highway system and expanding it to accommodate population growth is forecast to continue to widen. 

Among the reasons for the gap is a switch to more fuel-efficient cars and a decrease in driving that many transportation experts believe is related to the economic downturn. Electric cars and alternative-fuel vehicles that don't use gasoline are expected to start penetrating the market in greater numbers. 

The mileage system would require all cars and trucks be equipped with global satellite positioning technology, a transponder, a clock and other equipment to record how many miles a vehicle was driven, whether it was driven on highways or secondary roads, and even whether it was driven during peak traffic periods or off-peak hours. 

The device would tally how much tax motorists owed depending upon their road use. Motorists would pay the amount owed when it was downloaded, probably at gas stations at first, but an alternative eventually would be needed. 

Rob Atkinson, president of the National Surface Transportation Infrastructure Financing Commission, the agency that is developing future transportation funding options, said moving to a national VMT would take about a decade. 

The Associated Press contributed to this report.