Congress might want to consider a mileage fee for trucks and electric vehicles when it looks for ways to strengthen highway funding, says the Government Accountability Office.

GAO said that while privacy concerns and other issues limit the usefulness of such fees for automobiles, Congress should consider a pilot program to test them for trucks and electric cars.

The agency was responding to a 2011 request for an analysis of vehicle mileage fees from the House Transportation Appropriations Subcommittee.

The legislators' concern arises from concern that fuel taxes are not producing enough money to keep up with the demands on the national highway system. Transportation experts have been suggesting for some time that as vehicles become more fuel efficient, a mileage-based system would be a better way to raise the needed revenue.

GAO's analysis found that a mileage system would reflect actual use of the road and can be adjusted to cover true costs.

Citing a study by the Federal Highway Administration, the agency said heavy trucks generally pay less in taxes than the cost of the damage they inflict. But that study, done in 2000, needs to be updated, the agency said.

Overcoming Mileage-Based User Fee Challenges

Mileage systems need to overcome several problems before they can be used, the agency said.

A major concern is privacy. Some of the systems that have been tested by states use global positioning systems to track distance traveled, a technique that has triggered strong opposition from motorists. This opposition means that widespread implementation of mileage systems using GPS for all cars is not likely now, the agency said.

It is not clear that GAO had the latest information from an Oregon mileage pilot when it was finishing this aspect of its report. Oregon offers participants in its pilot the option of a non-GPS system that simply track miles, not location.

According to James Whitty, manager of the Oregon DOT Office of Innovative Partnerships, this approach solves the privacy problem.

"Privacy simply disappears as a problem because people can choose something that does not require location capability," he said.

GAO also notes that mileage systems can be expensive to administer, a point that trucking interests emphasize. American Trucking Associations contends that the solution to the highway funding shortage is to raise fuel taxes, rather than switch to a mileage system.

But the agency cited the experience of Germany and New Zealand as evidence that mileage fees can work for trucks.

Germany, for instance, imposed a mileage fee on its national highway, the Autobahn, as a way to recoup costs from trucks transiting from one part of Europe to another. The fee structure varies according to the fuel efficiency and emissions output of the truck.

This has produced substantial revenues to pay for road wear and reduce emissions, and it poses fewer privacy concerns, GAO said.

The agency said a federal pilot project could be structured to address these and other mileage system issues.

Rep. Earl Blumenauer, D-Ore., introduced a bill in the last session of Congress to require the Treasury Department to study a national mileage system. The measure would require Treasury to evaluate mileage systems, ensure that privacy is protected and make sure that the system is easy to administer, Blumenauer said when he offered the bill.