Northern Virginia Del. Jim LeMunyon filed legislation that would require any future fare increases on the Dulles Toll Road to be approved by the Fairfax and Loudoun county supervisors, but experts say the bill could threaten the multibillion-dollar extension of Metro service to Washington Dulles International Airport. LeMunyon, R-Chantilly, said he is looking to the General Assembly for help because he's concerned that plans to increase the tolls on the road tenfold by 2028 could divert more drivers from the toll road and onto already congested highways like Interstate 66 and Route 7. Since 2009, traffic on the toll road dropped 4.4 percent as tolls rose.

The toll at the main toll plaza jumped 25 cents to $1.25 on Saturday, and officials estimate the new rate will raise about $97.1 million in 2011, an 11 percent increase over 2010. Ramp fares remain 75 cents.

The Metropolitan Washington Airports Authority took over the toll road from the Virginia Department of Transportation in 2008 in hopes of recouping the $3.7 billion it's spending to extend Metro train service to Dulles. And limiting how much the tolls can be raised, as LeMunyon proposes, could undercut the authority's ability to recover that money, officials said.

LeMunyon argues, however, that a toll road too expensive to use will only add to the region's traffic woes.

"A zero-sum plan that limits use of the toll road to pay for rail seems inconsistent with the objective of reducing congestion and improving regional mobility," LeMunyon said. "Requiring approval of toll increases by local elected boards will help ensure that the toll road contributes to solving our traffic problems rather than making traffic worse."

Fairfax County Chairwoman Sharon Bulova said she supports the bill's intent, but shares the airports authority's concerns that it could undermine the extension of Metro service.

"We need to keep the tolls reasonable because we don't want to discourage ridership on the Dulles Toll Road. That's counterproductive to this region's transportation goals," Bulova said. "But the last thing we'd want to do is adopt a bill for what seems like good reasons and find out it jeopardizes a project that is one of our highest priorities in both Loudoun County and Fairfax County."

The legislation would likely undercut funding for the second phase of Metro construction and temporarily shut down the project, said Bob Chase, president of the Northern Virginia Transportation Alliance.

Metro trains are scheduled to begin running to Dulles in 2016.