Fairfax County supervisor John W. Foust said tolls should be installed on the access highway to Dulles International Airport to help pay for the cost of extending Metrorail and to hold down costs on motorists who use the adjacent Dulles Toll Road.

The issue came up Tuesday at a meeting of the Board of Supervisors’ legislative committee as county staff suggested that the board support a possible change in federal law that would allow the Metropolitan Washington Airports Authority to levy “passenger facility fees” to help finance the second phase of the Metrorail extension.

Foust urged county staff to explore whether it would also be possible for the airports authority to install tolls on the access highway, especially since the county, the airports authority and other stakeholders in the project are searching for ways to secure additional financing for the second phase of construction.

“I think it’s a fair solution,” Foust (D-Dranesville) said following the committee meeting. “It would raise a significant amount of money.”

The airports authority is leading efforts to build the 23-mile Metro line extension to Dulles and Loudoun County. Fairfax County is helping pay for it.

The first leg, from Falls Church through Tysons Corner to Reston, is under construction and could open as soon as December 2013. The second phase would extend Metro to the airport and Loudoun County, with funding from the commonwealth, the federal government, the Dulles Toll Road, and Fairfax and Loudoun counties.

Foust said it’s not fair that commuters who use the Dulles Toll Road must endure toll increases — projected to be as much as $13 for a one-way trip in 20 years — to pay for the new rail line while people who fly from the airport or work there can travel on the Dulles Access Road for free.