Dulles Toll Road users could see “heartburn level” rates to help pay for the $6 billion Metrorail extension to Loudoun County, according to the chairman of the Finance Committee of the board controlling the highway.

The Metropolitan Washington Airports Authority (MWAA), which oversees Dulles International and Reagan National airports, is also managing the construction of the 23-mile Silver Line. More than half of the cost for the project will be funded by toll revenue. The rest is from a combination of federal, state and local contributions.

After a Wednesday board meeting, Finance Committee Chairman Bob Brown said toll road drivers can expect to see increa­ses in “2013 and beyond.”

“I don’t envision the next few years will be years without toll-rate increases,” Brown said. “Tolls on this type of facility keep going up.”

Watchdogs have worried that in 20 to 30 years, toll rates could rise to $17 round trip. Brown and other MWAA board members said the way to keep toll rates down is to get more money from other sources to help pay for the new rail line.

“There is no silver bullet,” Brown said. “Maybe a little more from the private sector, a little more from Virginia, a little more [from the federal government] would help. The toll road users and the counties are tapped out.”

Toll road users have already seen increases.

Starting Jan. 1, the rates for the main toll plaza will rise 25 cents. It is the last of three annual increa­ses approved by the authority in 2009.

Under the terms for building the Silver Line, toll road users pay nearly 54 percent of the project’s cost. Virginia pays about 5 percent; federal grants make up 16 percent; the airports authority puts in 4 percent; and the rest comes from Loudoun and Fairfax counties.

The first phase, which stret­ches from Falls Church to Wiehle Avenue in Reston, is under construction. It is expected to open for passengers in late 2013.

During Wednesday’s meeting, authority staff members provided a progress report on construction of Phase 1 , which they said is on time and on budget. So far, $1.58 billion of the $2.8 billion has been spent. There is $54.1 million left in the contingency fund for the project. Bechtel, the lead contractor, Metro and MWAA are still figuring out who will pay for some changes to the project, according to Patrick A. Nowakowski, the executive director of the Dulles rail project. The transit authority will operate and maintain the line.

The final price tag is “dependent on us meeting mitigation,” he said. “If we don’t, there’s a chance it will go over budget.”

Nowakowski has said the worst-case scenario is going as much as $150 million over.

MWAA has hired an outside consultant to examine possible future rates, study transportation patterns and forecast toll road traffic. The study is due early next year.

Many watchdogs and toll road users who attended a public meeting Tuesday night on the rate increases said they worry that other roads, including routes 50 and 7, will become clogged with increased traffic as drivers try to avoid the tolls.

“Why should people who drive on the highways pay for people who take the subway?” asked Marv Rosenberg, a retired economist who lives in Vienna and attended Tuesday night’s event.

Jack Boese, a retired lawyer who lives in Reston, said Tuesday that he and his wife would change their patterns of taking the Dulles Toll Road three to four times a week if rates continue to rise.

“I’m worried tolls are going to get so high my grandkids are going to have to move from the area. It’s a disaster.”

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