Rates along the Dulles Toll Road could double starting next year if Virginia doesn’t deliver on a promise to contribute $150 million for the second phase of Metrorail’s new Silver Line.

In that case, tolls for a one-way trip that now costs $2.25 could increase to $4.50, according to a consultant’s report prepared for the Metropolitan Washington Airports Authority and presented during a board meeting Wednesday. If the state money comes through, tolls for the same trip could still rise to $2.75.

MWAA operates Reagan National and Dulles International airports and oversees construction of the new 23-mile subway extension to Dulles and Loudoun County.

James T. Taylor II, a consultant with Mercator Advisors, said the increased toll rates would be in line with those on area roads such as the Dulles Greenway and the Intercounty Connector.

“In terms of the time savings and the wealth in the area, these are reasonable rates,” he said. “The rates are low compared to new toll roads in the area and nationally, but for those who have been here for 20 years, they may be a shock.”

The board is expected to decide on future toll rates by September, according to the consultants and MWAA officials. The board would have to hold public hearings before any toll changes are made.

Virginia Gov. Robert F. McDonnell (R) promised $150 million to help pay for the second phase of the nearly $6 billion Dulles rail project as part of a compromise negotiated by the federal government to reduce overall costs.

The first phase of the rail project is under construction from Falls Church to Reston and is expected to open in 2014. Dulles Toll Road revenue is designated to pay for about half the cost of the Silver Line, and many board members have worried about the burden on drivers who use the roadway.

Many Virginia state lawmakers have protested the $150 million payment because of MWAA’s “project labor agreement,” which they say favors union-friendly companies. The state House and Senate recently passed bills that would withhold money from the Silver Line project if there is a mandatory agreement governing working conditions on the project.

The MWAA board approved a mandatory project labor agreement last year, but the board voted 11 to 2 at its meeting Wednesday to retract that requirement. Instead, MWAA will give bidding firms a 10 percent bonus in scoring their proposals if they implement an agreement.

Ben Brubeck, director of labor at the Associated Builders and Contractors group, said the MWAA board’s reversal was a sign that members are “playing games.”

“They’re trying to give the impression that there’s no favoritism towards contractors that enter into a project labor agreement,” he said. “But this is a de facto project labor agreement.” He said the board was trying to “create a facade” that there is no mandatory project labor agreement so “they can get the state’s money.”

MWAA officials said they expect eight firms to bid on building the second phase. They plan to reduce that list to five and then pick a contractor in January 2013. Construction is expected to take five years.

Construction of the first phase of the Silver Line is about 75 percent completed, MWAA officials said. Project supervisors said they expect to finish on schedule if they can make up weather-related delays. But they have warned that the project could be as much as $150 million over budget. There is $44 million remaining in what was originally a $312 million contingency fund for the first phase.

Revenue consultant CDM Smith and financial advisers Mercator Advisors and Frasca and Associates reported that projected revenue from the Dulles Toll Road is expected to decline by 3.4 percent, to $103.5 million, from estimates done in 2009. That trend is expected to continue to 2028 because of lower employment and population projections, consultants said.