Virginia lawmakers have included language in the state budget for a $2 million study of how Metro’s Blue Line could be extended through Fairfax into Prince William County to expand mass transit in the heavily congested and rapidly growing Interstate 95 corridor.

If the directive remains in the General Assembly’s approved state budget, the study would explore the cost of extending the line by about 15 miles to Fort Belvoir, Lorton, Potomac Mills and Marine Corps Base Quantico. It also would examine the potential ridership, location of stations and effects on those areas’ development.

Many Northern Virginia leaders have long sought such a study. However, advocates say new Democratic control of the General Assembly and the Prince William Board of County Supervisors shifted support toward transit investments in an area that has long focused on expanding roads.

Sen. Scott A. Surovell (D-Fairfax), who pushed for the study, said 90 percent of his constituents want more mass transit options.

“Every time we bring it up, almost every hand in the room goes up,” Surovell said. “I-95 in Prince William and Stafford [counties] has the worst traffic in the U.S. People in Prince William are desperate for real transit.”

The Senate Finance and House Appropriations committees did not appropriate $2 million specifically to a study. Instead, both added language to the budget directing the Virginia Department of Rail and Public Transportation to spend that amount to analyze a Blue Line extension, as well as “other multimodal options,” such as Bus Rapid Transit, for the I-95 and Route 1 corridors.

Even if the study recommended extending the Blue Line, Surovell and other transit advocates say, they would not expect any construction for at least 10 to 15 years. However, supporters were encouraged because money for the state study would mark the furthest the idea has come.

“The study included in the Senate Budget Amendment has been a long time coming,” Sen. Jeremy S. McPike (D-Prince William) said in a statement. “We finally have provisions to create a framework for transportation projects down the line. Typically, Metro expansions like this will take decades but we shouldn’t be disheartened. This study is the first step on our pathway to a modern mass transit future.”

However, there also is the question of whether the Metro system, which is nearing completion of its Silver Line extension, could handle more capacity. Phase 2 of the Silver Line will extend Metro to Dulles International Airport and into Loudoun County.

Metro officials have said the system cannot be expanded until they first increase capacity at crowded stations in the core and in tunnels that become chokepoints — an effort expected to take years and cost billions.

For example, Metro has said the Blue Line cannot accommodate more trains without a second tunnel at Rosslyn, a well-known bottleneck in the system where Blue, Orange and Silver line trains back up to travel beneath the Potomac River.

Any capacity expansion, Metro officials have said, would come after the agency addresses a backlog of unfunded work to make the system safer and more reliable.

“Metro remains focused on addressing the safety and reliability needs of a now-44-year-old system,” Metro spokesman Dan Stessel said in an email. “It’s truly gratifying to hear support for expanding public transit — and hopefully that can happen someday — but Metro and the region will first need to address physical capacity constraints, particularly in the downtown core, before any jurisdictionally funded extensions beyond a 129-mile system.”

Jason Stanford, executive director of the Northern Virginia Transportation Alliance, said the transit study would jibe with the state’s recent efforts to help expand Virginia Railway Express commuter rail and Amtrak service. But he questioned the timing of studying a Metro extension before transit agency officials have determined how and when they will expand the system’s core, and how the region will pay for it.

“You have to focus on the core capacity issues first,” Stanford said. “Otherwise, this will probably be a study that will sit on a shelf waiting until these other things happen.”

The push for more transit comes as some Prince William leaders have advocated for the Washington Redskins to build a new stadium in the I-95 corridor, which now has relatively little mass transit. Local officials say they also want to attract major employers, many of them seeking sites near transit, so fewer residents need to leave the county for work. In addition to shrinking commutes, they say, transit stations would spark office construction and other commercial development that would add to the tax base.

Surovell said the state should explore a Blue Line extension even as Metro addresses its capacity constraints so a project could be ready to compete for federal transit construction money. It also would provide the counties time to allow for denser development in the corridor, which would increase future transit ridership.

“One of these lines typically takes 10 to 20 years” to plan and build, Surovell said. “If we wait until Metro solves its core capacity problems, we won’t get a new line until 20 years after that.”

Prince William Board of Supervisors Chair Ann B. Wheeler (D-At Large) said she, too, believes the state must begin studying what she called a potential “game-changer” for the county.

“We want to be a destination and bring people in,” Wheeler said. “If it takes 15 years [to extend the Blue Line] and we start today, then we could have it in 15 years. But if we don’t start today, we’ll never get it.”

A study would begin after July 1 and is expected to take about 18 months.

Supervisor Margaret A. Franklin (D-Woodbridge) said Woodbridge’s roads weren’t designed for the population that has swelled over the past few decades. The area also needs the economic development that Metro stations bring, she said.

“Extending the Metro into Woodbridge will create economic opportunities that Prince William County has never seen,” Franklin said.