Maryland’s Board of Public Works approved major changes Wednesday to Gov. Larry Hogan’s plan to add toll lanes to the Capital Beltway and Interstate 270, allowing the state to begin soliciting proposals for one of its largest highway expansion projects.

The board, which had to approve the plan as a public-private partnership, voted 2 to 1 to move it forward. Hogan (R) and Comptroller Peter Franchot (D) voted for it, saying the additional lanes would provide badly needed traffic relief and boost the economy. State Treasurer Nancy K. Kopp (D), who represents the General Assembly, opposed it, saying the state had not shown how the plan made financial sense or how it would not hurt the environment.

The board approved the proposal for toll lanes in June but had to reconsider it after the Hogan administration proposed changes in the fall.

Hogan said the toll lanes will prove “transformational” and called Wednesday’s approval “the most historic vote we’ve taken in 20 years at the Board of Public Works.”

The additional lanes “will begin to solve what has been the number one problem in the Washington capital region for decades,” Hogan said, adding, “This truly is a monumental and historic achievement, not just for Maryland but for the entire capital region.”

The proposal appeared to have stalled last month after Franchot objected to several of the changes. However, he and Hogan reached an agreement Friday, providing the governor with majority support on the three-member board.

In the most significant concession, the Maryland Department of Transportation agreed to limit the first phase of construction to the Beltway between the Virginia side of the American Legion Bridge and the Interstate 270 spur, and to the lower part of I-270 between the Beltway and Interstate 370.

That left the future of the most controversial Beltway section — between I-270 and Interstate 95, where widening could destroy homes and public parkland — to be decided at a later, unspecified date. The state also is putting off toll lanes for the northern part of I-270 between I-370 and Frederick because the environmental study for that portion is further behind.

State officials have said the contracts will be worth more than $9 billion and part of one of the largest public-private partnerships in the country.

Franchot said that he has heard concerns from environmentalists and supports calls for more mass transit but that the state still needs more capacity on the Beltway.

“We simply have to do it,” said Franchot, who plans to run for governor in 2022. “Right now the congestion is damaging our economy — not just the regional economy but the whole reputation of the state. . . . It’s becoming unacceptable to just stand still and not do anything.”

Under the state’s plan, companies will build up to four lanes on each highway and finance their construction in exchange for keeping most of the toll revenue long-term, at no cost to the state. The existing lanes would be rebuilt and remain free.

Maryland State Highway Administrator Greg Slater said he expects to seek the board’s approval for the first contract in the spring of 2021. MDOT officials said they plan to begin soliciting companies next month.

After the meeting, Slater said the lanes will be built in phases, starting on the Virginia side of the American Legion Bridge and gradually working around the Beltway before extending up I-270. He said such construction typically takes three to five years but will depend on how the winning companies propose to work while keeping traffic moving.

Slater, whom Hogan recently nominated to replace outgoing transportation secretary Pete K. Rahn, said the state agreed to start on the highway segments that local officials supported.

“It just made a lot of sense to bring people together in places where we can move forward and in areas where we can show real congestion relief,” Slater said.

Emmet Tydings, who has served on a citizens advisory committee for the region’s Transportation Planning Board, told the board that Maryland’s new lanes will jibe with long-term plans for a regional network of toll lanes.

“We citizens want this,” said Tydings, who also heads Citizens for Traffic Relief, which backs the toll plan.

At a rally outside the statehouse before the meeting, about 18 protesters chanted “Trains not tolls!” and “Wrong way on climate change!”

Josh Tulkin, director of the Maryland Sierra Club, said expanding highways ignores the fact that transportation sources are the largest contributor to carbon pollution in Maryland.

“If climate change is a priority,” Tulkin said, “we wouldn’t be moving forward without data showing this won’t have an impact.”

Slater told the board the Maryland Transportation Authority will set a range of possible toll prices after soliciting public input. Toll prices will increase as the lanes become more congested, to keep them flowing at a minimum of 45 mph, Slater said.

Kopp, the state treasurer, said Maryland transportation officials still had not provided enough details, such as why a public-private partnership would be the best approach for the state.

“I share folks’ feeling that we should see more data,” Kopp said.

Kopp said she agreed that the Beltway, and particularly the American Legion Bridge, needs traffic relief but questioned how expanding highways would help the state meet its greenhouse-gas emissions goals.

In addition to agreeing to stop the first section of the Beltway expansion at I-270, the Hogan administration also won Franchot’s support by beefing up a commitment to expand public transportation in Montgomery and Prince George’s counties. The state said it will work with both county governments to decide how transit funding should be included in the contracts for the toll lanes. Previously, the state had agreed to allot 10 percent of its share of net toll revenue to transit, but only after contractors had recouped construction costs, which could have taken years.