The next phase in Virginia’s plan to expand its network of toll roads is well underway as work on the 66 Express Lanes outside the Capital Beltway picks up.

Ramp and bridge girders are appearing, and crews throughout the corridor are removing trees and relocating utilities and sound walls to make way for the new toll lanes.

The 66 Express Lanes inside the Beltway — 10 miles of rush-hour, peak-direction lanes between the Beltway and the District line — opened in December 2017. This project will add 22.5 miles of toll lanes, from University Boulevard in Gainesville, in Prince William County, to the Beltway.

State officials say the project, expected to be completed in 2022, will relieve congestion and provide drivers in the Interstate 66 corridor with a more reliable trip. Motorists will be able to choose between the general lanes, which will remain free, or the new high-occupancy toll (HOT) lanes, which buses, carpoolers and motorcyclists can use free.

Rebuilt interchanges at the Beltway, Nutley Street, and Routes 123 and 28 will improve traffic flow, and new park-and-ride lots will encourage carpooling and transit use, officials say.

“We cannot build our way completely out of congestion, but we can give people a travel choice where they can either carpool, ride a bus, or pay for a ride that is not congested,” said Susan Shaw, director of Megaprojects for the Virginia Department of Transportation. “Or they can use the general-purpose lanes, which will be congested some, but not nearly like they would have been without the project.”

Critics contend, however, that adding more lanes will only incentivize more solo driving. Others fear that the tolls, which will fluctuate based on traffic, will be too high.

The Transform 66 Outside the Beltway project is among the largest and most expensive transportation undertakings in the state — an investment of $3.7 billion, according to official figures. It’s being built through a public-private partnership and Express Mobility Partners, and the consortium of investors financing it will also maintain and operate the toll lanes under a 50-year concession.

The project includes two HOT lanes and three general-purpose lanes in each direction, along with new and expanded transit service. Two new park-and-ride lots under construction, plus others planned, will add up to 4,000 parking ­spaces. The project also leaves median space reserved for future rail service along parts of the route.

Officials say the toll lanes, combined with the other road and transit improvements, are needed to address the congestion in a corridor that experiences eight to 10 hours of gridlock daily, including weekends, and carries an average of 200,000 vehicles a day.

Heavy construction started last year with utility relocation, clearing and grubbing. The project requires work at 12 interchanges, 65 bridges and overpasses, and more than 13 miles of new bike trail. More than 1,000 workers are on the job, which will require more than 3 million tons of asphalt and 57 million pounds of steel.

“We will be in and out in places through the corridor for the life of the project,” said Nancy H. Smith, a spokeswoman for the contracting firm FAM Construction, a joint venture of Ferrovial Agroman and Allan Myers. “We have a short time frame to build a very large project. . . . That’s why you see construction taking place throughout the corridor.”

At the Beltway interchange, a mammoth operation is underway to build nine new bridges and widen three existing ones. The structures will connect the new eastbound and westbound 66 Express Lanes with the express and general-purpose lanes on I-495.

“That is a challenge at 495, where we are building new bridge support and between existing bridges and around traffic,” Smith said. “All of this will be done while keeping traffic moving.”

A new park-and-ride lot will open in Gainesville later this year, adding 960 spaces to the corridor. That will be expanded to a total of 1,920 spaces when the toll lanes open in 2022, and will provide direct access to the express lanes. Another park-and-ride in Prince William, at Balls Ford Road, will add 1,300 spaces when it opens in 2022.

As part of the improvements at the Route 28 interchange, four traffic signals between Westfields Boulevard and Route 29 in Centreville will be removed next year. The plan, officials say, will help reduce traffic backups to I-66 from Route 28. A new overpass will connect Braddock and Walney roads to create new access to I-66.

A redesign of the Nutley Street interchange includes the construction of two roundabouts that officials say will improve traffic flow to and from Vienna. The change should address a trouble spot prone to crashes and congestion, officials said.

Bridges and ramps under construction at the Route 123 interchange will create new traffic patterns in that area later this summer.

Officials are asking motorists to be patient, and are encouraging drivers to sign up for alerts on upcoming lane closures.

In the next year, as the pace of the work increases, those who travel in the corridor will experience even more slowdowns and traffic disruptions during the day and overnight. Metro’s plans for a summerlong shutdown of a portion of the Orange Line next year will likely send transit riders back to their cars, possibly worsening the I-66 commute. But crews will use the time the rail line is shut down to advance work that is now limited by rail operations, construction officials said.

As work progresses on the interstate itself, more than a dozen other smaller projects are under construction or in the design phase along the corridor, funded through a $500 million upfront concession payment to Virginia from Express Mobility Partners.

They include widening projects, trail work, another commuter park-and-ride in Fairfax County, a new bus garage and maintenance facility in Prince William, improvements along Routes 50 and 29, and funding for VRE.

“These projects will help reduce congestion, help increase travel options and improve transit service, offer new and additional transit and enhance transportation connectivity,” said Monica Backmon, executive director of the Northern Virginia Transportation Authority, which helped select the projects funded with the concession fee. “That allows us and the region to move more people.”

The plan is for the smaller projects to be completed by the time the toll lanes open, she said.

The lanes will grow Northern Virginia’s toll road network to nearly 100 miles. In the past decade, the state has built a network of express lanes in Northern Virginia that includes 10 miles on I-66, from the Beltway to the District line, 14 miles on the Beltway from Springfield to McLean, and 31 miles on Interstate 95 from Stafford to Fairfax. The 66 Express Lanes inside the Beltway are run by VDOT, but the 95 and 495 express lanes are operated by ­Transurban, in another public-private partnership.

Eight miles of toll lanes are set to open this fall on Interstate 395, as part of an extension of the 95 Express Lanes. And earlier this year, the state announced an agreement with Transurban to extend the 495 Express Lanes by three miles, to the American Legion Bridge. Last month, Transurban began construction to extend the 95 toll lanes to Fredericksburg, adding another 10 miles.