Maryland and Virginia will partner to rebuild and widen the American Legion Bridge in a ­billion-dollar project to relieve congestion at the Washington region’s worst traffic bottleneck, the states’ governors announced Tuesday.

In an unusual example of interstate cooperation, Virginia has agreed to help pay for the project even though most of the bridge — like the Potomac River flowing beneath it — belongs to Maryland.

The plan marks a breakthrough in a years-long impasse over widening the bridge on the northwestern stretch of the Capital Beltway. In the past, Maryland has said it didn’t have enough money for the undertaking, and Virginia said the bridge was its neighbor’s responsibility.

The new bridge will have four express toll lanes, in addition to eight free lanes as on the current span. The tolls will fluctuate based on congestion, rising to keep traffic flowing freely.

Construction is expected to begin in 2022 and to be completed and open to travelers in five or six years, officials said. They said it was too early to say how traffic will be affected while construction is underway, but significant disruptions are likely.

Both states said the undertaking will be built through public-private partnerships, in which the government teams up with private companies to finance, construct and operate big infrastructure projects. Typically, the private consortium pays some of the upfront costs and shares the financial risk with the government in exchange for future toll revenue. In this case, both states said, toll payers, rather than taxpayers, will foot the entire bill.

Maryland Gov. Larry Hogan (R) and Virginia Gov. Ralph Northam (D) made the announcement in a surprise joint appearance at a regional transportation forum in the District.

“A new bridge means commuters will get to work and back home faster,” Northam said. “This is about helping people see their families more, grow their businesses, and further unlock the region’s vast economic potential.”

Calling it the “Capital Beltway Accord” and a “once-in-a-generation achievement,” Hogan said that “a bipartisan common-sense interstate agreement such as this has eluded elected leaders throughout the region for many decades.

“Together with our partners in Virginia, we are building a foundation for even greater economic growth, greater opportunity for our citizens and advancing real, lasting, transformative improvements for the entire Washington metropolitan region.”

The American Legion Bridge connects Montgomery and Fairfax counties, the region’s two largest jurisdictions, and is the site of frequent miles-long backups. Delays on the span increased 40 percent between 2010 and 2017 as the region’s population grew. In March, an overturned tanker on the bridge blocked the northbound lanes for 12 hours, triggering rush-hour gridlock throughout the region.

In a related development, Maryland said it will speed up the process of picking a ­private-sector operator to build new express toll lanes on the Capital Beltway stretching northeast from the bridge to the Interstate 95 interchange in Prince George’s County.

Montgomery County officials and traffic experts have voiced concerns that an ­already-approved plan to add lanes to Interstate 270 before widening the Beltway and the bridge would make traffic worse. They warned it would dump more vehicles from I-270 into the same smaller funnel on the Beltway and the bridge.

Because of those concerns, the private operators will be picked simultaneously for the I-270 and Beltway projects, Maryland Transportation Secretary Pete K. Rahn said.

“I-270 does not work without the American Legion Bridge,” Rahn said. “The American Legion Bridge does not work without [Interstate] 495 over to 95. We have to be approaching this as a system.”

But Rahn also said the private-sector operators will determine in what sequence the actual construction takes place. That appeared to conflict with an email sent Tuesday to Montgomery County Council members. In that message, the governor’s office said the state would continue its plan to first add toll lanes to the southern stretch of I-270, while the Beltway construction would remain part of the second phase.

Rahn told reporters, “The successful [private] proposers will be determining the sequencing of construction within those corridors.”

The project will be required to undergo a federal environmental review and is likely to face opposition — especially in Maryland — from local officials and grass-roots groups who would prefer money be spent on public transit rather than roads.

“The only winners from this plan are the P3 financiers and the few wealthy drivers who can afford to pay $40 tolls every day,” said Ben Ross, chairman of the Maryland Transit Opportunities Coalition. “Everyone else will sit in traffic jams — otherwise drivers won’t be willing to pay the high tolls the P3 financiers need to make a profit.”

Josh Tulkin, director of the Maryland Sierra Club, said the Hogan administration is pursuing a “single-minded strategy” by focusing on “massive” highway expansion rather than investing in transit, telecommuting and other more environmentally friendly traffic-relief measures.

Widening the Beltway in certain spots also could face resistance because it would require the demolition of homes. Rahn said the state’s entire express lanes project — including the bridge, I-270 and the Beltway — risks affecting up to 34 homes.

Northern Virginia residents were encouraged by the news, said Fairfax County Supervisor John W. Foust (D-Dranesville), who represents neighborhoods near the bridge that have long sought expansion to address traffic gridlock. Residents have written to leaders in both states urging action on what is a top priority for the area.

“It is something we have been advocating for many years, and it is something that is very much needed,” Foust said. “Congestion is so bad to get across the bridge that Waze and other ­traffic-routing programs send commuters through neighborhood streets to avoid the Beltway. So it is not just a nightmare on the Beltway, but is a nightmare through the community.”

Officials said the project will cut commuting time in half for many travelers, reduce congestion in the regular travel lanes by 25 percent and boost lane capacity by 40 percent. The bridge carries 235,000 vehicles daily.

New bicycle and pedestrian access will connect trails on both sides of the Potomac.

The Potomac River belongs to Maryland under agreements dating to the Colonial era, when a 1632 charter granted by King Charles I awarded the river to the state.

As a result, Maryland owns 79 percent of the bridge. Virginia owns the remaining 21 percent because part of the bridge structure is on Virginia’s side of the river.

Under the new agreement, the two states’ public-private partnerships will divide the approximately $1 billion cost of the new bridge using a complicated formula. Maryland will bear more than half of the overall expense, officials said, but Virginia will cover significantly more than a 21 percent share.

Under the formula, Maryland will pay 79 percent and Virginia 21 percent of the cost to replace the eight existing free lanes.

To add four new express toll lanes, however, each state will pay for two. Virginia will pay for the ones running north from the George Washington Parkway to River Road, and Maryland will pay for the two running south.

In any case, toll payers and not taxpayers will pick up the entire tab, officials said.

“As a taxpayer, there will be no cost,” Rahn said. “It will be a cost to someone as a driver if they choose to use the express toll lanes.”

Virginia Transportation Secretary Shannon R. Valentine added: “There will be no public subsidy for this infrastructure project.”

Northam said a 2015 study of all Potomac River crossings found that the American Legion Bridge was the busiest and most congested and is expected to see the most growth in traffic over the next 20 years.

“You don’t need me to tell you what the problem is,” Northam said. “Right now in the afternoon, it can take 15 minutes or more to drive the last three miles in Virginia from where the express lanes end to the American Legion Bridge. That is bad for commuters, it is bad for our regional economy and it is simply not sustainable. So we are reaching across the river and across the aisle to do something about it.”

Hogan said the accord is the result of months-long negotiations between the two states held since the summer. He said Maryland will take the lead and be responsible for the construction of the new bridge and that Virginia will contribute nearly half the cost of the overall project.

“The result will be less traffic, more peace of mind, cleaner air and a better quality of life for hundreds of thousands of area residents and commuters for decades to come,” Hogan said.

For the Beltway, the project represents a kind of closing chapter to a project of two decades ago: the replacement of the Woodrow Wilson Bridge. That span, which is the counterpart to the American Legion Bridge on the southern side of the Beltway, was for many years the region’s most notorious choke point.

After years of effort, however, the old span was replaced by a wider bridge that opened in two phases in 2006 and 2008. The new plan would do the same for its upriver sibling.