Three bridges are set to close in Somerville in the coming months as the next phase of the Green Line Extension swings into gear.

City and MBTA officials are urging motorists to follow detours for motorists, bicyclists, and pedestrians that are being put in place because of the 8- to 12-month bridge closures.

The shutdowns will begin with the Broadway Bridge between Ball Square and Magoun Square, which will close to traffic for approximately a year starting Friday, March 22. The second will be the closure in two separate phases of the Washington Street Bridge — an underpass — near McGrath Highway in East Somerville, from April to the fall, and from spring 2020 to fall 2020.


The third closure is the shutdown of the Medford Street Bridge in Gilman Square behind the high school, from July through spring 2020.

Details of the bridge closures at each location, including detours, can be found at greenlineextension.org.

“This is a major step forward to bring Green Line service to the communities of Cambridge, Somerville, and Medford,” MBTA general manager Steve Poftak said in a statement. “I want to thank the residents and businesses along the project corridor for their patience while construction takes place.”

The $2.3 billion Green Line Extension involves extending the rapid transit line from a relocated Lechmere Station in East Cambridge to Union Square in Somerville and College Avenue in Medford. Seven new stations are planned, as well as a vehicle storage facility. Additionally, the Somerville Community Path will be extended to North Point Park, near the Charles River in Cambridge.

“There’s no question that the Green Line Extension will benefit our residents, our businesses, and our climate for decades to come, just as there’s no question that its construction will try everyone’s patience for the next few years,” Mayor Joseph A. Curtatone said in an e-mailed statement.


“There’s no sugarcoating it: GLX construction and the bridge closures will be tough, but the reward at the end will be the seven new T stations that our community has fought for for decades as well as a Community Path that extends to the Charles River,” Curtatone added.

Officials said the closures of the Broadway and the Medford Street bridges are necessary to accommodate new Green Line light rail tracks. Currently, both bridges are only wide enough to allow for the existing commuter rail service that operates beneath them.

The Washington Street Bridge requires a complete reconstruction of its abutments and structural span to accommodate the addition of the new light rail transit service and the extended Community Path, officials said.

To help ease impacts on pedestrians, the city will provide two separate shuttle services, one around the Broadway Bridge and one around the Washington Street Bridge.

Officials and public safety officers will also be onsite as each detour is implemented to monitor traffic flows, collect data, and make any needed adjustments.

The completed first phase of GLX included reconstruction of the Harvard Street Railroad Bridge in Medford, and the Medford Street Railroad Bridge in Somerville, as well as demolition of MBTA-owned facilities at 21 Water St. in Cambridge.

John Laidler can be reached at laidler@globe.com.