A year-long bridge closure is happening for Green Line Extension work. Here’s what to know.

Officials believe the closure will bring "significant" traffic impacts.

The Green Line extension will go from Lechmere into Medford.
–Barry Chin / The Boston Globe file

When the Green Line Extension, or GLX, project is complete, riders will be able to take advantage of two new subway branches — the Medford and Union Square branches — without having to use a bus and then transfer at Lechmere or to the Orange or Red lines.

But completion is not for about another two years, and the one-year closure of the Broadway Bridge in Somerville is set to begin Friday, March 22.

This closure is the first of three planned — the Washington Street underpass is set to close this April for eight months with a second closure planned for five months next year, from the spring through the fall. 

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The last of the bridge closures is Medford Street, which runs behind Somerville High School. This closure is planned for July 2019 into Spring 2020, according to the city.

State Department of Transportation officials anticipate closing the Broadway Bridge will have “significant” impacts, according to the project site.

“Traffic congestion for motorists and pedestrians in the area is expected,” the department said. “Extensive night and weekend work will be required to take advantage of narrow time windows when commuter rail lines are temporarily shut down or not operating.”

The bridge needs to be closed because the current one needs to be demolished — a new one is needed to allow the new train lines to run underneath. The current bridge needs to be taken out in one go, it can’t be done in phases, MassDOT said.

To help motorists as well as pedestrians and bicyclists get around the project site, the department has devised a list of detours

When complete, the GLX will bring seven new T stops plus a relocated Lechmere station, according to the MBTA.

Daily ridership is projected at 45,000 by 2030, MassDOT said, while also reducing traffic congestion on roads — vehicles are projected to drive 25,728 less miles regionally due to the new train availability.

A map of the Green Line Extension project. —MBTA