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The carpool lane on Interstate 93 southbound from Medford to Boston’s Zakim Bridge opened to all vehicles on Monday, a move meant to ease congestion while construction continues on the Tobin Bridge.

The lane will be open to all vehicles at all times as a temporary measure to reduce traffic, said Patrick Marvin, a spokesman for the state transportation department. The plan is not meant to be permanent, he said.

Chris Dempsey, director of Transportation for Massachusetts, an advocacy coalition, said his group opposes the move because it “penalizes carpoolers and bus riders who have long used this lane for faster trips into Boston.”

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“MassDOT should monitor impacts and reconsider the decision after a short trial period,” he said.

Route 1, which connects Boston to the North Shore, passes through Chelsea on the Chelsea Viaduct and the Tobin Bridge, which are both in need of “significant repair and rehabilitation,” according to the state. Construction and lane closures began last month.

On Tuesday, the project will cause lane closures on the Tobin Bridge that could snarl traffic, officials said. Officials warned bus riders and drivers to expect “increased travel times through the work zone on Route 1 north and southbound, especially during peak hours.”

On Route 1 north, two of three travel lanes will be open through the workzone from 5 a.m. to 10 p.m., while at least one travel lane will be open during overnight hours, from 10 p.m. to 5 a.m. On Monday evening, the construction zone was extended north to Country Road Overpass.

Also on Monday evening, the state closed one lane on Route 1 southbound from the County Road Overpass to the Tobin Bridge on a 24/7 basis. Two of the three travel lanes will be open through the workzone from 5 a.m. to 10 p.m. and at least one travel lane will be open during overnight hours, from 10 p.m. to 5 a.m.


Danny McDonald can be reached at daniel.mcdonald@globe.com. Follow him on Twitter @Danny__McDonald.