The state Department of Transportation is backing a proposal long championed by advocates to build an underpass for pedestrians and cyclists on the Boston side of the Anderson Memorial Bridge, which crosses the Charles River and connects Allston with Harvard Square.
Despite the backing, however, there is no clear timetable for when — or if — construction will begin on the underpass, which activists say will separate cyclists and pedestrians from vehicular traffic at a busy intersection.
In a letter dated July 31 to Ron Axelrod of the Charles River Conservancy, a nonprofit that has been strongly in favor of the proposal, Transportation Secretary Richard A. Davey wrote that his agency was “happy to support the underpass through the Boston abutment” of the bridge.
To that end, Davey wrote, his department is retaining Needham-based Gill Engineering to complete work for the “25 percent design stage,” which includes sketch plans and a cost estimate for construction.
Davey added that the project would require a number of steps, including “authorization from the Army Corps of Engineers, the Massachusetts Historical Commission, and other agencies. . . . If permitting is successful, we are committed to building this underpass.”
In addition, Davey wrote that if the project is feasible, his agency will support future efforts to include underpasses at the River Street and Western Avenue bridges, which also cross the Charles and connect Cambridge with Boston.
Davey did not say how much Gill Engineering would be paid for the work or when it will be completed. He also did not say when the Anderson underpass may open, if the project wins final approval.
Michael Verseckes, a Transportation Department spokesman, said in an e-mail that “the letter outlines MassDOT’s commitment and that the next steps in the design and permitting process will define what the scope, cost, and time elements will be.”
Axelrod, a conservancy board member, praised Davey for offering his support.
“Secretary Davey has set a great example in his support of underpasses and we are pleased to be taking the next step,” Axelrod said in a statement.
Building underpasses for all three bridges on the Boston side would create 7 miles of uninterrupted pathways for pedestrians, joggers, and bicyclists, according to Axelrod and other advocates.
State transportation officials have previously voiced concerns about the plan.
In 2010, they cited the difficulty of securing the federal permits needed to disturb parkland and alter the appearance of bridges in the Charles River Basin, a National Register Historic District.
They also said at the time that planning and building the underpasses would delay scheduled repairs to the bridges and add $10 million or more to the expected $80 million cost of rehabilitating the three aging spans.
Davey did not say in his letter why the Transportation Department is now backing the underpass push, but he wrote that “advancing the underpass proposal is a policy decision that is aligned with MassDOT’s goals.”