The Massachusetts Bay Transportation Authority announced on Aug. 2 that it plans to delay construction of the $1-billion Green Line extension to Somerville and Medford until fall 2018 or as late as summer 2020.

It plans to purchase the required land and obtain permits before putting out a bid for design and construction to avoid loss of time and money—which happened to a commuter-rail project completed south of Boston in 2007. A day after the announcement, the city of Somerville circulated a petition stating that a four-year delay was unacceptable and demanded an accurate timeline for the project. It noted, “The state must do better than declare it is going to miss the legally mandated 2014 deadline for the extension.”

The authority was unable to estimate how much added cost would be incurred because of the delay. MassDOT and MBTA are considering staged construction of stations and other ways to accelerate project completion.