The Inner Belt was a proposed eight-lane highway that would have connected U.S. Route I-93 to U.S. Route I-90 and I-95 through a ring road through Somerville and Central Square and across the B.U. Bridge and beyond through Boston to the Southeast Expressway. A group of city planners, community activists, universities, and politicians formed a coalition to block the construction of this road. Their actions preserved much of Cambridge and attracted national attention
as one of the earliest community efforts that blocked an infrastructure development. It became the rallying cry of many later political movements in Cambridge.

The Inner Belt project was first proposed in 1948. Opposition to the project grew through the 1960’s, and Governor Sargent eliminated the Inner Belt system from further consideration in November, 1972. 

History Cambridge chronicles much of the 24-year history here: https://historycambridge.org/history-hubs/inner-belt-hub/