Series of public parks in Boston. The most famous are the Boston Commons and Boston Public Gardens. Looks like a green necklace on a map, hence the name.

Has a few breaks, including the Charlesgate Overpass and Arborway which have been rebuilt for cars.
The Emerald Necklace roughly follows the old route of US 1, which until the late 1980s went on Storrow Dr and the whole sequence of Charlesgate, Fenway, Riverway, Jamaicaway, etc.