1995hoo got it. As noted in the picture of the plaque, the bridge is officially the Korean and Vietnam Veterans Memorial Bridge, but still commonly known as the Lagoon Pond Bridge, which was the name of the original bridge at this location. This is one of only three actively operating single-leaf bascule highway bridges that I know of (the others being the Neil R. Underwood/Hampton Harbor Bridge and some bridge in northeastern Michigan that I don't know the name of).
For the record, I intentionally distorted the photo to blur the name on the Town Line sign - it's clearly visible in the original photo. Since I know that MA has distinctive signs like that, this was one of those where a hint was contained within the picture itself.