The light at this intersection is not for Sacramento Ave -- it's primarily for a pedestrian crossing that connects with a pedestrian underpass that goes the railroad tracks. The underpass is the only pedestrian connection to Cambridge between Oxford Street and Park Street. It also gives Spring Hill residents access to Star Market. It gets quite a lot of commuter and shopper foot traffic.

The traffic signal doesn't change unprompted; the light only turns red when a pedestrian pushes a button to get the walk signal. The duration of the red light is determined by the time it takes a someone to walk across the fairly wide intersection, and that can't legally be changed. After the walk light is finished, it takes a minute or two before it will let a pedestrian cross again. You don't want to make that time too long, or people will start jaywalking and getting hit by cars.