staties and speed traps on the Longfellow bridge

I crossed the Longfellow Bridge this morning and saw a couple of State Police cruisers stopped at the curb at 1 Main Street. One of the officers was issuing a citation to a driver, and another officer was standing with (what I presume was) a speed gun.

The officer taking speed readings was making hand motions to drivers coming from Boston into Cambridge to slow down. The official speed of the Longfellow Bridge is 30mph, but I don't think I can remember the last time I saw a car stick to that speed (or under it) on the Boston to Cambridge side. The Cambridge to Boston side is a different story, as cars often gets backed up due to the traffic in Charles Circle.