When coming over the Mass Ave. bridge (over the Charles River) into Boston, there commonly is at night an officer in a patrol car watching for speeding vehicles. The limit over the bridge is 35 (I believe) but it is common to see vehicles going 50-55+ on this bridge during late night when there is minimal traffic on the bridge. The officer will sit on the first small road on the right side between the river and the first traffic light, and you do not see him until you are just passing by him.