Between exit 11 and 14 on I-575 (north of Atlanta) the City of Holly Springs will have their local patrol cars park under the overpass bridges (obscured from traffic) for both northbound and southbound traffic. The speed limit is 65mph and is enforced strictly by the Holly Srings police but not by the Georgia Highway Patrol which uses the "flow of traffic" rule (if traffic is moving smoothly and no one is weaving in an out, no one is singled out.) I drive this way daily and have noticed that the Holly Springs police are most numerous the last week of the month, and not present at all the first week of any month.