Empty cop cars slow traffic

My drive to work takes me about 10 miles down Loop 1 (called the MoPac Expressway because it follows the Missouri Pacific railroad) every morning.  As you would expect, there often are police cars parked under the overpasses, ostensibly looking for speeders.  But there’s a catch:  the cars are empty!  I can’t remember the last time I actually saw a police officer sitting in one of those cars.  But they do have a calming effect on traffic.  Every day we’ll be clipping along at 75 MPH until the guy in front can see the empty police car sitting under the bridge.  He probably knows as well as I do that there’s nobody in the car, but he can’t be sure.  Presto, instant speed control.  It’s an interesting lesson in how peoples’ expectations control their behavior.