https://marco.org/2007/10/02/the-police-have-little-portable-trailers-with
The police have little portable trailers with radar and a display sign. They leave it unattended on side streets. The radar tracks your car and the display tells you how fast you are going. They’ve discovered that motorists slow down when they realize the device is tracking their speed. This doesn’t work on me. When I see that trailer, I speed up because I know the police won’t put both the trailer and a real policeman in the same neighborhood.
— The Dilbert Blog: Knowledge of economics is a mild superpower