My college roommate once got a ticket and went to defensive driving to get it dismissed. The instructor was a moonlighting police officer. He told the class about one of his department’s scams.

One police officer would park by the crest of a hill. A second officer would be waiting a half mile away on the other side of the hill. As motorists passed car #1 and then the hill, they would frequently flash their lights at oncoming traffic to warn them of the cop hiding just over the hill. The second officer would see the blinking headlights and pull the driver over. He would explain that he was pulling the motorist over because he observed the lights flashing on and off.

If the driver said that he had noticed the other car, and was deliberately flashing the lights to warn oncoming traffic to remind them to check their speed, the officer would thank them for their concern in keeping traffic to a safe speed and send them on their way. There was nothing illegal about their conduct.

Usually, however, the motorist would be afraid that warning other traffic would be frowned upon by law enforcement. These drivers would play dumb, and say they hadn’t flashed their lights. They would be written up for defective headlights, because if the driver wasn’t blinking them and the lights were blinking, they would certainly be malfunctioning, right?