Question: where speeding is (in most cases) not criminal, and the purported goal of speed traps is for safety, not revenue collection, drivers flashing their headlamps achieves the oft-stated goal: getting drivers to slow down.
I have to disagree with this. Flashing the lights only gets drivers to slow down for 5-10 seconds. For example, they are going 100 in an 80 zone, someone flashes coming the other way flashes their lights, so the driver knows there is a speed trap and they slow down to 80. Then, right after they pass the police, they'll start speeding again. So, like I said, flashing your lights at oncoming drivers only causes people to stop speeding for about 10 seconds max. However, if that speeding driver were to get caught speeding, there is more of a chance they won't speed at all next time since they'll be concerned about a fee or loss of license. So not knowing where speed traps are acts as a deterrent.