I don’t know who it was that told me about the unwritten and unspoken rule of the road — if you saw a police officer sitting and waiting to catch people going quickly in the opposite direction, and you saw people coming in that direction that you were encouraged, if not obligated, to warn the drivers headed toward the speed trap by flashing your headlights a couple of times.
Under ordinary circumstances, the person receiving the warning can only respond in one of two ways. Either they are aware of what the warning means and slow down, or they are not aware of what it means and continue driving as they were and possibly get pulled over for speeding. If they are wise enough, at that point they figure out why someone flashed their lights at them. I would imagine if they do not quite get it at that point, they never will.
It has never been on the books as a legal thing to do, however — not that it was on the books as an illegal thing, either. One driver got pulled over for unlawful use of his high beams and when he filed a lawsuit after getting ticketed for this reason, it was considered to be quite a big victory for civil rights.
If you were walking down the street and saw a police officer crouching behind some bushes, waiting to jump out and arrest passersby, would it be considered illegal to write “POLICE OFFICER IN BUSHES” on a large sign and to walk in the area with it? It absolutely would not be — it most certainly would fall under the banner of free speech, just as flashing high beams can be considered a form of expression regardless of what is being expressed.
Hopefully the victory in this lawsuit means that it will be recognized in other states — if you see a police officer, you can flash your high beams to warn other motorists. After all, we have been told for well over ten years that if we saw something to say something — and why would they want us to stop at just the things that they deem relevant?