For many years, I have tried to warn motorists on the other side of a highway that there is a speed trap ahead of them. After I pass a cop lying in ambush, I wait a few moments (to catch people before it's too late for them) and then do a quick flash-flash with my lights. Do people get what I'm doing there, or would you, dear reader, assume I was telling somebody to turn on his lights?
Also, I wondered about the ethics of it. I think you could tell a plausible story where motorists announcing the presence of the cop, in effect just magnifies his effectiveness. I.e. if you did a huge model that had the police and motorists, and could show the different results from plugging in different strategies, then I think you could calibrate the payoffs such that everybody is happier when a certain fraction of motorists consistently warn others of a cop. It would be giving the police the option of deputizes a thousand citizens, as it were, to crack down on speeding, but the hitch is that the deputies can't issue fines. But lo and behold, people always slow way down when the deputies flash their lights. In this way, one squad car can slow down many miles of highway.
But the motorists like this outcome better, too, because now they never get a ticket. There are advance scouts, scouring the highways in all directions in order to provide a warning to each driver.