Is it against the law in any state to flash your headlights to let other drivers know that there is a police cruiser up ahead of them?
This took place in Australia, so it doesn’t really answer your question, but years ago, my father passed a police radar unit stationed on the opposite side of the road. A few minutes later he flashed his lights to warn a truck driver.
As he did it, he noticed a police car travelling behind the truck, but it was too late. The cop did a U-turn and pulled my father over. The policeman was clearly trying to think of a way to charge him, and said " Are you aware that… uh… it’s an offence to uh… distract other drivers?"
My father was thinking a bit quicker than the cop, and said “I was letting him know he’d left his headlights on”
“Are you aware there is a radar patrol back down the road?”
“Umm… yes, I think I might have seen something like that”.
The cop tried to remember if he’d seen the truck’s tail lights on, couldn’t, and let my father go with a warning “to be more careful next time”.
Here’s a link to a column on this topic in the Pittsburgh Post-Gazette:
http://www.post-gazette.com/columnists/19990429brian.asp
It details the case of a man who was cited for flashing his brights to warn oncoming motorists of a speed trap ahead, and took his case to the state Supreme Court, and won.
The fact that this case was in the state Supreme Court suggests to me that the law probably varies from state to state. Also, interestingly enough, what the guy actually got cited for was not warning other people of a speed trap, but rather “shining his brights within 500 feet of another motorist.” He successfully argued that the brights-shining law was designed for nighttime driving, when you could reasonably say that shining your brights could blind oncoming drivers, and not for daytime courtesy flashing.