So now you know. If you pass a police officer at a speed trap, you can flash your headlights to warn upcoming drivers of their presence and not get pulled over at least for just that. It is constitutionally protected under the first amendment.

This ruling stems from a driver who was ticketed for doing just that, flashing their headlights to warn on coming drivers of an officer waiting for drivers passing illegally.

According to a Federal judge, flashing your headlights is a driver’s way of telling someone to conform to the law, for example telling them to turn on headlights, warning drivers of an emergency and or to just slow down.

Federal law trumps state law, so you’re good to go. You're welcome.