Having driven cross country a few times, I have been guilty of flashing my headlights to warn drivers of an upcoming speed trap. Maybe, you have done this too. In 2009, a man in Florida did the same thing, but it is illegal in that State and he was ticketed. In the last 5 years over 2,400 drivers have been given a ticket in the 'Sunshine' State. He fought the ticket and won-his case was thrown out of court. But, now he has turned around and is suing the State, saying that flashing your lights (to warn other drivers) is an expression of free speech. If he wins, Florida will have to refund the fines to all 2,400 drivers.
By the way, there is no law in Florida banning this behavior.