A federal judge in Missouri, Henry Autrey, has ruled that this practice—warning other drivers of speed traps—is legal and protected by the First Amendment. The issue came up because a man (Michael Elli) in Ellisville, Missouri was pulled over and cited for flashing his lights to notify other drivers to slow down. He challenged the citation and even filed suit against the Ellisville Police Department. They dropped the charges against him (which could have resulted in a $1,000 fine), but they continued to fight Elli’s suit.About time someone clarified this - but unless you live in Missouri, don't expect to get away with this, unless you, too, are prepared to fight in court.
But Autrey ruled in Elli’s favor. His comments are refreshingly reasonable:
Autrey said in his ruling that the flashing of headlights was essentially a good thing, sending “a message to bring one’s driving in conformity with the law — whether it be by slowing down, turning on one’s own headlamps at dusk or in the rain, or proceeding with caution.”
I'll keep on doing this as long as I can still drive.