Judge: It's Legal to Flash Your Headlights to Warn of Cops It's within your First Amendment rights, he rules By Kevin Spak, Newser User Posted Feb 6, 2014 1:26 PM CST 67 comments Comments (Shutterstock) (Newser) – Drivers of the world, unite—because the coppers can't stop you from doing so, a federal judge in St. Louis ruled this week. It's a common custom of the road to flash your headlights at oncoming traffic to warn them if they're about to pass a police speed trap. But when Michael Elli did that in Missouri in November 2012, the Ellisville police cited him for violating a city ordinance, the AP explains. The charges were eventually dropped, but not before the ACLU had filed a suit on Elli's behalf. On Monday, US District Judge Henry Autrey agreed that prohibiting the headlight flash violates the First Amendment. "Detaining, ticketing, or arresting someone for the content of their speech is illegal," he reasoned, according to the St. Louis Post-Dispatch. Ellisville says it's since dropped the ordinance, but there's a similar state law, and drivers elsewhere in Missouri have reported similar stops. "This has sweeping implications for the First Amendment," says one law professor. "What this citizen is doing is warning other citizens about the violation of law."