The Associated Press reports:

It’s a common practice among drivers who pass through a speed trap:  Flash your headlights at approaching cars as a warning to slow down.  Now, a federal court judge in Missouri says penalizing drivers for the headlight flash violates their First Amendment right to free speech.

U.S. District Judge Henry E. Autrey in St. Louis on Monday issued a preliminary injunction prohibiting the St. Louis County town of Ellisville from citing and prosecuting drivers who flash their lights to warn of radar and speed traps.  Ellisville’s city attorney said there are no plans to appeal.

The order stems from a lawsuit filed by the American Civil Liberties Union of Missouri on behalf of Michael Elli.  On November 17, 2012, Elli flashed his headlights to warn oncoming vehicles of a radar set up by Ellisville police.