ST. LOUIS — 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 Nov. 17, 2012, Elli flashed his headlights to warn oncoming vehicles of a radar set up by Ellisville police.

An officer saw the flash and pulled Elli over, citing him for a city ordinance violation, though a similar statute exists in Missouri law. Elli was told he would likely face a fine of up to $1,000 if convicted.

The case was eventually dropped. Ellisville City Attorney George Restovich said the city changed the policy and no longer pulls over people for flashing headlights.

"The reality is that the injunction doesn't change the way the city has been operating for the past 12 months," Restovich said.

Still, Ellisville continued to fight the suit. At a hearing last year, Ellisville officials made the case that flashing headlights could interfere with a police investigation.

But 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."

Jonathan Turley, a criminal attorney and a professor at George Washington University Law School, said courts across the country are dealing with the same issue. In virtually every case except those still being decided, the person cited has prevailed, Turley said.

"This has sweeping implications for the First Amendment," Turley said. "What this citizen is doing is warning other citizens about the violation of law. People regularly warn others about the possibility of arrest. There's no difference between a verbal warning and a mechanical warning. Both are forms of speech."

Jeffrey A. Mittman, executive director of the ACLU of Missouri, said that since filing the suit, his office has heard from many other drivers across Missouri complaining about similar police practices.

"It is important that law enforcement officers in other jurisdictions take note of this federal court decision and the ACLU-MO's commitment to free speech," Mittman said in a statement.