ST. LOUIS -- A federal judge has issued a permanent injunction prohibiting a St. Louis County town from ticketing drivers for flashing their headlights to warn other drivers police are nearby.

U.S. District Judge Henry Autrey issued the injunction Monday.

The American Civil Liberties Union had filed suit on behalf of Michael Elli. In 2012, Elli flashed his headlights to warn oncoming vehicles of radar set up by Ellisville police. An officer saw the flash and cited Elli for a city ordinance violation.

Elli could have faced a fine of up to $1,000, but the case was dropped. A message left with Peter J. Dunne, the attorney handling the case for Ellisville, was not immediately returned Wednesday. Ellisville city attorney George Restovich has said previously that the city changed the policy and no longer pulls over people for flashing headlights.

The ACLU contended Ellisville was violating First Amendment rights of drivers.

"The police cannot retaliate against drivers who have done nothing wrong and are simply exercising their right to communicate with other drivers," Jeffrey A. Mittman, executive director of the ACLU of Missouri, said in a statement Wednesday.

Drivers often flash lights to warn approaching drivers to slow down for a variety of reasons, ACLU attorney Tony Rothert said. Among those reasons: To warn them a police car is nearby, perhaps using radar to catch speeders.

Autrey issued a preliminary injunction in February.

Autrey said in his February ruling 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."