ST. LOUIS — The American Civil Liberties Union is hoping that a federal judge’s ruling Monday that temporarily barred a Missouri city from punishing drivers for using their headlights to warn others of speed traps will itself serve as a warning to other cities who try to do the same. ACLU Legal Director Tony Rothert said Monday that it was the first federal court ruling on the issue anywhere in the country. “It is legal in Missouri to communicate in this manner,” he said, “and detaining, ticketing or arresting someone for the content of their speech is illegal. “ U.S....