You will be given a passage, and your task is to generate a Yes/No question that is answerable based on the given passage.

In the United States, although the legality of headlight flashing varies from state to state, a federal court ruled that flashing headlights was a constitutionally protected form of speech, issuing an injunction prohibiting a police department from citing or prosecuting drivers who flash their lights to warn of radar and speed traps.
is it illegal to flash lights to warn of police?