Michael Elli, a resident of the City of Ellisville, Missouri, observed a speed-trap in the City of Ellisville, and communicated to other drivers approaching from the opposite direction by flashing his headlamps to warn them that they should proceed with caution. The flashing of headlamps is commonly understood as conveying the message to slow down and proceed with caution. Of course, an officer in a marked police vehicle saw him and issued a ticket. When he appeared in municipal court, he was advised by the judge that the fine for using headlamps to communicate the presence of speed-trap is $1,000!
Elli went to Federal Court and requested a preliminary injunction to stop the practice of ticketing individuals who flash their headlamps.
Decision: Preliminary Injunction Granted. The judge stated that the loss of First Amendment freedoms for even minimal periods of time, unquestionably constitutes irreparable injury.
So it looks like flashing your headlamps is protected free speech, at least in the Eastern District of Missouri!