From Yahoo News: Is flashing your car’s headlights protected by the first amendment?

Missouri resident Michael Elli wanted to let others on the road know to slow down because they were about to drive into a speed trap, so he did what many kindhearted souls do: He flashed his headlights as a warning.

Police didn’t take at all kindly to warnings of this 21st century Paul Revere. They flashed him a ticket of his very own for obstruction of justice. Prosecutors eventually dropped the case, but Mr. Elli has now filed a class action lawsuit against the city because he says that the city retaliates against drivers who exercise their right to free speech–and that the government is trying to prevent it because it doesn’t like the message.

More at the link. While tickets of this sort are almost always thrown out by prosecutors and judges, the winning of lawsuits against cops or cities that issue them never get very far.