I can’t imagine under what possible doctrine this could have been prohibited unless it were creating some kind of immediate traffic hazard. Course they never let stuff like that stop then.
Suppose you had a police scanner, and you heard that the cops were coming to pick up your neighbor. So you run over to your neighbor’s house, and speak, using your 1st amendment right to free speech “Hey, the police are coming to arrest you” — and your neighbor hops in his car and gets away.
Do you think this is OK, or have you aided a criminal to evade capture?
I think you might be able to distinguish that from flashing lights by an argument that you don’t KNOW that anybody on the other side is speeding, plus speeding isn’t really a criminal violation, more of a civil thing.