re: Do you flash oncoming traffic after passing a cop?

There are some cases that say it is protected speech, but I don’t think the US Supreme Court has ruled on it.

This is from a 2019 federal case from Wisconsin:

one federal district court has addressed conduct similar to that in this case and found that it is entitled to protection under the First Amendment. Elli v. City of Ellisville, Mo., 997 F. Supp. 2d 980, 984 (E.D. Mo. 2014) (“Even assuming, arguendo, that Plaintiff or another driver is communicating a message that one should slow down because a speed trap is ahead and discovery or apprehension is impending, that conduct is not illegal.”). In addition, at least two state circuit courts also have found that drivers have a constitutional right to flash their headlights. See State of Oregon v. Hill, Citation No. 034117 (Jackson Cty. (Ore.) Justice Ct. Apr. 9, 2014) (flashing vehicle headlights to warn others about presence of law enforcement is protected free speech under state constitution); State v. Walker, No. I-9507-03625 (Williamson Cty. (Tenn.) Cir. Ct. Nov. 13, 2003) (accepting First Amendment defense to charge of knowingly interfering with officer where defendant flashed headlights to warn oncoming motorists about speed trap).

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As discussed at length above, the law is far from settled about whether drivers have a First Amendment right to flash their headlights to convey a warning to oncoming vehicles. Neither the Supreme Court nor any federal court of appeals has addressed the issue.

Obriecht v. Splinter, No. 2019 WL 1779226, at *5 (W.D. Wis. Apr. 23, 2019)