Have you ever flashed your headlights at an oncoming driver in order to warn them of a possible speed trap ahead? If your answer is yes, you may be given a ticket for obstruction of justice, as Missouri citizen Michael Eli recently was. These “flashing” laws vary greatly from state to state. Mr. Eli had his charges dropped by the county prosecutor, but has recently teamed with the ACLU (American Civil Liberties Union) in a class action lawsuit seeking to establish that flashing your headlights is protected speech under the First Amendment. But what if Mr. Eli had warned drivers while in Ohio?
In Ohio, Mr. Eli would not have faced any charges. Drivers have already brought individual cases in the past, and courts have ruled the flashing of one’s headlights does not constitute obstruction of justice or obstruction of official business.
In Warrenville Heights v. Wason, the court ruled that:
“flashing headlights to warn oncoming motorists of a radar speed trap is not proven to have obstructed an officer in the course of his duties where there has been no proof that the warned vehicles were actually speeding.” 
The Court then went on to say that:
“In order to establish the elements of the offense charged it had to be established that there was an illegal act that generated a police officer’s duty to enforce the law and that defendant interfered with that duty.”
Therefore, unless there is a way to prove that the warned vehicles were actually violating the law, there is no way to establish the grounds for obstruction.
It was even found in Village of Kirtland Hills v. Garcia that a town ordinance against “flashing lights” would not apply to vehicles warning other vehicles about a speed trap or police car ahead.  Indeed, many Ohio ordinances outlaw “flashing lights” but this phrase is often construed by the courts to be oscillating “emergency lights” as it will list exclusions to the ordinance as ambulances, emergency vehicles, and snow plows.
Around the U.S.
If you tend to flash your headlights, you may want to look up state and local traffic laws regarding the matter. In Ohio, Florida, Pennsylvania, Utah, and New Jersey, you can flash away as you won’t be ticketed. But in states such as Alaska and Arizona, you can be ticketed for just flashing your lights, regardless of if you’re warning oncoming traffic. These laws vary from state to state, and are increasingly being challenged as violative of First Amendment free speech.