Ever since headlight beam selectors began being placed on the steering column of automobiles, drivers have engaged in headlight flashing, or the practice of warning other drivers of hazards on the road or police speed traps by flicking their lights. Though a common practice, depending on what jurisdiction drivers who flash their high beams are in, their conduct could subject them to penalties.
On November 17, 2012, Missouri driver Michael Elli flashed his headlights to warn oncoming vehicles of a speed trap by police in the St. LouisCounty town of Ellisville. Pulled over and ticketed, Elli risked a fine of up to $1,000 if convicted. Despite contentions by the Ellisville police that flashing headlights could interfere with a police investigation, U.S. District Judge Henry E. Autrey nevertheless recently found in favor of the American Civil Liberties Union of Missouri and Elli, issuing a preliminary injunction prohibiting the town from citing and prosecuting drivers for flashing their headlights.
Like some other states, the Missouri federal court likened mechanically warning others about the possibility of getting ticketed to verbally warning another about the possibility of an arrest, holding that penalizing drivers for headlight flashing violates their First Amendment right to free speech. According to Judge Autrey, flashing headlights is actually beneficial to safety on the road, sending "a message to bring one’s driving in conformity with the law — whether it be by slowing down, turning on one’s own headlamps at dusk or in the rain, or proceeding with caution."
Missouri is not unique in tackling the flashing headlights matter. Courts across the country have been faced with the same issue in recent years, with nearly all cases being decided in favor of drivers cited for the act.
In New York, the practice of flashing high beams is not illegal. Rather, New York Vehicle and Traffic Law Section 375 (3) merely states that headlamps "shall be operated so that dazzling light does not interfere with the driver of the approaching vehicle." In a 1994 decision, the Appellate Division, Second Department held that flickering high beams do not amount to "dazzling lights." People v. Lauber, 162 Misc.2d 19, 617 N.Y.S.2d 419 (2d. Dept. 1994). In 2009, the Fourth Department declared more directly that the flashing of lights by itself is not a violation of the New York Vehicle and Traffic Law Code, and that stopping a vehicle based upon the driver flashing his or her high beams is illegal. People v. Rose, 67 A.D.3d 1447, 889 N.Y.S.2d 789 (2009). However, drivers leaving the State of New York should be wary of reaching for their headlight beam selectors, as the practice of flicking headlights remains a ticketable offense in many other states.
(Sources: http://www.vosizneias.com/154439/2014/02/06/st-louis-mo-headlight-flashing-ok-missouri-judge-says/; http://en.wikipedia.org/wiki/Headlight_flashing#cite_note-38)