Headlight flashing refers to the act of either briefly switching on the headlights of a car, or of momentarily switching between a headlight's high beams and low beams, in an effort to communicate with another driver or drivers. This signal can be intended to convey a variety of messages, including a warning to other drivers of road hazards or of speed traps, and it can also be a form of aggressive driving. The legality of headlight flashing varies by jurisdiction.
Headlight flashing might have come into use as a common means of attempting driver-to-driver communication by the mid-1970s, when cars began to come with headlight beam selectors located on the steering column—typically activated by pulling the turn signal stalk—rather than the previous foot-operated pushbutton switches. The signal stalk configuration permits the incorporation of momentary activation of the high beams regardless of whether the headlamp switch is turned on or off.
Headlight flashing is commonly used to warn other drivers of dangers on the road, such as crashed cars or police speed traps. It can also be used to inform drivers of problems with their car, such as headlamps left off after dark, burned out or misaimed lights, or misuse of high beam rather than low beam in traffic, or to berate a driver who is driving poorly. Headlight flashing coupled with blowing the car's horn can help clear deer from a driver's path. Drivers often flash headlights to indicate the intention to pass another driver, or to signal a driver who has just overtaken that he or she is far enough ahead to change back into the lane of the overtaken vehicle. Flashing is also commonly used to request or insist that a leading driver speed up or change lanes to get out of the way of a faster following driver. Headlight flashing may also be a part of aggressive driving, and can be used in an attempt to intimidate others into speeding or otherwise driving unsafely. Headlight flashing can let other drivers know of one's presence. Some drivers flash their headlights as a signal that they are yielding the right of way to another driver. Headlight flashing is also commonly used in attempts to warn oncoming drivers of police speed traps in the area. Headlight flashing can be used to give thanks to another driver. For example, in situations where a driver is warned of police activity in the area, it is sometimes considered courteous to flash one's lights in response. Headlight flashing may also indicate protest or celebration of an event or position.
Headlight flashing as an effective mode of driver communication has been questioned, and researchers have found the ability of drivers to communicate with one another is about the same as the communication abilities among insects. However, when used as a means of increasing conspicuousness to other drivers, as in the case of motorcycle high/low beam flashers, it is highly effective.
Flashed headlamps can have contradictory meanings with no way for the observing driver to tell for sure what the flashing driver is trying to say. It may mean, for example, the flashing driver intends to yield the right of way, or that he intends to take it. Misinterpretation of the flashing driver's intent can cause crashes. With the case of motorcycles however, the mere fact that other drivers see the flashing lamp, regardless of interpretation, proves the practice effective.
In the United States, the legality of headlight flashing varies from state to state. Historically, law enforcement officers give citations for headlight flashing under three types of laws: (1) laws prohibiting a person from obstructing a police investigation, (2) laws prohibiting a person from having flashing lights on their vehicle, and (3) laws prohibiting shining a vehicle’s high beams at oncoming traffic. The specific language of each law varies by state along with courts’ holdings on whether their respective laws prohibit headlight flashing. Additionally, although not legally binding, the state driver’s manual of some states suggests flashing high beams under specific scenarios (e.g. if an oncoming vehicle is using its high beams, driver’s manuals suggest a motorist flash his or her high beams).
In California, headlight flashing is legal in some situations and illegal in others. It is legal for a driver to flash his headlights to indicate intention to pass on a road which does not allow passing on the right. However, headlight flashing on multiple-lane highways is illegal.
In Florida, headlight flashing is protected free speech pursuant to the First Amendment. Additionally, on 1 January 2013, §316.2397(7), Fla. Stat. was amended to legalize headlight flashing. In 2005 and 2011, judges in County Court held that flashing a vehicle's headlights is not a violation of §316.239(7), Fla. Stat.
In Maryland, police officers ticket drivers for flashing car headlights under a law which prohibits driving in a vehicle with flashing lights and laws prohibiting "obstructing a police investigation". The American Civil Liberties Union of Maryland challenges the current interpretation of the law, contending the law refers to an adjective and not a verb; automatic flashing lights on non-emergency vehicles are illegal, but the act by a driver of flashing a vehicle's headlamps is not. Though ticketing was common in the 1990s, Maryland and Washington, D.C. police say that flashing one's headlights was not against the law in either place.
In Massachusetts, the practice of headlight flashing is technically not forbidden. A clever police officer though can ask a motorist if they were flashing their lights to warn oncoming motorists of police. If the motorist says no, the officer can ask if the vehicle has defective lights—which is a violation of Massachusetts General Laws Chapter 90, Section 7.
In New Jersey, drivers are allowed to flash their headlights to warn approaching drivers about a speed trap ahead. In 1999, The Superior Court of New Jersey Appellate Division held that a statute limiting how far high beams may project is not violated when a motorist flashes his or her high beams to warn oncoming motorists of radar. The Court also concluded that a stop by a police officer based upon high beam flashing is also improper.
In New York, headlight flashing is not illegal. New York Vehicle and Traffic Law Section 375  requires that headlamps "shall be operated so that dazzling light does not interfere with the driver of the approaching vehicle". In 1994, New York Supreme Court, Appellate Division held that flipping or flicking high beams at approaching vehicles is insufficient to cause the "dazzling lights" prohibited under New York Vehicle and Traffic Law Section 375 . In 2009, the New York Supreme Court Appellate Division, Fourth Departmentheld that the flashing of lights alone is not a violation of New York Vehicle and Traffic Law Section 375 , that stopping a vehicle based upon that is illegal, and all evidence gather as a result of the illegal stop should be suppressed.
In North Dakota, when an oncoming vehicle is within 500 feet, high-beam flashing for any length of time (including momentary flashes) and for any purpose at night is illegal under N.D.C.C. Section 39-21-21.
In Ohio, courts have held that the act of flashing one's headlights so as to alert oncoming drivers of a radar trap does not constitute the offense of obstructing a police officer in the performance of his duties, where there was no proof that the warned vehicles were speeding prior to the warning. In another case, where a driver received a citation under an ordinance prohibiting flashing lights on a vehicle, a court held that the ordinance referred to the noun of flashing lights and did not prohibit the verb of flashing the headlights on a vehicle. In a difference case, a court held that a momentary flick of the high beams is not a violation of Ohio R.C. 4513.15 (which prohibits drivers from aiming glaring rays into the eyes of oncoming drivers).
In Virginia, headlight flashing to warn of police activity is not illegal, even though other evasion techniques like radar detectors are outlawed. Virginia motor vehicle code specifies an "audible or light signal" to indicate overtaken vehicles should yield in certain situations 
In Washington, flashing your high beams is not technically illegal. Driving with your high beams on within 500 feet of oncoming traffic or 300 feet of traffic in front of you is illegal. This being said, don't flash your high beams in Washington as overzealous law enforcement officers commonly misconstrue this law. http://seattletimes.com/html/localnews/2003712164_ontheroad18e.html Under section 46.37.230 of the Revised Code of Washington, flashing one's headlights is not expressly forbidden but may result in a $124 traffic infraction. http://apps.leg.wa.gov/rcw/default.aspx?cite=46.37.230 
On some occasions, motorists who flashed their headlights to warn of police activity have unwittingly helped fugitives evade police. In 2008, one of Jamaica's most wanted men went around police checkpoints which had been set up on his most likely routes after a driver had flashed his headlights to warn of police ahead. Drivers were warned that flashing headlights may result in "unwittingly facilitating criminal activity".
Headlight flashing to warn drivers of traffic enforcement cameras is illegal in the state of Queensland, carrying a $30 fine and one demerit point, or a $1500 fine if the fine is unsuccessfully challenged in court. Officers may either fine a driver for improper use of headlights, or may even arrest and prosecute for hindering police.
In NSW, headlight flashing by regular drivers (that is, not a police officer, etc.) for any means is illegal, except in emergency purposes and except for immediately before overtaking (Rule 218(2)).
In Ontario, the Highway Traffic Act does not prohibit "flashing head beams". Some have brought tickets to court, claiming the law only regulates the use of alternating lights in an attempt to impersonate emergency and law enforcement vehicles, and not a driver's manually flashing his car's headlamps to communicate with other drivers.
Though not all of its rules represent law, the Highway Code states "Only flash your headlights to let other road users know that you are there. Do not flash your headlights in an attempt to intimidate other road users". Drivers warning about speed traps have been fined in the past for "misuse of headlights".
Beginning in the early 1980s, a widespread rumor regarding flashing headlights was spread mainly through fax, and later on the internet. The rumor states that various gangs across the United States carry out an initiation wherein the initiate drives around at night with his headlights off. Whichever driver flashes his headlamps in response to the unlit car becomes the target; to complete the initiation, the prospective gang member must hunt down and shoot, kill, assault, or rape the target. The story was widely spread by many government organizations, including the New Mexico State Police. This rumor has been proven an urban legend.
The story originated in Montana in the early 1980s, where it was rumored that the Hells Angels bike gang was initiating recruits in this way. By 1984, the story had spread to Eugene, Oregon, where it had morphed into a story of Latino and black gangs targeting whites. In August 1993, the story once again appeared, this time spread through fax and email forwarding. Warning of a "blood initiation weekend" on 25 and 26 September, the rumor this time compelled some police departments to issue actual warnings after having received the fake ones. In February 1994, Ann Sibila of Massillon, Ohio reinstated the rumor by issuing flyers which claimed that killings would take place at Westfield Belden Village. After a night of sending faxes to local businesses, Sibila was arrested for inducing panic.
The rumor once again spread in October 1998, when a new fax, this time claiming to originate with a Drug Abuse Resistance Education officer in Texas. The rumor spread further when officials in the San Diego government circulated the fax among city agencies; this version of the fax, though quickly dismissed within city government when it was found that the Sheriff's office had no real connection to it, now appeared to be a legitimate government-issued document. Also in the fall of 1998, the Sheriff's office of Nassau County, Florida sent a warning about such gang initiation to the county fire department, who subsequently spread the fax to all county agencies. Police dispatcher Ann Johnson had thought the message urgent enough to send, but had not bothered to check its legitimacy.
The rumor provided inspiration for the 1998 film Urban Legend, and served as a plot device in Mark Billingham's 2008 novel In The Dark. An incident inspired by this legend is the catalyst for the 2007 film Death Sentence, starring Kevin Bacon.
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