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 is sometimes called an optical horn, since it draws the attention of other drivers.
The 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 more common use as a 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 was wired to permit the momentary activation of the high beams regardless of whether the headlamp switch was turned on or off.
In the most basic sense, headlight flashing is a way of attracting attention, for any of a number of purposes. In this way, it can be perceived as the visual equivalent of blowing the horn, again for any of a number of reasons. This equivalency is underscored by some car owner's manuals, which identify the steering column control lever which allows headlight flashing as the "optical horn". Headlight flashing can be used simply to let other drivers know of one's presence, or as a response to acknowledge the presence or actions of other drivers. Some drivers flash their headlights as a signal that they are yielding the right of way to another driver, for example at an intersection controlled by stop signs.
Headlight flashing is commonly used to warn other drivers of dangers on the road, such as crashed cars or police speed traps. 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 own lights in response.
It can also be used to inform drivers of problems with their car, such as headlamps left off after dark, burned out or misaligned lights, or misuse of high beam rather than low beam in traffic, or to berate a driver who poses a risk to traffic. 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 overtake or 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.
Some drivers attempt to communicate "I will continue my current behavior!" when flashing their headlights. For example, if such a driver flashes his or her headlights while slowing down, they intend to communicate to another driver who is waiting to merge in traffic: "Go on, I will let you merge!" On the other hand, if such a driver keeps his or her current speed and flashes headlights, they intend to communicate "Stay back, I am not slowing down!"
Effectiveness and ambiguity
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 modulators, it is highly effective.
Flashed headlamps can have ambiguous or 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, that the flashing driver intends to yield the right of way, or instead that he intends to take it. Misinterpretation of the flashing driver's intent can cause crashes.
Legality and meaning
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 the state of New South Wales, headlight flashing by regular drivers (that is, not a police officer, etc.) for any means is illegal, except in emergency purposes, and except immediately before overtaking (Rule 218(2)).
Headlight flashing is common in Bangladesh, where roads are sometimes narrow and lane discipline is lax. It is done by large vehicles such as buses or trucks to alert smaller, more maneuverable vehicles to their presence and to encourage them to make way, for example by moving to the side of the road.
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. The section that deals with alternating headlights in Ontario is Section 169 of the Highway Traffic Act. It is an offence to improperly use high-beams at night, which is dealt with by way of section 168 of the Highway Traffic Act.
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".
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 others about speed traps have been fined in the past for "misuse of headlights".
Headlight flashing in the United Kingdom is mostly used as a signal that the driver flashing you is offering to let you go first. Using it to indicate that you are coming through and the other driver must wait, could lead to an accident.
In the United States, the legality of headlight flashing varies from state to state. Some states consider that drivers have a First Amendment right to flash their headlights. In other states, 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 momentarily).
In Arizona, flashing high beams or headlights is a violation of A.R.S. Section 28-942.1 (Failure to Dim Headlights). However, A.R.S. Section 28-942.2. states: If the driver of a vehicle follows another vehicle within two hundred feet to the rear, except when engaged in the act of overtaking and passing, the driver shall use a distribution of light permissible under this article other than the uppermost distribution of light specified in section 28-941, paragraph 1.
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 sometimes 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; that automatic flashing lights on non-emergency vehicles are illegal, but the act by a driver of manually flashing a vehicle's headlamps is not. Though ticketing was common in the 1990s, Maryland and Washington DC 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 suspicious police officer can ask a motorist if they were flashing their lights to warn oncoming motorists of police presence. If the motorist denies this, 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 Department) held 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 different 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 Oregon, a court ruled that flashing a vehicle's headlights to warn others about the presence of law enforcement is protected free speech under Article I, section 8, of the Constitution of Oregon.
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 high beams is not technically illegal, while driving with high beams on within 500 feet (150 m) of oncoming traffic, or 300 feet (91 m) of traffic in front of a driver is illegal. Drivers are advised not to flash their high beams, as overzealous law enforcement officers commonly misconstrue the law. 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.
Beginning in the early 1980s, a widespread rumor regarding flashing headlights was spread mainly through fax, and later on the internet. The rumor stated 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 led some police departments to issue warnings after having received the fake ones. In February 1994, Ann Sibila of Massillon, Ohio revived 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 (DARE) 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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