Headlight flashing is the act of a driver flashing their headlights at an oncoming driver by briefly turning the headlights on or off, or from high beam to low beam, or low beam to high beam. There is no set or clear message transmitted by headlight flashing, but it often means the driver who flashes their headlights believes the oncoming driver’s headlights are inappropriately on high beams or are not turned on or illuminated when they should be.

Headlight flashing may also be intended to convey other messages, such as warning another driver of the flashing driver’s presence; telling another driver to proceed in front of the flashing driver (yielding the right of way); or warning an oncoming driver of danger ahead—which may include a deer or other animal in the road, an automobile wreck, or a police officer hidden from view who is checking the speed of passing drivers and issuing speeding tickets (a speed trap).

Police officers have sometimes issued traffic tickets to drivers for headlight flashing, and in some instances a traffic stop for headlight flashing has led to more serious charges and an arrest for DUI/DWI, for example. Tickets for headlight flashing are often issued based on an alleged violation of a municipal ordinance or a state statute that prohibits:

• high beam headlights within a certain distance (500 feet, for example) of an oncoming driver;

• aggressive driving;

• having flashing lights on a vehicle; or

• obstructing a police investigation.

Drivers who have received tickets for headlight flashing have sometimes challenged the ticket—or the traffic stop that led to an arrest and additional charges—on the ground that headlight flashing is a communication protected by the First Amendment to the United States Constitution.

Laws and ordinances vary from state to state and among cities and towns (municipalities), and state and federal courts have arrived at different conclusions when applying the First Amendment and other laws and ordinances to traffic tickets or stops based on headlight flashing.

In Massachusetts, headlight flashing is not explicitly prohibited by state law, but drivers may still face penalties if their actions are interpreted as a violation of other traffic regulations. For instance, Massachusetts law requires that headlights must be used from 30 minutes after sunset to 30 minutes before sunrise, when visibility is reduced to 500 feet or less, or during adverse weather conditions. The state also mandates the use of low beams when approaching or following another vehicle within 500 feet. While headlight flashing could be seen as a courtesy or warning to other drivers, it could also be misconstrued as aggressive driving or interference with a police investigation, especially if it's perceived as an attempt to warn others of a speed trap. Drivers who are ticketed for headlight flashing may argue that their actions are a form of speech protected under the First Amendment. However, the outcome of such challenges can vary, as interpretations of the law differ among jurisdictions and individual circumstances of each case.