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 California, headlight flashing is not explicitly prohibited by state law, but it can be subject to interpretation under various vehicle codes. For instance, California Vehicle Code Section 24409 states that drivers must dim their high beams when approaching another vehicle within 500 feet. While this code does not directly address headlight flashing, it could be argued that flashing high beams within this distance violates the statute. Additionally, flashing lights are generally reserved for emergency vehicles and certain service vehicles as per California Vehicle Code Section 25252. However, there is no specific prohibition on headlight flashing to communicate with other drivers. The practice of headlight flashing has been debated in the context of the First Amendment, with some courts recognizing it as a form of protected speech, particularly when used to communicate messages such as warning of hazards. Nonetheless, if a police officer interprets headlight flashing as aggressive driving or an attempt to interfere with a police investigation, it could potentially lead to a traffic stop and citation. Drivers who receive tickets for headlight flashing may challenge the citation on First Amendment grounds, but outcomes can vary based on the circumstances and the court's interpretation.