are (usually) a mode of operation of turn signals
(a.k.a. trafficators, turn indicators) on an automobile
. They are used to increase the visibility of the vehicle displaying them, typically because the vehicle is moving slowly or has stopped in a location where fast(er) traffic will be forced to avoid it. They are also known as blinkers, hazard flashers
or just hazards.
Some vehicles, especially truck
s or service vehicles, have dedicated hazard warning flasher lamps.
Since modern automobiles are required to have highly visible turn signal lamps mounted in order to indicate driver intentions, those lamps are nearly always dual-purpose and also serve as the hazard flashers. When operating as hazards, both lamps at the front or rear must flash simultaneously (or all four together) in order to avoid confusion with turn indicator operation. This serves to alert other drivers to the vehicle's presence, and potential hazard.
In the United States, hazard flashers are mandatory equipment. According to 49 CFR 571, Federal Motor Vehicle Safety Standards:
S5.5.5 The vehicular hazard warning signal operating unit on each vehicle shall operate independently of the ignition or equivalent switch, and when activated, shall cause to flash simultaneously sufficient turn signal lamps to meet, as a minimum, the turn signal lamp photometric requirements of this standard...
S5.5.10 The wiring requirements for lighting equipment in use are:
(a) Turn signal lamps, hazard warning signal lamps, and school bus warning lamps shall be wired to flash;
Standard J910, first issued in 1966 and revised in 1988, lays out the requirements for the Hazard Warning Signal Switch, and SAE J945 lays out the standard for the flashers themselves. Alas, I have not been able to find the text of these standards available for free; apparently, you gotta pay to comply.
Generally, though, the switch for the hazard signals is in one of the following location on modern cars:
Iron Noder 2010