Wikipedia
A wig-wag is a device for flashing an automobile's headlamps at a preset rate. In its traditional form a wig-wag constitutes the illuminating of the right and left headlamps alternately, with each lamp lit for around half a second at a time. In the United Kingdom the wig-wag is only seen on the road on emergency vehicles. The 'standard' wig-wag is often used within a cycle of other illumination patterns, such as swiftly alternating the left and right headlights, alternating the left and right headlights slowly, or flashing both headlights together, and using a mixture of high- and low-/dipped-beams.
Although the use of flashing headlights does increase the visibility of any vehicle, it can also create problems. In situations when highbeam headlights are flashed, the wig-wag may create glare or may otherwise temporarily blind the drivers of oncoming vehicles.
Generally, wig-wags are prohibited on all vehicles except emergency vehicles,. However the road rules in New South Wales, Australia, and some areas in the United States allow school buses to have flashing headlights.
A wig-wag is the red light (also known as a "red-eye") near each door of a motion picture sound stage. It flashes to indicate that cameras are rolling inside the stage and all people and vehicles outside should remain quiet. A flashing wig-wag is accompanied by one long buzzer when the cameras start rolling, and two shorter buzzers when cameras stop rolling.