A wig-wag is a device for flashing an automobile's headlamps, in its simplest form, so only one of the two headlights operates at a time, with the two flashing at a preset rate. In its traditional form a wig-wag emits 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,^{[citation needed]} it can also create problems. When highbeam headlights are flashed, the wig-wag may create glare or temporarily blind the drivers of oncoming vehicles.

Generally, wig-wags are prohibited on all vehicles except emergency vehicles.^[1] However, the road rules in New South Wales, Australia, and some areas in the United States allow school buses to have flashing headlights.^[2] In New South Wales specifically additional wig-wag lights (separate from the headlights) are used on all public transport buses. In the United States, motorcycles are similarly allowed to be equipped with a headlamp modulator to increase conspicuity during the daytime. In the United States specifically, additional Wig-Wag lights (separate from the headlights) are used on all public transport buses, and can use white (clear) lights to increase conspicuity during the daytime.