trafficlight

Traffic lights, or traffic signals, are devices at road intersections that regulate the flow of vehicles and pedestrians. They use colored lights and sometimes arrows to indicate when it is safe to proceed, wait, or yield. The primary aim is safety and efficiency.

The usual colors are red, yellow (amber), and green. Red means stop, green means go, and yellow warns that the signal will change soon. The exact sequence and timing vary by country and jurisdiction; some places use signals with additional arrows or pedestrian phases.

A signal system comprises signal heads, a controller, and a cabinet or electronic controller. Vehicle signals may be accompanied by pedestrian displays. Modern installations often include sensors such as inductive loops or cameras to detect approaching traffic and adjust timings accordingly.

Control strategies range from fixed-time cycles to actuated and adaptive systems. Fixed timing runs on a set schedule; actuated systems respond to demand, while adaptive signals adjust timing to real-time traffic conditions and coordinate signals along a corridor.

History: The first gas-lit signal appeared in 1868 in London. Electric signals began in the 1910s in the United States, with the three-color design standardized in the 1920s by William Potts and colleagues.

Accessibility and variants: Some signals include pedestrian countdowns, audible signals for the visually impaired, and dedicated indicators for bicycles or buses. International standards guide visibility, placement, and timing, though details differ by region.