Why do traffic lights use red, yellow, and green only? The global standard that changed road safety forever

You could be the most powerful person in the world, or even its creator, but if you plan to step onto a road, you still have to obey the same three colours as everyone else: red, yellow, and green. The rule is universal; you stop at red lights, pause at yellow, and go at green, without questioning why. These three colours could literally change the trajectory of your journey. In a world full of colours, why were these three chosen to manage traffic flow? While these colours seem obvious now, they were not chosen randomly. They are the result of carefully considered optical physics, human psychology, and historical evolution.The first traffic light appeared in London in 1868, a gas-lit signal designed more for railway use. It had only two positions: stop and go. Later, as traffic volumes exploded, the need for better systems grew. Back in the 1910s and 1920s, roads were far from organised. Major cities were crowded with drivers competing with trains, trolleys, horse-drawn carriages, wagons, buggies, bicycles, and pedestrians. Traffic control relied on police officers standing at intersections. This human-directed system was not without problems. Accidents were common, some even fatal. In 1924, traffic accidents caused 23,600 deaths and 700,000 injuries in the United States alone.The first electric traffic light was installed in Cleveland, Ohio, in the early 1900s, but it only had two colours: red and green. The real breakthrough came in 1920, when police officer William Potts created the inevitable—the first four-way, three-colour traffic light in Detroit, Michigan. The introduction of traffic signals transformed road safety. Accidents and fatalities dropped dramatically. In the early days, traffic signals differed between cities, creating confusion for drivers, which eventually led to standardisation. Today, the red–yellow–green system is universally recognised, keeping millions of people safe.If you see red on traffic lights, it means only one thing: you must stop. The choice of red is intentional. From railway signals to naval flags, red has long been used as a signal to stop. When the railway industry standardised its signalling systems in the 19th century, it chose red because it is the colour visible from the greatest distance in fog and dim conditions. Its wavelength allows it to penetrate atmospheric haze better than other colours.Green was not always used as the ‘go’ signal. Early systems used white, but this led to confusion and chaos. White lights were easily mistaken for stars or street lamps, leading to dangerous misinterpretations. As a result, green became the safer alternative. Green is associated with calmness and reassurance from a psychological perspective. More importantly, the human eye is particularly sensitive to green wavelengths, making it easier to see.Yellow solved a practical problem: the need for a transition. Early systems had only red and green, which prevented drivers from safely preparing for sudden changes. Later, in the 1930s, the yellow signal was introduced as a cautionary phase. Yellow sits in the middle of the visible spectrum, making it highly visible in both daylight and darkness. It prepares drivers for change—to slow down and proceed with care.What seems like a simple solution is, in fact, one of the most effective innovations in the history of humankind.