Red for stop, green for go, yellow for attention. This rule seems so natural that we rarely think about why these three colors have become a universal language on the road. The story begins not with cars, but with railways in the 19th century. Trains travel long distances and it is necessary that the signals be visible from afar and understood without words. Red gradually established itself as a sign of danger and stopping. Its long wavelength helps the light to be seen relatively well in different conditions.
From railway systems, the idea moved to city streets. As traffic increased, engineers and authorities needed a way to regulate cars, carriages, trams, bicycles and pedestrians without a policeman standing at every intersection. The red signal now has the necessary meaning and logically takes on the role of a stop command.
The green signal begins to indicate permission to move, creating a clear visual contrast with the red. Yellow appears later, because a transitional signal is needed between “you can continue“ and “you must stop“.
Without a yellow light, the driver could switch from permission to move to stop almost without warning. Therefore, the main message of the system is summarized by the simple rule: “red means stop, green means go, and yellow warns of a change“. The exact obligations at a yellow signal are determined by the laws of individual countries, but its purpose remains similar. The colors are also selected because of the conditions under which they must be recognized. Traffic lights operate in bright daylight, at night, in rain and fog. Therefore, the signals must be distinguished quickly, and not require prolonged observation.
The first known traffic light appeared in London in December 1868, in front of the British Parliament building. It was designed to control horse traffic and pedestrians moving around a busy intersection. The device had little in common with today's electronic traffic lights. During the day, it used movable mechanical arms, and at night - red and green gas lamps. It was manually operated by a police officer.
The experiment ended quickly. In January 1869, the gas traffic light exploded, injuring its operator. The case was reported by the Italian agency ANSA, and the Smithsonian describes the first London system as an early combination of a railway signal and urban traffic regulation. Electric systems turned out to be more practical. As the number of cars increased in the early 20th century, cities began to introduce increasingly complex solutions for intersections, while the combination of red, yellow and green gradually became the standard.
Today's traffic lights can include LEDs, cameras, car sensors, pedestrian buttons, timers and adaptive systems that change the duration of the signals according to traffic. However, the basic colors have been retained. An important reason is that drivers understand them almost instinctively. Additional help is provided by the arrangement - in vertical signals, red is usually on top, yellow in the middle and green on the bottom. This way, the signal can be recognized even when the colors are not clearly distinguishable.
Sources: motor1.com, ansa.it, smithsonianmag.com