Red, yellow, and green are now the colors of traffic lights, but that wasn’t always the case. The first (and disastrous) attempt to install a traffic light occurred on December 10, 1868, which also marks the official date of the installation of the world’s first traffic light. It was located in front of the British Parliament in London and consisted of two movable signs mounted on lever-operated arms, illuminated at the top for visibility at night. Unfortunately, the unlucky scoreboard didn’t last long, as it exploded less than two months later, killing the police officer who was putting up the sign.
Traffic lights started to be installed worldwide with the advent of electricity in cities. Cleveland (USA) was home to the first two-color traffic light operating on electricity. Detroit and New York added yellow between red and green in 1920. In Europe, Paris saw the first traffic lights in 1923, followed by other major cities. A year later, Berlin, Milan, Rome, London, Prague, and Barcelona also installed traffic lights.
The first treaty on the unification of road signals was signed in Geneva in 1931 with the goal of enhancing road traffic safety and facilitating international traffic on roads with standardized traffic lights. Most of the signs seen on streets today were established by this agreement, making the three colors of red, yellow, and green the standard for traffic lights. The choice of colors for traffic lights was inspired by railway traffic conditions, where red indicated stopping, white indicated moving, and green indicated caution. The introduction of yellow was to address the confusion caused by white, which could easily be mistaken for starlight at night.
When it comes to color perception, red has the longest wavelength, followed by yellow and green. Longer wavelengths travel farther, which is why colors with longer wavelengths are chosen for signal transmission. As a result, red light can travel the farthest, making it visible even in conditions like rain or fog.
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