The traffic light we know today has remained mostly unchanged for a century, true to the initial vision of two American policemen:
The traffic signal was first invented in 1912 — by a Detroit policeman named Lester Wire — as a two-color, red-and-green light with a buzzer to warn pedestrians ahead of the impending transition. In 1920, this basic design was modified (by another policeman named William Potts) to include the tri-colored red, amber, and green lights widely used today. This simple, three-color icon has endured for nearly a century with relatively little change, save for the incorporation of modern technologies such as automatic timers, diode lights and motion sensors.
On the color:
Red is classically seen as a color representing danger or caution. (There are countless phrases and idioms that use “red” as a message of the bad or unknown — “in the red,” “seeing red,” and “red herring,” among others.) Green, on the other hand, is a reassuring color in most cultures — the color of nature and growth; of harmony, freshness, and fertility. Green has a strong emotional correspondence with the idea of safety, and was intuitively chosen to guide pedestrians responsibly through an intersection.
The signal has endured except where it hasn’t, in concepts such as “shared spaces” — like this one in Drachten, Netherlands — where all traffic lights have been removed. People rely on other signals, human ones, to decode traffic transitions.