For drivers in many parts of the globe, the red, yellow, and green sequence of traffic lights has become an instinctual part of how we move through the world. But why these three colors? How did they come to govern the movement of billions of people across the planet every day? As it turns out, there are a few factors in play, including the history of transport, color psychology, and basic physics.  Red Lights and Railroads The colors used in traffic lights today were not invented for roads. They were largely inherited from the railway industry, which began developing light-based signaling systems in the 1830s (based on even older semaphore flagging). But in the early days of railroads, the colors were slightly different: While red was used to signal “stop,” white meant “go,” and green was the color used to indicate caution. The …Read More