Curious for more?

Red symbolizes danger, which makes sense, considering it has the longest wavelength of any color on the visible spectrum, meaning you can see it from a greater distance than other colors. With the ironic exception of stop signs, red has meant stop since long before cars existed, with train signals. use of red dating back to the days when mechanical arms lifted and lowered to indicate whether the rail ahead was clear.

Yellow means "caution" because it's almost as easy to see as red. From the earliest days of motoring up until the mid-1900s, not all stop signs were red- many were yellow, because at night it was all but impossible to see a red stop sign in a poorly lit area. it's second only to red in terms of visible wavelength to the domain of "caution." That's why school zones and buses, crosswalks, and other important warnings are yellow today.

Green's role in lights has actually changed dramatically over time. Its wavelength is next to (and shorter than) yellow's on the visible spectrum, meaning it's still easier to see than any color other than red and yellow. Back in the early days of railway lights, green originally meant "caution," while the "all-clear" light was, well, clear or white. Trains, of course, take an interminably long time to stop, and legend has it that several disastrous collisions happened after an engineer mistook stars in the night horizon for an all-clear. Thus, green became "go,"