Photo: Niels Sienaert/flickr/CC-BY-NC-ND 2.0

Why Are Stoplights Red, Yellow, and Green?  

Candice Darden
2.5k views 8 items

Have you ever looked at a traffic light and wondered, "Why are traffic lights red, yellow, and green? Is there a specific reason for this color code?" We associate red with stop, green with go, and yellow with caution, but why is that and when did it first develop?

The standard stoplight system of today first came into existence because of the system the British developed to prevent ships crashing into each other at night - this two color light system was extremely basic and simply had green for go and red for stop. The railway also developed a system of its own, which then went through trial and error before being adapted into the stoplights we see on the road today.

Read on to discover why these specific colors are used and what happened during the early railroad days that caused the 'go' color to be changed permanently.
The first lights were displayed on British steam ships in 1848.
British steamships displayed colored lights at night to prevent crashing into each other. The light on the right side was green, and the one on the left was red; these colors were chosen for high visibility. If your vessel approached another on the right with a green light, you could pass, but if approaching on the left - with a red light - you had to wait for direction. 
A similar system was used by the railway, first developed in 1830.
The original panel of colors for the railway was red for stop, green for caution (or yield), and white for go. The colors were later changed to the standard we have today after several crashes and mishaps as a result of the original colors.
Red is a universal color for "caution."
Probably because it's the color of blood, red has been used for warning signs for centuries. This made it a natural choice for the official traffic light color for "stop." In fact, it's the only color that has remained consistent from the beginning stages of the development of stoplights.
White used to be the color to indicate "go," but there were serious issues.
Using the color white for "go" in stoplights led to major mishaps. A major accident occurred when a red lens fell out of a railway stop light in 1914, causing the light to show white instead of red. Since white meant "proceed" at the time, both trains went and collided. Luckily this was sorted out before the color system was adapted for traffic lights, and the color white was changed to the green as we know it today as a direct result of that railway accident.