The other day the traffic light near my home was broken. It got me thinking why traffic lights are designed the way they are today.

For example, why do traffic lights use three separate color? Could we use a single-colored light bulb to control traffic? It’s quite easy to see that single-light system is not reliable. Let’s consider a couple options:

  1. Single (green) light to mean go. When the light is off, people might just think there is no light and keep going.
  2. Single (red) light to mean stop. The light might stop working and all cars would believe it is safe to go.

Nowadays we can easily make LED lights that change color. However it is not widely used because the positions of the lights are useful for colour-blind peope.

Ok single light bulb doesn’t work. Why red green and yellow though? The design originally came from railway signal lights. Red has always been used to signal danger. At the beginning in railway lights, red was used for stop, green for caution and white for go. However, white turned out to be a terrible choice because it is visually similar to a bunch of other light sources. For example, if the lens of a red or green light falls out, the light would appear as white. This happened in 1914. As a result, the railroad changed to a system where green means go and yellow means caution.

It was also not always three colours with traffic lights. In the 1910s when traffic light first got adopted on the road in US. Only green and red was used. At the time, officers had to blow a whistle to let drivers know when the signal was about to change. Note that blinking light was technically infeasible when traffic light was first invented. In 1920, William L. Potts, a policeman from Detroit, invented the three-colour traffic signal based on the railway signals.

References:

  1. The origin of the green yellow and red color scheme for traffic lights
  2. Why do traffic lights have three separate light bulbs instead of having one