In this video, we explore the question, "why are traffic lights red and green?" It explicates on how with ended up with this specific color scheme.

Back in 1865, a railway manager and engineer named John Peake Knight, proposed the idea of using green and red lamps as traffic lights. It was installed outside the House of Parliament in London. The system worked well for about a month. That’s when one of the gas lines that was supplied to the lights began to leak.

Unfortunately, the policeman who was operating the lamp was unaware of the leak and ended up being severely burned when the lamp exploded. Thus, despite its early success, the semaphore traffic system was immediately dropped in England.

Obviously, we've come a long way in traffic light technology, but we are stuck with the same color scheme. What is it about red and green that gave them such lasting power?

Red, the color of blood and fire, has been a danger signal since long. So, it makes a good sign for a warning signal.

Rayleigh scattering is the scattering of light by particles much smaller than the wavelength of the incident light such as air molecules or fog or mist. It is inversely proportional to the fourth power of the wavelength of light, which means that the shorter wavelengths such as (violet and blue) are scattered stronger than the longer wavelengths such as red. If you do the calculations, it shows that violet scatters almost 10 times more than red.

When the first primitive railroad signaling devices were developed in the 1830s and 1840s, red meant "stop," green meant "caution," and clear (i.e., white) meant "go." This system had several defects. One obvious problem was the fact that the white signal could easily be confused with an ordinary white light. What was worse, however, was the fact that the system wasn't fail-safe. This was tragically demonstrated sometime around 1914. The red lens supposedly fell out of a signal so that it erroneously showed a white indication. This caused a train to sail through the "stop" signal, resulting in a disastrous crash. The railroads subsequently decided to drop white and make green "go" and yellow "caution." Yellow presumably was chosen because it was readily visible and offered the most striking contrast to the other two colors. When the first electric traffic signals were installed in Cleveland, Ohio, in 1914, they used red and green indications. When the first modern automatic traffic signals were put up in Detroit in the early 1920s, they used red, yellow, and green, and that's what we're stuck with today.

Tags: traffic lights, traffic lights invention, traffic lights color scheme, why are traffic lights red and green, the motivated engineer, traffic light convention, traffic light colors, why red and green, why red green and yellow, red and green, red green and yellow

No copyright intended.

The information in this video is picked up from the following resources. All credit to them.
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Animation on Rayleigh Scattering comes from djd14d's channel, all credits to him:
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I'd like to specially thank Nikhilendra Gudisa for giving me the idea to make this video. I'd like to thank NIkhilendra Gudisa, Sharan Erukulla and Vinod Srinivasan for reviewing and validating the information presented in this video.

Music Credits:
Vinod Srinivasan (srinivasanvinod429@gmail.com)
Vinod makes great music which I use in many of my videos. Contact him by email if you need music for any of your videos.

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