Why Are Traffic Light Signals Red, Yellow, and Green?

Rancho Labs
4 min readJun 6, 2021


Have you ever been puzzled with the thought as to why blue doesn’t indicate go and brown denotes a halt? More than simply traffic lights have been influenced by the concept that red indicates halt and green signifies go.From an early age, we’ve been taught that the color red denotes danger and the color green denotes that it’s safe to proceed. But why were those specific colors picked for traffic signals? It turns out there’s a perfectly reasonable answer.

The Early Traffic Lights

Because of an increase in traffic on the road, the first traffic lights were built in the United States. Concerned about traffic accidents, towns and cities would construct traffic towers to assist traffic flow. Officers stationed themselves in the towers, blowing whistles and flashing red, green, and yellow lights to alert traffic when it was time to stop and go.

William Potts invented the first tri-color, four-direction traffic light in 1920. This made it easier for cars to stay safe at junctions. But, there were still a number of traffic signal and pattern systems in existence around the country. The Federal Highway Administration published “The Manual on Consistent Traffic Control Devices” in 1935 to define uniform standards for all road signs, pavement markings, and traffic signals, mandating them to all employ red, yellow, and green light colors.

History of the colors

There were traffic signals for trains before there were traffic lights for cars and other automobiles. Railroad firms used red to indicate stop, white to indicate move, and green to indicate caution at first. Train conductors, as you might imagine, had some issues with the color white indicating go — bright white could easily be confused for stars at night, leading train conductors to believe they were all clear when they weren’t.

Because it’s easy to differentiate from the other colors, railway companies finally adopted the color green to signify go, and the color yellow to imply continue with caution, and it’s been that way ever since, and when traffic lights were installed, it became standard for them as well.

Why are these colors chosen?

  • We recall that the seven colors of the light spectrum are VIBGYOR (Violet–Indigo–Blue–Green–Yellow–Orange–Red), specified with the wavelengths increasing in order. Violet has the shortest wavelength, whereas Red has the longest. Also, longer wavelengths have shorter frequencies and tend to travel larger distances when encountering obstructions.
  • If different colors are considered, red has the longest wavelength, followed by yellow, and finally green. The colors with the longest wavelengths are chosen for Signals because longer wavelengths may go a greater distance. Because Orange looks too much like Red in bright light, it might cause confusion. As a result, Red light can travel the greatest distance in rain, fog, and mist, and remains visible the majority of the time since it has a high priority, followed by Yellow, and finally Green.
  • The three signs of traffic light: Stop. Caution and Go, were assigned in order of priority to be notified to the driver. Stop, for instance, should be alerted to a driver as soon as possible so that he may respond and come to a complete stop . As a result, it was assigned the red color, which has the longest wavelength and can thus be seen from a great distance. Similarly, the Caution sign was chosen next.Because yellow has a little shorter wavelength than red, it was used to warn drivers. School zones, some traffic signals, and school buses are still painted yellow since it is visible at all hours of the day. This was followed by the Go sign which was assigned green color , as per its wavelength, which comes after yellow color.

Long before vehicles were ever invented, the color red has always been associated with danger. Because red has the longest wavelength of all the colors, it can be seen from a longer distance than the others. Hence it has been assigned STOP signal, and other colors , Yellow and green are used to signify Caution and Go respectively as per their wavelengths. There’s science in everything, you know!



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