# I Traced the History of the Traffic Light. It Began 35 Million Years Ago Source: https://www.youtube.com/watch?v=8HUEAjTHjFM Recap page: https://rapidrecap.app/video/8HUEAjTHjFM Generated: 2026-03-10T15:09:21.563+00:00 --- ## Quick Overview The selection of red and green for traffic lights, which seems counterintuitive based on human color perception, is actually rooted in physics, biology, and railroad history, ensuring red's superior visibility in adverse conditions due to its longer wavelength and less scattering, while the historical development of traffic signaling involved early gas-lit semaphore systems that eventually led to the standardized three-color electric light system we use today. **Key Points:** - Red light was chosen for stop signals because it possesses the longest wavelength in the visible spectrum, resulting in less scattering by atmospheric particles like fog and rain, maximizing visibility. - The color red is biologically associated with danger, anger, and arousal in humans, making it an ideal, universally recognized warning signal. - Early traffic control systems involved manual signaling using flags or colored glass lamps, such as the gas-lit semaphore invented by John Peake Knight in 1868 in London. - The first electric traffic light, invented by Lester Farnsworth Wire in 1912, used red and green bulbs, which caused confusion when drivers with color blindness could not distinguish between them. - William Potts, a Detroit police officer, is credited with inventing the four-way, three-color traffic signal in 1920, adding the crucial yellow light to signal caution. - Primate color vision, particularly in species originating outside Africa, often involves only two color-sensitive cones, making distinguishing between red and green difficult, which is why the visible spectrum's red and green wavelengths are so distinct for human vision. - The prevalence of red as a danger signal across nature (poisonous animals) and culture (warning signs) reinforces its instinctive use for stopping. ![Screenshot at 00:09: An AI search result explains that the choice of red is a mix of physics, biology, and railroad history, setting the stage for the video's multi-faceted explanation.](https://ss.rapidrecap.app/screens/8HUEAjTHjFM/00-00-09.jpg) **Context:** This video explores the historical, physical, and biological reasons behind the choice of red, yellow, and green for modern traffic lights, addressing the common misconception that these colors were chosen purely for scientific reasons. The narrative traces the evolution from early railroad signaling using semaphore arms and colored glass, through early electric prototypes that caused confusion due to only having red and green, to the standardized three-color system introduced in the 1920s, linking these conventions to physics (Rayleigh scattering) and human biology (trichromatic vision). ## Detailed Analysis The video explains that the choice of red for stop signals and green for go signals is not arbitrary but has deep roots in physics, biology, and railroad history. Red light has the longest wavelength in the visible spectrum, meaning it scatters less in the atmosphere (Rayleigh scattering) due to air molecules, dust, and fog, making it visible from a greater distance, especially in poor weather. Biologically, red is associated with danger and arousal in humans, reinforcing its use as a warning, while green is associated with safety and calm. The history begins with early railroad signals in the mid-1800s, like John Peake Knight's gas-lit semaphore in London (1868), which used red for stop and white for go. When electric signals arrived, early inventors like Lester Farnsworth Wire in 1912 used only red and green, which confused colorblind individuals. William Potts, a Detroit police officer, patented the three-color system in 1920, adding yellow as a caution signal, which was not universally adopted immediately, with the London system taking decades to fully transition. The video also touches on color vision, noting that many primates only have two color-sensitive cones, making them unable to easily distinguish between red and green, which reinforces why these two colors were chosen as opposites in signaling. ### The Physics of Light - Red light has the longest wavelength in the visible spectrum - Red light scatters less by air molecules, dust, and fog compared to blue or violet light - This allows drivers to see a red light from a much greater distance, giving them more time to react in poor weather conditions. ### Historical Development of Signals - Early railroad signals used moving arms or colored glass (red for stop, white for go) - John Peake Knight invented the first gas-lit traffic light in London in 1868, which exploded in 1869 - Lester Farnsworth Wire, a Utah police officer, invented the first electric, two-color (red/green) traffic light in 1912. ### The Three-Color Standard - William Potts, a Detroit police officer, added the crucial yellow/amber light in 1920 to signal caution - Potts's design was based on railroad signals but featured red/green/yellow lights vertically - The resulting system proved more reliable than hand signals or early two-color lights. ### Biology and Perception - Human vision is trichromatic, allowing us to easily distinguish between red and green wavelengths - Many primates outside of Africa are dichromatic (only two cone types), which means red and green look similar to them, reinforcing the need for distinct wavelengths like red and yellow/green. ### Cultural Associations - Red is culturally associated with danger, blood, and anger, making it an intuitive stop signal - Green is associated with calm, life, and safety, making it intuitive for 'go'. ![Screenshot at 00:05: A physical traffic light is shown with red, yellow, and green lights illuminated, prompting the central question of why these colors were chosen.](https://ss.rapidrecap.app/screens/8HUEAjTHjFM/00-00-05.jpg) ![Screenshot at 00:23: A graphic illustrating the visible spectrum shows red light having the longest wavelength \(around 700-750 nm\) compared to shorter wavelengths like violet/blue, explaining why red penetrates fog better.](https://ss.rapidrecap.app/screens/8HUEAjTHjFM/00-00-23.jpg) ![Screenshot at 00:54: A collage of historical documents and images highlights early signaling methods, including patents for automatic traffic signals and early semaphore arms.](https://ss.rapidrecap.app/screens/8HUEAjTHjFM/00-00-54.jpg) ![Screenshot at 08:14: A comparison between William Potts's four-way electric traffic signal \(left\) and an earlier two-color signal \(right\) shows the introduction of the yellow light.](https://ss.rapidrecap.app/screens/8HUEAjTHjFM/00-08-14.jpg)