The Origin of the Green, Yellow, and Red Color Scheme for Traffic Lights

Today I found out the origin of the green, yellow, and red color scheme for traffic lights.

While some of the specifics have been lost to history, it is known that this color scheme derives from a system used by the railroad industry since the 1830s. At this time, railroad companies developed a lighted means to let train engineers know when to stop or go, with different lighted colors representing different actions.  They chose red as the color for stop, it is thought, because red has for centuries been used to indicate danger. For the other colors, they chose white as the color for go and green as the color for caution.

The choice of a white light for go turned out to cause a lot of problems. For instance, in an incident in 1914 a red lens fell out of its holder leaving the white light behind it exposed. This ended with a train running a “stop” signal and crashing into another train. Thus, the railroad decided to change it so the green light meant go and a caution “yellow” was chosen, primarily because the color is so distinct from the other two colors used.

So how did this system transfer to the road?  In London, England in 1865 there was a growing concern over the amount of horse-drawn traffic causing danger to pedestrians trying to cross the roads. A railway manager and engineer named John Peake Knight, who specialized in designing signaling systems for the British railway, approached the Metropolitan Police with the idea of using a semaphore/lighted system for road traffic.  In the daytime, this semaphore method used an arm or arms that could be raised or lowered by a police officer, notifying carriages when they should stop when the arm(s) stuck out sideways.  At night, his system used the red and green colors for stop and go.

Modern Railroad Semaphore

His proposal was accepted and, on December 10, 1868, the system was put in place at the junction of Great George and Bridge Street in London, near Parliament. The system worked extremely well… for about a month. That’s when one of the gas lines that supplied the lights began to leak. Unfortunately, the policeman who was operating the arm 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.

On the other side of the pond, signaling traffic in the United States also used policemen as it was thought that people would not follow a set of rules unless there was some form of law enforcement present. Towers that allowed officers a better view of the traffic became commonplace in the 1910s and 1920s. During this time, officers could either use lights (usually red and green after the railroad system), semaphores, or simply just wave their arms to let traffic know when to stop or go.

In 1920 in Detroit Michigan, a policeman named William L. Potts invented the four-way, three-color traffic signal using all three of the colors now used in the railroad system. Thus, Detroit became the first to use the red, green, and yellow lights to control road traffic.  Many inventors continued to come up with different designs for traffic signals, some adopting the red, yellow, green color scheme and some not. Most usually needed a person to push a button or flip a switch to change the light. As you might expect, this man-power intensive way to change the lights proved costly.

In the late 1920s, several “automatic” signals were invented. The first ones used the simple method of changing the lights at specific timed intervals. However, this had the drawback of having some vehicles stopped when there were no cars going in the other direction. An inventor named Charles Adler Jr. had an idea to get around this problem. He invented a signal that could detect a vehicle’s horn honking. A microphone was mounted on a pole at the intersection and once the vehicle stopped, all they need do is honk their horn and the light would change. To keep people from continually honking to get the light to change, and thus causing havoc, once the light was tripped, it wouldn’t change again for 10 seconds, allowing at least one car to get through.  Presumably people walking by and living in nearby homes and businesses were not fond of this system.

A less annoying automatic signal was invented by Henry A. Haugh. This system used two metal strips that sensed pressure. When a passing car pushed the two strips together, the light would soon change to allow that car to go.

All of these different types of lighting systems began to present a problem. Drivers could drive through different areas and encounter several different types of systems, causing confusion and frustration. Thus, in 1935, the Federal Highway Administration created “The Manual on Uniform Traffic Control Devices.” This document finally set uniform standards for all traffic signals, road signs, and pavement markings- pertinent to the topic at hand, on the traffic signal front, it required using the red, yellow, green light indicators.

Bonus Facts:

  • Current traffic systems use a variety of methods to optimize throughput in intersections.  For instance, some use such things as lasers or rubber tubes filled with air to sense pressure (often the bane of motorcyclists and small car owners); however, the most common is the “inductive loop” method. You’ve probably seen the groves cut in the roadway just at the stop line of traffic lights. The common misconception is that there is a scale under these grooves, sensing the weight of a vehicle.  In actuality, embedded in these grooves are what is known as an inductive loop. Inductive loops work by detecting a change of “inductance” or magnetic field. It uses a wire wrapped around some metal with a power source. When the wire wrapped around the metal is powered, it begins to build up a magnetic field. Sensors known as inductance meters continually check the inductance of the coil. Once a car, which contains a lot of different types of metal, enters the inductors’ magnetic field, the inductance rises and lets the system know a vehicle is parked over it. From here, different municipalities will use different algorithms to tell the lights how to use this information, thus how long lights stay red or green.
  • Older incandescent traffic light bulbs typically used 175 watt bulbs.  New LED traffic lights use only around 10-25 watts.
  • In the early police officer manned traffic control systems, police officers often used red for stop and green for go, but rather than have a yellow light, they simply blew a whistle to indicate that they were about to change the signal.
  • Another early traffic light system, developed by Earnest Sirrine, threw out the whole red/green paradigm and instead had lit words saying “Proceed” and “Stop”.
  • The word “semaphore” comes from the Ancient Greek words sêma, meaning “sign”, and “phoros”, meaning “bearer” or “bearing”.  So, essentially, “semaphore” translates to “sign bearer”.
  • The railroad semaphore system was originally patented by Joseph James Stevens in the 1840s.
  • In the U.S. and some other countries, modern traffic signal lights are either 8 or 12 inches in diameter and must be visible in every kind of weather and lighting condition.
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  • I think it’s worth noting that Garrett A. Morgan invented a semaphore type traffic signal as well (patented in 1923). Though there is some speculation as to whether (or how often) it was put into use, he still contributed to the invention that we use today.

  • When was it agreed upon that red would be on the top with green on the bottom and yellow in the middle? Near my small hometown we had a stoplight that for years used only three bulbs to control a four way intersection. The upper bulb; when lighted; would shine through a green lens in two opposing directions and a red lens in the other two; thus signalling N/S traffic to go while E/W traffic was stopped. Then all four would shine the yellow caution light, then when the bottom bulb was lighted the opposite two directions would be seeing green and red. I’ve heard that it was due to color-blindness of some drivers that the locations of the colors in relation to each other were standardized. Curious if this is the case and if so when it occurred. Thanks!

  • I was told in my drivers ed class that red is used for stop because it is the color that can be seen from the furthest distance.

  • how are the lights timed to prevent accidents?

  • In Germany the traffic lights turn yellow for a few seconds before they turn green so you can get a head start. I love that. I think it’s because a lot of people drive manual transmission cars, so it gives them a chance to shift into gear.

  • Do you know if any thought has been given to introducing a system which would be friendly to color blind people?

    • YES, the red lights are the TOP lights, no brainer

      • There’s no Federal law that says so, nor in my state. A few small towns in my state have “upside-down” traffic lights, which regularly collect fines from the 15% of males passing through who are red/green color blind.

        Then there are states like Texas, which love to put their lights sideways…

    • The red and green are not 100% red and green. The red has some orange and the green has some blue

      • Not enough for me to tell the difference. And now that most local lights have gone LED, all three lights are the same color instead of one yellow and two not-yellow.

  • Love the article – fun and well documented.
    Some more interesting facts that you might have not known:
    1. The yellow light actually came into proper use in the 1920s
    2. You can see the red light sooner than you can see the green light. It is not an illusion! Red light has a longer wavelength than green, so it can be seen from farther away.
    3. There was a sitcom called ‘Traffic Light.’ (ok lame fun fact – but it’s true and it ran for 51 episodes). Married with kids was red light, moving in with girlfriend was yellow light, and and forever single was green light.
    4. The world’s oldest traffic light is in Ashville, Ohio installed 1932.

  • I think it is also of note that the first pure lights used on the railways had a reversed direction of Green on the top and Red on the bottom.

  • My father was a pharmacist born in rural Texas in 1911. This is what he learned from several old timer druggists that were born in the 1800s. Have you ever heard of apothecary show globes or bottles? These show globes or jars where made of clear glass with a ground glass stopper and the first ones he said always sat in their own metal stands. They were not made to stand on their own. They were rather like ancient amphora jugs but made of clear glass. These jars were filled with colored water for a reason. There should have been 2 for each store but one would have done but would have necessitated changing the color of the water more often. A bottle would be placed in the front window so it could be seen from the street. The water was either red or green. Red meant an epidemic like small pox or any other type of deadly disease was present in the town and countryside and was a warning to NOT STOP here. The green meant it was a safe place to stop. These bottles and use of these colors were brought to this country from the “Old World” where they were used in the same way but were more often a warning against plague or a way of signaling that a village was free of disease. The bottles were tall in their tall stands so they could be easily seen from horseback or a horse drawn vehicle hence the name, “Show Bottle”.
    Apothecary/Chemist shops is the “Old World” term for what we call here drugstores or pharmacy’s. Note: I just asked a friend (born in 1952) who was raised in Ireland about “Show Bottles” and if they were in use there back then and now and he said “yes”. However he says he had no idea as to their meaning. They use the term Chemist for Pharmacy. I don’t know how to go about verifying this. I will leave that to someone else and too when the yellow color came into use. I’ve never seen any “Show Bottle” use yellow colored water. I hope somebody finds this of interest.
    PS Pharmacy’s always had and used what today we call food coloring in the preparation of their elixirs, tonics and cough remedies. That is why they were the ones chosen to use “Show Bottles”.
    There’s more but I won’t bore you anymore. Thanks for reading this. Sincerely, Chaney Noe (born in 1952)

  • Erwin van der Meulen

    Really interesting. When I should make a direction of the colors, it should be from the top ; Red – Yellow – Green. The reason is that from a distance on the horizon I always see the top first. Most important is the STOP signal. Here in The Netherlands yellow is orange which means “caution”. When the light are malfunction the orange will always start blinking on all lights in the system and you must slow down or stop on a intersection. Also in the late evening and night some lights are switching over to blinking-mode. The bus and tram in all cities like Amsterdam and Rotterdam has there own lane. There own specific lights you will only see in The Netherlands calles Negenoog (Nine Eye) See Wikipedia (In Chrome on that page left click mouse, Translate English)

  • OK, some documented facts. Garrett Morgan did file the first recognized patent in 1923 (bought by GE,) however Lester Wire (Salt Lake City) installed the first recognized electric traffic signal in 1912. Cleveland followed in 1914 using Wire’s design. In 1920 William Potts installed the first electric 4-way 3-color signal in Detroit. Then all sorts of people and companies got into the act. For the record, Morgan may be best known for the first actual patent of a traffic control device, however he invented all sorts of things that we actually use today.

    With respect to colors and color orientation, green was originally on the top and red on the bottom, similar to railroad signals. I have a fully functioning 1927 Harrington-Seaberg that displays this arrangement.

    A number of early economy signals (popular in small towns and many still present in the 1960s and beyond) used one bulb in each section that shone in all four directions. Thus red had to be installed in the top for one street and green in the top for the cross street, and vice versa for the bottom section. The yellow interval displayed both when the light was changing from green to red and when changing from red to green. W.S. Darley was the most prolific manufacturer of these simple but sturdy signals, selling thousands of them. Their weakness was if one side of the signal faced low angle sunlight, it would shine through to the opposite side and inadvertently illuminate all of the indications on that opposing side. Some towns had to get creative and extend the visors on some signals to eliminate these “phantoms.”

    Common belief is that eventually red was standardized as the top most indication (or left most on horizontal signals) as those positions were more easily seen over or around vehicles in the lanes ahead.

    In 1935 the Federal Highway Commission created the Manual of Uniform Traffic Control Devices (MUTCD.) It served as more of a guide than a regulation. In 1966, Congress passed the Highway Safety Act that required states to create highway safety programs and adopt contemporary standards or lose a portion of their federal highway funds. From that point all new intersections had to meet the standards, although older equipment was grandfathered until it was no longer functional.

    Another influential body was the Institute of Technical Engineers (ITE.) ITE took on the issue of traffic signal lens designs and colors. ITE adopted 8-3/8-inch and 12-inch round lenses for signals, established standards for the beading to improve visibility and reduce phantoms from reflected sunlight, and standardized colors… a little blue added to the green, a little orange added to the red and true yellow (as opposed to amber) so as not to be confused with the slightly-orange red.

    Some states required motorists to idle in neutral at red lights and the signals displayed a yellow interval before switching to green, advising motorists to “get it in gear.” A handful of those signals are still in service although as they are replaced, the new installations typically conform to MUTCD standards.

    As far as the oldest still functioning traffic signal? Ashville, OH makes the claim but truthfully it’s just the most unusual traffic signal still functioning – and it’s in their museum and definitely worth seeing. It went into service in 1932. There are functioning signals in collections made by companies that went out of business or sold out during the crash of 1929 so it’s not the oldest. Call up a Google Street View of N. 2nd and E. North Streets in Albemarle, NC and you’ll see a Crouse-Hinds Type T (refurbished and hanging from a new mast) that predates the Ashville signal, still in service, still directing traffic.

  • Why is the red light put at the top?????????????

  • Why does it now take under a second after the red light has gone out before motorists start using their horns if you haven’t moved off….?

  • My grandfather from Sweden used to say:

    “If ‘dey put da’ brake v’ere ‘da horn is, da’ vur’ld vud’ be a safe’ah place…”