Tech In Plain Sight: What Does A Yellow Light Mean?

Ghigleri’s traffic light

The traffic light is a ubiquitous feature of modern life and is quite old — dating back to 1868 London, although that device was a modified railroad semaphore operated by a policeman, but it was the same idea. The initial test of the signal proved disastrous.

The semaphore had gas lamps to illuminate the signs in the dark. A gas leak caused one of the lamps to explode, badly burning the operator and ending the nascent invention for a while. In 1910, American inventor Ernest Sirrine worked out an automatically controlled traffic signal. Two years later, Lester Wire, a police officer, developed a different version powered by overhead trolley wires to light the signal. A 1917 patent by William Ghiglieri also had two lights — red and green. But where was the yellow light?

Time To Slow Down

The fundamental problem with the red/green system is that of inertia. Cars can’t stop instantly, and if the side with green starts moving before the side with red stops, they collide. Borrowing from the rail system, the first yellow light appeared when policeman William Potts added it to signals in Detroit in 1920.

Yet another system to include a pause state was invented by Garret Morgan. Morgan was the first African-American to own a car in Cleveland.  He also invented several things, including a hair relaxer that came from a solution used to prevent sewing machine needles from overheating, and a firefighter’s safety hood that routed cool air from floor level to the user in 1914. He and his brother would use the hoods to rescue two people trapped underground after a gas explosion in 1916. The Army used these hoods in World War I.

But back to traffic lights, Morgan was driving through Cleveland one day and saw an accident. He noted that the traffic signals of the day would abruptly change from “go” to “stop” and decided a third state would solve the problem. From his patent:

One of the objects of my invention is the provision of a visible indicator which is useful in stopping traffic in all directions before the signal to proceed in any one direction is given. This is advantageous in that vehicles which are partly across the intersecting streets are given time to pass the vehicles which are waiting to travel in a transverse direction; thus avoiding accidents which frequently occur by reason of the over-anxiety of the waiting drivers, to start as soon as the signal to proceed is given.

Morgan’s traffic signal relied on using arms labelled “stop” and “go”, but importantly also included the pause to stop traffic in all directions until the intersection could clear. This “caution period” between go and stop would become a standard for traffic signals of all kinds. Morgan’s invention was cheaper to produce than the Potts design, and he sold the rights to General Electric for a cool $40,000 — quite the sum in those days. The original prototype is in the Smithsonian’s American History Museum.

Like many inventions, there wasn’t a single inventor for the modern device. Charles Adler is another name in traffic light history. In 1928, he perfected a system that allowed drivers to honk to change the light. That would seem to have some problems operationally, but it did serve as a foundation for lights that respond to vehicle traffic. Adler also came up with the pedestrian push button in 1929.


Then there are all the technical issues the public never thinks about. How do you maximize incandescent bulb life? You derate the bulbs and keep a slight current flowing through them all the time so the filaments stay warm, minimizing thermal shock when you turn them on again. Or, these days, use LEDs. Then again, LEDs don’t melt snow off the lights like the old bulbs did. Of course, you need a wide viewing angle, and there is usually some sort of visor to help prevent the sun from washing out the color.

Then there is control. The original signals were operated by policemen. Automated or timed lights started to appear around 1922. The Crouse Hinds company, based in Houston and known for railroad signals, added timers to Houston traffic lights around that time.

Cities found it enticing to automate lights. New York City, for example, had 6,000 officers working traffic control and, thanks to light automation, reduced that number to 500, saving over $12 million a year. One problem was that drivers thought a human would do a better job controlling traffic. Many thought it was a fad, although, obviously, that wasn’t the case.

Coordinating lights was another issue. Turning all the lights on a major street green seems like a good idea for traffic flow. But in practice, drivers would race to make as many lights as possible, resulting in Philadelphia seeing an increase in accidents after implementing such a system.

The answer was to change the lights in lockstep so that a vehicle going at some speed would, in theory, never have to stop after making one green light. General Electric pioneered this in Washington, D.C., in 1926. Of course, back then, the timing mechanisms to pull this off were finicky. Eventually, computers and sensors would add sophistication to lights. That’s only gotten better with time.


One interesting bit of engineering trivia is that the “standard” traffic control signals were not standardized for many years. They did inherit from the railroad system, but — mostly — the people working on traffic control were a tightly knit bunch, and they valued uniformity and not confusing the public. For example, in 1923, there were well under 1,000 traffic lights in the United States. Someone realized that 10% of drivers were color blind, seeing both green and red as a gray color. The proposal to change to yellow and blue was quickly dismissed.

Not that there were no deviations or attempts at deviations. They were just short-lived. Famously, an Irish neighborhood in Syracuse, New York, defaced traffic lights because red on top of green was seen as a symbol of British domination over Ireland. Authorities briefly switched the lights in those areas. China’s Red Guard proposed making lights in that country so that the red light indicated “go,” but the government rejected the proposal. Of course, these days, there are standards at the national and international levels.

Of Course, There’s More

If you want a higher-level history of traffic lights — including the yellow light — check out the video from [The History Guy] below. If you want to know what the yellow light really means, check out the clip from the TV show Taxi below that. Want a peek inside?  Then there are always traffic circles.

Featured image: “Traffic Light Tree, Canary Wharf London” by Martin Pearce

58 thoughts on “Tech In Plain Sight: What Does A Yellow Light Mean?

  1. Eyes scan article title.
    Brain hears “SLOW DOWN”. . .

    Not disappointed to see the reference in the writeup. :)

    1. Today I realized I had never seen Christopher Lloyd when he looked younger than Doc Brown and to be honest until he started talking I didn’t recognize him. Soon as he did though I could see it.

      1. Reverend Jim AKA Jim Ignatowski AKA Iggy was a great character portrayal by Lloyd.
        I haven’t seen the actual show Taxi in a long time. I wonder if it holds up. I was much younger then.

        I’ve seen the driver’s test outtake numerous times and it holds up for me. I don’t know if it would be so funny if I weren’t already familiar with the character.

        It took a bit of watching the Doc Brown character before I could *not* see Rev. Jim. I’m unsure if I ever completely rid my brain of the association.

        1. “I wonder if it holds up”

          It does for me. I’ve got the entire five season series on DVD (and it’s unusually cheap at only $28).

          When I saw him as a Klingon in one of the Star Trek films, I just couldn’t get Iggy out of my mind.

          Trivia: He showed up for the Taxi audition for the part in exactly what he wears on the show. The ultra-faded denim jacket was found caught in a bush on a friend’s ranch, undoubtedly blown there by the wind.

    2. It’s funny, but it’s also wrong. Yellow light actually means STOP. Specifically, it means stop unless you are unable to do so safely. Red pretty much means stop at all costs. If you just slow down, you’re really interfering with traffic. You only need to slow down to watch for unpredictable events, like around construction or children. Stop lights are there specifically to make traffic predictable. There’s actually a Vienna convention for this:

      The weak enforcement of yellow means we in the US have the worst of all worlds — a long yellow light to let green traffic clear, followed by a long 4-way red to let all the traffic violating the yellow rule to clear. The long delays just reinforce the value of “running” a yellow, thus the yellows and 4-way reds keep getting longer.

      I really don’t understand why we have yellow at all. As a driver, all I can do is go or stop. Having a “maybe” signal in the middle doesn’t help anybody. And since “drive safely” is pretty much the #1 rule of driving, then “stop if safe” is really what red means, and yellow adds nothing. Maybe flash the red a few times just to catch drivers eyes, then use the 4-way red to clear the intersection.

      1. Maybe one reason we need the yellow because it makes it easy for police to prove that you were acting unsafely while running a red light (i.e., you always are acting unsafely, because the yellow gave you enough time to stop). If we didn’t have yellow, they’d have to record the exact timing of your entry into the intersection to prove you were unsafe.

      2. With smart traffic lights, having no yellow means that you can suddenly find yourself 5 meters from the intersection when the light decides to turn red. They can switch states at unpredictable times when the system fails to detect an approaching vehicle, so the yellow light acts as a warning.

        In doing so, the meaning of “yellow” has become “this light might turn red OR green as you approach this intersection, so slow down.”

      3. I mean, they could start the red before starting the next green. But by using the yellow, people know the green is *about* to start but *hasn’t* started, so they need to clear the intersection in an orderly fashion rather than by any means available.

        As vehicles have changed, the distance and time at which it’s technically possible for most cars to stop from a given speed has shrunk, although for some vehicles or when the road is slippery, it’s almost as long as ever. It would be a bit much to expect someone to perform a emergency/panic stop on yellow just because it’s “safe” to stop. (Where safe means no injury, no damage, and stopping outside the intersection.) They could have just calmly proceeded through the intersection instead.

        It’s kind of like evacuating versus calmly exiting somewhere. There’s a *lot* of range between normal acceleration and deceleration versus maximum. It’d be bad if we just had two buttons on the floor instead of pedals, and choosing to go meant flooring it and choosing to stop meant mashing the brakes, with no in-between. A signal for “calmly get out of the way” is fine. It’s like when you see a slow down signal, or a move over signal, or a proceed with extra caution signal, etc – more information can be useful.

  2. “They were just short-lived. Famously, an Irish neighborhood in Syracuse, New York, defaced traffic lights because red on top of green was seen as a symbol of British domination over Ireland. Authorities briefly switched the lights in those areas. ”

    I’ve seen sideways lights as well. Interestingly enough color-blindness and being vertical.

    1. I mean, that’s the joke, right?

      Alien visits earth, and speeds through a traffic light and gets pulled over.
      Cop asks, “What were you thinking!?” Alien responds with, “I’ve been watching you humans. You drive normally on a green light, stop on a red light, and on the yellow, you drive as fast as you possibly can.”

      It’s an old, old joke.

      1. Simpsons did it too, when the local Mensa chapter took over running Springfield.
        “We have analysed traffic patterns and discovered drivers move the fastest through yellow lights. So now we just have the red and the yellow”

      2. There’s a variation on it too.

        For pedestrians if the sign says “Don’t Walk” that means you only have enough time to cross the intersection if you run, otherwise you can walk.

        Forgot which TV show it was, involving two out-of-state backpackers.

  3. >The answer was to change the lights in lockstep so that a vehicle going at some speed would, in theory, never have to stop after making one green light.

    I’ve always been curious about this because it only works for one direction of traffic, right? Cars going the opposite direction on the same street get the worst possible sequence of lights for traffic flow rate, and it also pre-empts all cross-traffic, so it seems to me like this can at best help 50% and at worst 25% of the traffic, while hurting all the other traffic.

    1. For a first approximation (assuming square blocks, traffic light at each corner,) make each traffic light operate 180 degrees out of phase with its immediate neigbours. A speed of 2 blocks per light in any direction should see nothing but green.

      This falls apart in the real world with traffic jams, pedestrians, turning vehicles, rectangular blocks, etc. but it is a starting point.

      1. Back in the day, it was possible to synch the lights for both sirections as I have experienced it, notably on the Mc Carran loop in Reno nv. Catch the first one mid green then do 2mph under the limit all the way around town until you get tired of it. They upgraded it to modern…….

    2. You’re not wrong. Some places try to figure out which direction has more traffic at different times of day, and switch the direction of the cycling. This is usually for something like the morning rush going one way and the evening rush going the other. Only one direction at a time gets the benefit, as you say, but the one with more traffic usually gets it.

    3. This is generally used on one way streets in downtowns or major cities. For instance the downtown in the city where I live has signs posted the lights are timed for 25mph. If you do the speed limit you’ll get greens all the way through. Go slow or fast you’re gonna catch a red.

    1. Well with two lights you could do Green for go, Blinking Red for “stop if possible” Solid Red for “STOP” and (like we used to do in Ontario) Blinking Green for advanced left turn.

  4. From the very outstanding 1984 film, Starman, starring Jeff Bridges and Karen Allen, when Allen considers letting Bridges drive for the first time (and Bridges explains that he’s been watching her drive, so he knows how…) :

    (Allen) “…What does a red light mean?”
    (Bridges) “Stop”.
    (Allen) “OK; what does a green light mean?”
    (Bridges) “Go.”
    (Allen) “Good. What does a yellow light mean?”
    (Bridges) “Go faster.”

    1. That channel can’t be recommended enough. As a person using all the different modes of transportation in the city, I applaud the shift away from cars as the answer to everything. <3

      1. If only public transportation wasn’t such a corrupt mess. Even with 2 € per liter gasoline prices, I can easily buy 50 km worth of fuel with the money for a trip downtown and back. The trick is that you have to be subsidized (student/unemployed/pensioner/child) to get reasonably priced transit.

        Without a car, my daily travel range would shrink by a factor of 10 in the winter. Not because it’s difficult, but because it’s too unpleasant – literally painful and difficult to breathe – in the freezing cold.

    2. Okay, that was a nice example of how things can be…. but certainly not an example of how things really are in the Netherlands. Rest assured that there are still PLENTY of traffic light with absolutely no intelligence at all. Or perhaps some of the light have become so intelligent that they’ve become bored and decide to annoy traffic on purpose (well it sometimes just feels like that).

      If I’d get a euro for every time I needed to wait a long time for a completely empty crossing, I did not need to ride a bike because I could afford a taxi. Don’t get me wrong, the infrastructure in the Netherlands is pretty great, especially when riding bikes. But it just isn’t as perfect as suggested in the video, which seems to ignore that the Netherlands is a lot bigger then just Amsterdam.

      1. Oh, good point!

        It does say that traffic lights are not the same everywhere in the Netherlands, and that there are some annoying ones, but not sure if it was on this or another of his videos.

        I wish someday all traffic lights worldwide will be like the good, intelligent ones shown there.

        Meanwhile, here is the relevant XKCD:

  5. The amount of time a light is yellow is really variable too. Longer yellows result in less traffic accidents apparently. Shorter yellows means people speeding through to make the light have less time before someone going the other way pulls out because green means go!!! and doesn’t simply look. I’m a conservative (ok old…) driver but the number of times I’ve had a passenger tell me “go!” when light turns green instead of what would be better “light is green” is quite a few. Had I just gone I would have either gotten T-boned at warp speed or ran over someone in a crosswalk.

    1. When I was in London (briefly) 1987, I recall the yellow/amber light lit up before the green light came on, as well as before the the red light.
      But I don’t recall that happening when I was there a few years ago.

      1. Red + amber means to get ready to go. Supposedly because majority of drivers uses manual transmission, the yellow light gives them time to shift gear and I dunno if it worked well or if it only encouraged driver to go vrooom on green.

        Not that it matters, I hear more and more people are using automatic transmission these days.

        1. Thanks,
          what I remember most was being in the crosswalk and seeing the amber/yellow light come on and Bentley taxis revving up their engines, I feared for my life!

      2. I may be incorrect but when I was in Germany in the 90’s, the yellow lights came on at the same time for intersections. So if no one was around you could go already but if there was traffic it was more of a “get ready, green is coming” situation. I haven’t traveled there in the last 20 years so I may be wrong entirely.

  6. My father, a lawyer, was defending his client who entered the intersection on a yellow. The other driver got a green light and drove into the client. The other driver’s defense: “I had the right of way.” The judge stated, “ A green light is not a command “…

    1. Both of those statements are true. If you’re already going on green and someone cuts in front of you, I don’t see why you would be at fault.

      People abuse rules like that all the time. For instance bus drivers around here. When they signal left, it means they’re going to pull in front of you no matter what, even if you’re already past their rear wheel, so if you see a bus at a stop with no indicators on, you have to assume they’ll wedge in the second you reach them. If you collide, it was your fault because they were signaling left and you were coming from “behind”.

  7. I was in Cairo, Egypt in the 1980s. It was common for drivers to completely ignore red lights. So at busy interchanges they posted policemen armed with huge baseball bats to whack any car that shot a red. None of the taxis had a straight panel on them!

    1. Should glue a few rock pieces to the top. If a car with multiple dents are seen blowing red light, whack the glass instead. The rock like crushed concrete or broken porcelain often have points that will easily shatter glasses.

  8. Here in Japan, a green light is actually called blue, for two reasons.
    Historically the green light was a more blue-green colour (you can still see older lights that are very blue compared to what is seen in the west), and also due to the cultural zeitgeist regarding colour perception.
    Green (midori) is a relatively new word and used for proper green things, the blueish green of the traffic lights is considered to be blue.

    Also, from my observations while driving here, there’s at least 2 seconds of green in every red light…..

  9. In germany there’s two different types of yellow: The “yellow after green” which is only the yellow light coming on and means *clear the intersection*, and the “red+yellow” where both those lights are lit, indicating that it’s about to turn green but somebody may still be on the intersection, doing a U-turn or the likes so don’t just dumbly floor it, watch out for folks.

  10. We had a Red/Yellow/Green /Yellow/Red light until the early 80’s. That was an oddity when all the other lights were just Red/Yellow/Green.

    As a cyclist I note that many lights change the green the moment the cross direction changes to red. A few lights have a delay so then a red on all directions. I still check left and RIGHT (yes, I’ve seen it) before proceeding through the light.

  11. One of the first “lights” was a clock dial with a hand showing how much rather than just sudden states. This makes the most sense. Our downtown lights have seconds counting down for the walk signals which adds time info on the rest of the light signal. Analog info is quicker to read and interpret than numbers though, but on the same street some change at 0 but some change several seconds later.

  12. “Of course, back then, the timing mechanisms to pull this off were finicky. ”

    Nowadays, of course, things are better – they’re just deliberately programmed to make you stop at every light no matter what your speed is;)

  13. So how do you factor for the people pawing a cell phone and not moving until the light is going back to yellow? A real treat when you’re in a left turn line and all of the cars in front are glowing like a reactor pool from the cellphones, so you know the delay is coming.

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