Why do traffic lights have three separate light bulbs instead of having one?

  • Recently, I checked out a concept design for a traffic light with only one light.

    Do you know the exact reason for having three lamps in a row, instead of one like in this concept?

    Because of position. What if someone is color-blind? Or uses a seeing-eye dog? The position of the lights is *always* the same. Red is always on the top of vertical lights, and on the left on horizontal lights.

    It's worth noting here that the UK *railways* **do** now use light signals where red, green and yellow can all use the same light (using LED technology).

    @AndrewLeach - are those signals facing train drivers and other railway staff? I would hypothesise one can't get a job driving a train or maintaining tracks if one is visually impared

    Yes: they are replacing the more traditional multi-light signals when replacements come round, or for new installations. And yes, you can't be a driver or signalman if you're colour-blind. Not sure about train guards, though.

    UK railways also use lights with red at the top instead of the bottom (unlike road lights that are always one way around). They are *usually* the one way around (I forget which is most normal of the two), but anywhere where there is a chance of the red being obscured by environmental factors they get swapped. They also have the concept of "double yellow" on high speed lines, so there is extra warning before a red.

    @RocketHazmat, agree on color-blindness. But seeing-eye dogs don't use stop lights; not by position, not by color. That's an urban legend probably fostered by TV and movies. The handler of the dog (the blind person) has to determine when it's safe to cross based on either an audible crossing signal or the sound of traffic patterns, and then command the dog to go forward. The best a dog can do is refuse to go and/or cross in front of the handler and block the way, which they are trained to do when they perceive an imminent danger.

    @AndrewLeach In the Netherlands bus lanes and tram lines with their own traffic lights which use a single set of LEDs still have different colours _and_ different layouts: http://en.wikipedia.org/wiki/File:Public_transportation_traffic_lights_in_NL_and_BE.svg. In fact, even for colour blind drivers and with any one of the LEDs failing the light is still unambiguous. I don't know what the UK railway lights look like, but I presume most systems in the world are designed like this.

    Not worth a separate answer, but I'd also venture a guess that it might simply be cheaper to build and maintain. It probably wouldn't be a major reason, but it's just another reason on top of the other very good suggestions here.

    @Luaan how can be a three unit lamp will be less expensive than from one unit. I think that the color-blindness is not the main issue for having such lamp but more it has a technology constraint in early stages of product definition. Frequency of blinking can be also added to LED powered ones.

    @Abektes It's incredibly simple - all the hardware is primitive, and it's connected in a very simple fashion. This means less opportunities for failure, and more simplicity in exchanging failed parts. That said, it's just a guess. It may very well be that we've progressed to a point where the more complicated solution will also be less expensive. Engineers do tend to pick the simplest solution possible (unless you're watching Star Trek's engineers). The reliability of each of the parts has to be much better to offset the fact that there's more parts total.

    @Luaan It is a bit off-topic to the real question but I think that it is not that simple especially if you are getting the lifecycle and distribution of the product into the equation. Each connection of the hardware indeed increases the opportunities for failure. From technology and engineering point of view, disco lights can be a good product for comparison. When you check the trends and prices of the products, you will have a better opinion about having RGB solutions is actually a better deal than separated ones..

    "The position of the lights is always the same. Red is always on the top of vertical lights, and on the left on horizontal lights." Nope. It varies by state (in the US). Some have red on the bottom or right. This can be very confusing if you pay more attention to position than color.

    @PhilPerry: While there may exist lights that don't have red at the top/left, they are technically illegal. There is a federal standard that mandates the arrangement of the colors.,

    I wanted to point out that positioning and number of bulbs are independent, you can easily have a pair of mirrors, and open/close a shutter in front of the windows, to show each light in a different position even with a single bulb. I think early traffic lights were like this because it's the simplest design, and later ones never changed because "don't fix what isn't broken". Also three light are less likely to fail all at once.

    @RocketHazmat, seeing-eye dogs do not look at stop lights, the blind person tells the dog when to cross based on auditory clues.

    @josh3736, there may indeed be a Federal standard, _but it is not legally binding on states_. It's only a recommendation. As such, there are states which continue to reverse the color order. There is a statement that "no strobes shall be on or near the signal light" -- New York frequently puts a white strobe around or across the red, to get the attention of approaching drivers. Every state makes its own rules, and while they're generally consistent, they're not exactly the same.

    Another interesting point about the UK, is that the traffic lights have a valid state where two lights are lit at once. This would be impossible with a single light configuration. (Lights show amber and red at once to show they are about to change)

    A single light with different colors for each state (go, stop, etc.) might be difficult for people with color blindness if it is always the same shape, but it would be pretty easy to display shapes when you're using a grid of multi color leds. So green could be an arrow shape, red a cross shape and a warning could be an exclamation mark. That way it wouldn't make any difference if you were color blind or not. Maybe it would even be better since it is more descriptive than three similarly shaped lights.

    Can you actually think of any advantage of a single 3 state light over 3 lights? I can't. Also, whoever put together that concept image highlighted a big problem, a very small bevel to the light and a green background would make it very hard to pick out, there's a reason current lights often have a thick black background.

    As a red-green colourblind individual this would create a living hell for me.

    I thought the UX SE network was for web UX only... I take it not?

  • Historically

    It is a simple fact that traffic signals were introduced in 1868. From The Origin of the Green, Yellow and Red Color Scheme for Traffic Lights:

    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 similar 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 to indicate the same type of thing, again traditional to the railroads.

    As traffic signaling evolved the use of lights or flags continues. From the same article:

    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.

    In 1920, there would be little option other then to provide 3 independent lights to show 3 different colors. While other options might have existed, their complexity may have limited their adoption.

    Technology & Economics

    To specifically call out the reason above: because of technology. Today's technology allows for amazing things, but to think that a single-light traffic signal would be both technologically and economically feasible in 1870, 1920, 1980 (or even 1990) is a very hard argument. You can argue the ability in the later years, but can you argue the economics?

    Technologically it is very easy to put 3 lights in a row and tell them when to turn on or off. Economically it is very easy to put 3 lights in a row and tell them when to turn on or off... and fix them when they break.

    The technology to do it "this way" vs "that way" is simply a lot cheaper.

    Color Blindness

    EDIT: Some comments on this answer were not taking this section in the context of historical precedence. I have attempted to clarify the points being made, in the greater context of my final answer at the end. I have also corrected some information on standards which where inaccurate.

    While "because of color blindness" is a popular belief of the historical, this is simply false. Looking back at the historical reasons, for the time when traffic signals were being developed, the idea of catering to a rarely known condition would be unheard of.

    A two color light, based on the railways, was first introduced in 1912 (per Wikipedia):

    The first electric traffic light was developed in 1912... It had two colors, red and green, and a buzzer.

    Later came William L. Pott's three-way traffic signal (mentioned above). Many other different types of signals also existed as the traffic signal continued to develop. This would obviously create confusion for many, so standards were put in place in 1935 (again, from The Origin of the Green, Yellow and Red Color Scheme for Traffic Lights):

    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. So, in 1935, the Federal Highway Administration created “The Manual on Uniform Traffic Control Devices.” This document set uniform standards for all traffic signals and road signs.

    Note that these standards were put in place for the benefit of everyone! This was not an action taken on behalf of "color-vision deficient drivers". No mention of such a population is made in the 1935 Manual on Uniform Traffic Control Devices (p. 71).

    The idea that the order (Top = Red, Middle = Yellow, Bottom = Green) is for color blind individuals is quickly debunked when you search for "sideways traffic lights".

    enter image description here

    ... or go someplace like Syracuse, New York:

    enter image description here

    Order is common, and mostly followed, but standards are not always followed and the original development of the traffic signal pattern was not guided by the standards that exist today.

    There were also no standards in traffic light shading of the red/yellow/green colors - meaning one traffic light may use shades that allow certain color blind individuals to distinguish red vs. green, but another may not. While standards exist today, these did guide the use of these colors or their shades during early development.

    Color blindness does not mean you can not see color (except for the very rare "achromatopsia" condition) - it is possible to provide shades and luminosity that do work for the majority of color blind people.

    But it does help! Certainly the positional nature helps colorblind people, as long as they know what order the lights are in. This is very likely a reason why a single-light concept would be very difficult to implement, given today's attitudes and awareness of safety (for all drivers). However, the point here is that color blindness is not a reason why they were made that way in the first place.


    Historically they were made that way; improving over time until the technology to create and maintain them became less and less; creating a situation where the economics to change them are prohibitive.

    But... User Experience!

    Yeah, I know - it's a UX site. Sometimes things can't be explained in terms of UX. Sometimes things exist simply because they have existed that way for so long.

    Thank you, thank you, thank you for dissecting the color blindness argument. "Because color blindness" is the most overused UX justification I see on this site, often being dragged out by people who know nothing about it.

    According to this article the upside-down traffic light in Syracuse is a very special case, controversial, and accompanied by special warning signs. I don't think you can claim it as evidence that colorblindness is not a consideration here; it appears more likely to me that it *is* a consideration, and the officials in Syracuse consciously chose to disregard it. As for the sideways lights, I think you'll find that in such cases, red is always on the left.

    In Belgium the law explicitly (Dutch link) states that the light have to be 3 colored and red has to be on top and green at the bottom. Thus to change the lights they'd first have to change the law.

    It's worth mentioning that nearly 10% of men (and fewer but still quite a lot of women, 1% or so) have some degree of red-green colour blindness, and these people usually judge red from green based on relative brightness (which is very difficult with a shining light on black). So, even if it was not in reality a consideration, with it being so common, it should be!

    (Illuminated railway semaphores) [https://upload.wikimedia.org/wikipedia/commons/7/7b/Rail-semaphore-signal-Dave-F.jpg] **did** use a single lamp with moving filters - the historical example at the start of this answer was apparently based on that, so I think it would have more to do with a transition from 9electro-)mechanical to all electrical signalling.

    The sideways traffic lights seem to be commonplace in North America, but I have never seen one in all the European countries I have been to. In vertical traffic lights, the order seems to be invariably red - yellow - green from top to bottom. So, I see no reason to call that explanation "debunked" - even if color blindness was not the original reason, it may well be an additional point nowadays for why we keep sticking with the three vertical lights as they are.

    I just checked, and the German traffic laws explicitly state that in traffic lights, red is on top, yellow in the middle, and green below - like the with the Belgian laws cited by @ratchetfreak, the point about sideways traffic lights doesn't hold here.

    Regarding the color blindness section -- it is simply factually incorrect, and you don't actually cite any authoritative sources to back your assertions. In fact, color blindness **has** long been a consideration in the design of traffic lights. See this answer for more.

    @josh3736, while standards have been created concerning the positioning & color this was not done until 1935, for everyones benefit (not explicitly for the "color-vision deficient"); luminosity and chromaticity not even later. As I point out in my closing statement the positioning *does* help color blind individuals, but there *original* creation was *not guided* by the condition.

    There is another reason for more than one bulb: failing bulbs. When you have three separate bulbs in two out of three cases you know exactly what situation you are in (as one of the unbroken bulbs is on) and you can thus also spot the broken bulb and know that it would be on (because the others are off as well) - you would only have to wait at most one light change to know which bulb is broken (assuming that the chance of all three failing at the same time is slim).

    @O.R.Mapper, indeed color blindness is a consideration *today* but was not a defining factor when the 3-light system was first developed. Such considerations are absolutely why sticking with the 3-light layout is such a strong argument now. The answer is not saying the 3-light system does not take color blindness into account, it is saying that the design is not originally to cater to color blindness. I have updated the answer to hopefully be clearer.

    Many railroad signals used one lamp and moving filters to indicate different colors; such a design could be adapted to operate without electricity using a gaslight and either clockwork or manually-operated control cables. Switching electric lights was probably easier than moving things, but when traffic signals had moving arms, having moving filters as well would have posed no difficulty.

    I like the design of the conceptual traffic lights. I think it's pretty easy to resolve the "color blindness" problem if they just use some standard traffic icons for stop/go, which can be implemented with the LED array.

    @MarjanVenema But you're assuming the technology is the same, which it wouldn't be if we were building the new type of signal. A LED display is not a "bulb" and does not simply die all at once, unless there is some electrical problem (which would kill a whole array of standard traffic lights as well). If anything, the proposed system would be more reliable because it would tend to display more warning signs that it needs to be replaced before dying completely.

    @nmclean: doesn't matter. Even if the technology used would fail differently or less frequently, the argument stays the same as if you use only one display for all three colors, it can still fail and you get less information than when using one display per color.

    @MarjanVenema It absolutely does matter how the technology fails, because your whole argument is redundancy. The fact is, a 2D array of a tens of LEDs that each dim as they fail is FAR more redundant than a row of three bulbs that each die instantly.

    @nmclean: Of course, but that it is far more redundant doesn't make it infallible.

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Content dated before 7/24/2021 11:53 AM