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?
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:
As traffic signaling evolved the use of lights or flags continues. From the same article:
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, there 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.
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):
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):
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".
... or go someplace like Syracuse, New York:
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.
There have been a couple of passing mentions of trains, where the constraints are interestingly different.
Train signals (in the UK at least, but I don't think they're massively different elsewhere) are also red-amber-green (or on higher-speed lines they're red-amber-doubleamber-green), and they're not consistently one way up; so sometimes the red is at the top, and sometimes at the bottom. This is partly, I understand, because a train driver may have to react to them from further away than a car driver would react to a traffic light, so that they wouldn't necessarily be able to distinguish them based on position, and there is therefore no utility in requiring them to be in a consistent position. It follows (I recall, but can't find a link to corroborate) that train drivers can't be colour-blind.
The order isn't random/whimsical, though. The lights are ordered so that the red light is nearest to the eye-line of the driver (see that Wikipedia link for lots more detail).
And just by the way, the reason the four-light sequence is red, amber, and then double-amber (whereas you might initially guess that double-amber is ‘more cautious’ than single amber), is so that it fails safe: if an amber light fails then the signal displays the ‘more cautious’ single amber.
Relevant to the original question, there are indeed now LED-based lights which have all the colours in a single housing, rather than one lamp per colour.
This question has several parts (at least implicitly).
The answers to most of these questions arise from the historical development of the system.
Many standards are the result of revision and refinement of a largely arbitrary precursor. It is often expensive (in human lives as well as financially) to change safety-critical conventions that have been learned by large numbers of people, even if there is a theoretically better system.
Road traffic signals are a development of railroad signals. (BBC)
Some early railroad signals do have several different coloured lights appear at the same single position. They had one lamp. A mechanical arrangement moved coloured lenses in front of the fixed lamp.
Railroad signals were initially semaphore signals where the position of a signalling arm indicated whether it was safe to proceed.
An important point is that, as a form of fail-safe, if the arm raising mechanism fails, gravity will drop the arm to the "stop" position.
OK so road usage followed railway usage, this begs the question - why were those colours chosen by railways? The answer may be that the railways followed established maritime conventions. Ships had red and green lamps on their sides so that other ships could tell which direction the observed ship was travelling. There had to be a convention about which of two converging ships should give way. The convention was (apparently) that if you see a red light, you are required to give way. Another explanation was simply that red, the colour of blood, has long been associated with danger and so became associated with the need to stop.
Railroad tracks are divided into sections. Following trains are provided with an indication that a train is on the section ahead (stop/red), has left that section and is in the section ahead of that (caution/yellow) or is much further away (proceed/green).
For night time use, a lamp was added and the motion of the signal arm brought different coloured lenses in front of the lamp.
Order: Failsafe at Red
You can see that with this arrangement, the "stop" indication is at the top position when the line of lenses are vertical. This is a consequence of the fail-safe arm arrangement discussed earlier.
Why separate positions?
With road traffic signals, the semaphore arm was eventually entirely replaced by the lights. The mechanical elements were eliminated, perhaps because they required more maintenance, and three electric lamps were then the simplest signalling system. The colour, number and physical arrangement obviously followed the previous system to avoid confusion.
It is obviously an advantage, where red-green colour-blindness is common, to have a positional reinforcement for the signal meaning.
The accepted answer contains some good information about the history of traffic lights, but it is dead wrong about the color blindness issue.
First, some facts:
Finally, from the FHWA: (emphasis added)
In other words, color blindness has long been a consideration in the design of traffic lights, and it's one of the reasons we keep three separate fixtures arranged in a particular order today.
A few answers have made the point that it would be difficult for colour blind users to see the signal. But the truth is, we are all colour blind at least in our peripheral vision.
From your eye's fixation point, at 30 degrees you start to lose trichromatic vision. In other words, you start to lose the ability to differentiate colour. At 50 degrees, you are essentially colour blind. The colours that you see right now in your brain aren't actually 'seen' -- your brain has coloured in the gaps for you.
However, your eyes are good at detecting movement in these extremes. Our response is to saccade to movements in our visual field so we can see them in more detail.
With a single-bulb system, we might be unable to see a change from green to red because, in our peripheral vision, nothing has changed. So it is not only colour that is essential to the signal, but the movement of bulb position which prompts us to fixate on the signal.
One reason for the three lights, in the UK at least, is that you have many states. Two of the lights can be on at one time, i.e. red and amber signalling that you should get ready to go, where as a single amber light would mean get ready to stop. The concept you link to doesn't cover this use case.
There are also historic reasons for the 3 light traffic signal, they used to be operated my humans and originally were gas burning lamps (briefly) - this was before LEDs even existed, so they needed 3 states. Traffic lights all started on the railway, the amber light was a signal to slow down and give the train driver advanced notice it's state was about to change.
With the specific implementation you mentioned, I can also see a few other use cases (on UK roads) that would be rather difficult to implement - traffic signals do more than tell people when to stop and go.
A few examples of other uses are:
@Steve Wortham also makes a great point that a moving light (much like something animating on a web page) is more likely to catch your attention than a single light changing colour, especially if you are not paying complete attention.
Another reason why we may not have changed the design so much could be that - if we were to change the traffic light system, it would cost a HUGE amount of money for a government to not only implement the lights themselves but to also teach and educate road users of this new system.
I think a big reason must be that it's easier to see that the light changed (even out of your peripheral vision) when one light goes off and another goes on in another position. The fact that the position AND color of the light is changing makes it easier to notice.
My best guess would be that if there was poor visibility or some sort of color blindness problem, one could identify what to do by seeing which light was on/or off.
With 1 light one might not be able to tell what the color is and that could cause problems.
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