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I remember when I was a kid. Each traffic lights had different shapes. Red = Square, Yellow = Triangle, Green = Circle. Now all the traffic lights that I see are round.

Is there a reason why they don't need to have different shape any more?
Isn't it harder for people that have trouble with colors?

Before

Now

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6  
Fun fact: green and red are easily confused when you have various types of colour blindness, so the green also contains blue light, making it easier to discern! –  Matt Sep 25 '12 at 4:34
    
I don't think I've ever seen traffic lights with those shapes (I'm in California). When and where did these exist? –  Keith Thompson Sep 25 '12 at 8:46
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@Keith Quebec/Canada –  the_lotus Sep 25 '12 at 11:59
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3 Answers

up vote 9 down vote accepted
  1. Its more expensive to manufacture and maintain different shaped lights.
  2. Round hoods on each light provide better shielding against sunlight and better protection against rain (quicker dissipation) and wind by being more aerodynamic (less resistance).
  3. Parts are more interchangeable. Examples: One hood fits on all the lights. The clear tempered glass cover on the red light also fits on the other lights. Clear glass is less expensive than colored glass.
  4. Even color-blind people know where the red, yellow and green lights are located.
  5. (not relevant to this question) Each light is now made up of a matrix of small LED's. If one LED fails, the rest keep on working. As we know, LED's are brighter, more energy-efficient and last much longer than regular bulbs.
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#5 was very interesting to know –  the_lotus Sep 25 '12 at 12:01
    
Interesting points. Do you have any links to the various sources where you've found this information? –  JonW Sep 25 '12 at 12:44
    
No links, these are my own common sense opinions. –  FrankComputerAtYmailDotCom Sep 25 '12 at 13:05
    
Thanks for accepting. Although I did not obtain this info from any source, my common sense made these assumptions. –  FrankComputerAtYmailDotCom Sep 29 '12 at 22:47
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I think your basic assumption that they are all the same now is wrong, or at least very locally biased. Where I'm from, there are plenty of different shapes in use. Most notable are the pedestrian lights: Basic green light Basic red light

However, there are many variations, even animating ones: Animating pedestrian light

and ones with different count-downs: Light with countdown 1 Light with countdown 2

There are also special versions for bikes: Bike light green 1 Bike light green 2 Bike light red

Then, of course, there are also the arrows-type lights, in use for cars: Left red sign for cars Set of arrow-type lights for cars

To conclude: it is hardly the case that there are only circles in use these days.

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I live in Canada, here's an example: dec-ced.gc.ca/docs/leddartech2.jpg –  the_lotus Sep 25 '12 at 12:02
    
Great examples! –  Christian Sep 25 '12 at 13:15
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In the United States, the U.S. Dept. of Transportation has uniform standards for traffic lights, and all other transportation devices, signs, etc. See (mutcd.fhwa.dot.gov/pdfs/2009r1r2/mutcd09r1r2editionhl.pdf) –  FrankComputerAtYmailDotCom Sep 25 '12 at 14:01
    
By The Mall, in London, there are pedestrian crossing lights and horse-rider crossing lights. Seriously. –  TRiG Nov 26 '12 at 19:18
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From a great distance, like traffic lights are often viewed from, it is hard to discern shapes--especially square vs. circle.

Glare and flaring tend to reduce the light to a blur anyway (except for those have 20/20 vision and perfectly clear eyeballs viewing them through a clean windshield on a clear day). The color becomes the predominant signal--and therefore it is quantity of light that becomes important.

The above examples all cut down on the amount of light leaving the signal-- especially the "bicycle" and "pedestrian" shapes (which are appropriate for the slower-moving clientele they serve, but would blur into "dim green"/"dim red" for car drivers, especially in the rain.

As @Matt mentioned in a comment to the question, the 'green' traffic lights often have a strong blue component so that RG-colorblind people have a better chance of distinguishing them, the major argument against relying solely on color.

Position provides an additional reinforcement, and a top-down arrangement is clearer than a left-right (or is it right-left?) arrangement, which was the other notably confusing thing about shaped Canadian traffic lights.

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The arrows for cars do serve a purpose. They are used here only in situations where there are different sorting lanes for cars turning in different directions, and these may be allowed to go at different times. For instance, turning right is allowed now, but turning left is not. Also note that the animation might actually better catch the eye than a solid light. I know it works like that in computer UI's where animations are really quite powerful at drawing the eye. That may make up for the reduction in the amount of light. –  André Sep 25 '12 at 15:44
    
Arrows definitely serve a purpose, and tend to be engaged at shorter distances than the round balls (because you tend to slow down to take a curve). Animation may draw the eye--especially blinking--and this may not be preferable, given the confusion of signals in front of a driver. We reserve blinking for specific meanings tied to the color of the lights; an animation that is confusable with blinking better have a similar meaning... –  Alex Feinman Sep 25 '12 at 16:27
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Note that the animated lights are used for pedestrians only, which is the one you thought suffered from dimished light and, as you rightly stated, is much slower traffic. Note that animations are used for car drivers as well though: the flashing light on police cars or other services, and things like closed off lanes on roads that may be signalled using an animated arrow pointing towards the open lane(s) next to it. –  André Sep 26 '12 at 9:09
    
@André. There's also blinking amber lights at pedestrian crossings, which basically mean that cars are free to move if the pedestrian has cleared the crossing. –  TRiG Nov 26 '12 at 19:20
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