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A friend of mine who is (completely) red/green color blind once told me that he can distinguish the green traffic light from the yellow and red one despite his color blindness and that he thinks to be able to remember having heard that this is not an accident. Is there some law or guideline for it or is it just niceness on the traffic light manufacturer's part and what exact green is it?

In case it matters, I'm living in Germany.

(Sorry, if this is the wrong SE; it seemed like the most appropriate one.)

Ernest Bredar
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  • There really is no such thing as an exact green. It depends on how you are measuring it. Different people will see the same color differently. – Eric S Nov 20 '22 at 17:14
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    in my city the color varies between different models of the traffic lights – jsotola Nov 20 '22 at 18:42
  • With "exact green" I mean something like wave length or important characteristics that a "green" color needs to posses, so that a color blind person can distinguish it. – Ernest Bredar Nov 20 '22 at 18:49
  • @jsotola Are you living in Germany? – Ernest Bredar Nov 20 '22 at 18:50
  • @ErnestBredar no I am not – jsotola Nov 20 '22 at 18:51
  • @jsotola Then I think this easily explains that phenomenon :) . Sad to see that not all parts of the world take accessibility serious. – Ernest Bredar Nov 20 '22 at 18:56
  • Something else to consider. Not all forms of red-green color blindness are the same. People with a different forms of red-green color blindness might see things differently to him. It would be interesting to know how your friend perceives the difference between red & green traffic lights & does the same distinction occur between other objects colored red or green. Is one darker or brighter than the other or is there a subtle change of hue? Is the difference the same during the day & night? – Fred Nov 20 '22 at 19:42
  • WIth the exception of lasers, the green light will have a spectrum, not a single wavelength. That spectrum will vary depending on the type of light (incandescent vs LED). The perception of that spectrum will depend on the observer's eyes. – Eric S Nov 20 '22 at 19:42
  • Accessibility was taken care of a long time ago in most jurisdictions, by also using a standard arrangement of positions for the lights. The green light is always on the bottom when mounted vertically, and on the right when mounted horizontally. Even a person with no color vision at all can read them correctly. – Dave Tweed Nov 21 '22 at 04:44
  • @DaveTweed: As a red-green color blind friend of mine admitted to me a long time ago, knowing the position of the different colored lights works well during daylight, but it an be problematic during the night, particularly when he was driving & he wasn't supposed to have a driver's license due to his color blindness. – Fred Nov 21 '22 at 07:06

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Red-green color blindness is a reduced ability to see color. It's not total. Traffic lights are not all the same color. LED and incandescent traffic lights look slightly different.

stretch
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  • There appears to be a translation problem. With "color blind" I (and he) mean complete inability to distinguish red and green. About your other point with the different colors: That just means that it appears to be either a more widespread phenomenon or that today here (quasi) all incandescent lights are phased out, not that my question is unanswerable. – Ernest Bredar Nov 21 '22 at 17:28