Why do we use Red/Yellow/Green for traffic lights?
At this time Red meant stop and Green meant caution, and this was brought on from the railroad system which got it off from the standards adopted in the electrical industry.
Well so far so good, until people got tired of using manually operated traffic signals (I am guessing the people who operated them). There were also gas lantern related accidents, such as explosions which got them thinking towards the modern traffic lighting systems.
Apart from these, we do have to mention the emotions attached. Red indicates danger or warning, and hence a perfect choice for traffic to “Stop“. Green has a more serene feel to it and hence the obvious choice of “Go”.
Finally there was this need for a color that could indicate warning, which could be a part of every sign post and traffic signals. The choice went to Yellow or Amber, for the sole reason that it contrasted in color to Red and Green. They screwed up here with the color-emotion thing as Yellow indicates warmth and comfort (I am pointing to the Sun), but hey, now Yellow is used throughout to indicate warning/watch out/ be cautious.
I am guessing these colors are going to stick for a long time, so it was actually nice to know a little about them, maybe you could use this info to get a green signal on your pick up line 😛