The above light is made for quick easy identification by anyone who can see shapes, or colors. Particularly in the case of color blind people.
The 1st traffic light ever made exploded, killing the cop that was operating the lever to direct it's light in the right direction. Jan 2nd 1869 was the day, and natural gas vapor was the source of the flame that made the lights visible... no electricity, nothing but a torch behind a colored lens.
The first traffic light in Ireland (1890's) was placed near the oldest pub in Europe (900 AD) because the first car in Ireland was owned by the pubs frequent patron, and he was Ireland's first drunk driver.
Photo and much more info at http://en.wikipedia.org/wiki/Traffic_light
But the best idea in traffic lights is a light that has a time indication of when it will switch to green http://justacarguy.blogspot.com/2010/04/two-good-ideas-id-like-to-see-in-world.html