Stoplights are red, yellow, and green because, early on, traffic officials copied the code system railroad engineers devised for track systems controlling the trains.

The goal of the railroad engineers in crafting this code was to prevent often fatal train collisions by giving the trains advance warning. Therefore, they did not take their task lightly in selecting the symbolic colors for the signals.

Red, the color of blood, proved a logical choice for the stop signal, as for thousands of years, this color indicated danger. The color alone, railroad engineers reasoned, should give people cause to pause, to abide by the signal, and to stop or suffer the consequences of death and destruction.

Engineers used the trial-and-error method in selecting the other colors. The first trial in the 1830s, which used green for the caution signal and clear for the go signal, failed miserably. Clear as a choice for the go signal varied only slightly from the light cast from a typical streetlamp or from the glare of the sunlight, and thus could quite easily be mistaken for the go signal… after the fact.

This failure prompted the railroad engineers to alter their color selection to red for stop, yellow for caution, and green for go. Traffic engineers, either lacking in ingenuity or a work ethic, scurried off with this system of color coding and instituted the very first electric stoplight in Cleveland, Ohio, in 1914. The first signal did not include the color yellow for caution but was added within a few years. Railroad engineers, not traffic engineers, should be credited for the lives saved in the interim by their system of coding warning signals red, yellow, and green.

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