Willis Lamm's
Traffic Signal Collection

  History of Traffic Signal Design
Part Six

  Evolution of Modern Signals

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Some of the early traffic signal designs that survived many decades weren't much different than William Potts' signal. They consisted of three chambers in which a single bulb in each chamber lit up all four directions. In most cases a simple electromechanical controller in the bottom of the signal controlled the signal's displays.

As with Potts' signal this design required the signal to display red - yellow - green from top to bottom on the main street, and green - yellow - red from top to bottom on the cross street. The signal also had to display a yellow interval when changing from red to green as well as from changing from green to red.

However at that time there had not been any standards adopted that specified as to which location each color was placed and many jurisdictions used yellow displays when lights changed from red to green to warn pedestrians in the crosswalk that the light was changing and for motorists to get their stickshift cars in gear. A few of these old signals could be seen in service as late as the 1970s.

There were also still advocates of two-color signals and thousands of these signals were placed into service. Two color supporters cited simplicity and cost. Most two-color signals warned motorists of impending changes in one of two ways. Some signals went dark for three seconds during signal changes (a scary prospect nowadays) or they overlapped red over green for three seconds to advise motorists that the light was changing. Last time we checked there were still a small number of two-color signals still in service in New York City.

Ultimately the prevailing wisdom was that yellow displays commanded greater attention from motorists when signals changed. National standards called for three-color signals with red placed on the top and green on the bottom (or red on the left and green on the right for horizontally mounted signals,) and these are the configurations that nearly every American traffic signal that remains in service today is based on.

Continue to Many Colors

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