4-Way Traffic Signal
The first four-way, three-color traffic light was created by police officer William Potts in Detroit, Michigan in 1920.
In 1920 William Potts, a Detroit Traffic Police Superintendent, designed the first 4-way three-color traffic control device that is recognized as the basis for the modern traffic signal. Potts built what was basically a rectangular box that was divided into three stacked chambers. Each chamber was illuminated by a single bulb. Railroad signal lenses were affixed on each side of each chamber and the signal was suspended from a cable so it could be more readily seen by traffic.
While the few traffic signals that already existed displayed only red and green colors, Potts' added a yellow "caution" interval to warn motorists that the traffic signal was changing. This idea turned out to be significant.
Because the four lenses in each compartment of Potts' signal were lit by a single bulb, two sides of the signal facing the main road had to be provided with red on the top section and green on the bottom while the cross street had to have green on the top and red on the bottom. Since at the time there were no standards as to where different colors were located, the positions of the red and green lenses were not confusing to motorists.
Potts' basic design was the basis for decades of fixed four-way signals. The compartmentalized "three bulb" design that required green lenses on the top and red lenses on the bottom for cross streets was used by some manufacturers through the 1940s, although the optics used in these later signals were much improved over Potts' original design.