Potts became the "superintendent, signal person police" for the city of Detroit.
The old system of police directing traffic had become increasingly outmoded; two-color signals, with green and red lights, already existed, but they did not leave drivers sufficient time to stop at high speeds.
Some municipalities experimented with leaving the green on for a few seconds after the red was illuminated, to caution the driver that the right of way was soon to change.
[2] In 1920, Potts designed the first four-way, three-color traffic signal tower, which was installed at the intersection of Woodward and Michigan Avenues in Detroit, in October 1920.
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