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William Potts, an officer with the Detroit Police Department, is credited with developing the tri-color - red, green, and yellow - traffic signal still used today. This four-way directional signal was first installed in Detroit in 1920.

Potts Four-Way Traffic Signal

Potts Four-Way Traffic Signal: Image courtesy of "Inventing History: Garrett Morgan and the Traffic Signal", 2006.

In 1935, the Manual on Uniform Traffic Control Devices (MUTCD) made this tri-color, four-way signal the national standard. The manual advised that one signal head be visible at each approach in an intersection. Often, as seen in the image below, the signals were placed at the center of an intersection, allowing the greatest visibility at each approach.

Traffic Signal in Intersection

Traffic Signal in Intersection: Image courtesy of the University of Vermont Landscape Change Program and the Windsor Historical Society.

After WWII, more families were able to purchase automobiles, resulting in the increased congestion of roadways in cities and suburbs. At this time, traffic lights were more heavily urged and underwent a number of changes. Many of these changes involved the mechanics of the signal head and are not visible in the landscape. However, manufacturers experimented with larger signal heads and lenses and taller poles, and many unites became more streamlined and less ornate. Traffic lights were removed from the center of intersections and placed on poles at each corner. Often, more than one signal head was interconnected on each pole, as seen in the images below, and the heads could be adjusted to face any direction. In 1954, the MUTCD required that two signal heads be visible at each approach.

Also note in the images below, which were taken at the same intersection roughly a decade apart, the difference in the signal poles. The signal heads in the earlier image, on the left, have been mounted directly onto the utility pole. In the later image, on the right, the signal heads have finally been moved to a traditional pole.

Mounted Traffic Signal Mounted Traffic Signal

Mounted Traffic Signals: Image courtesy of the University of Vermont Landscape Change Program and the Hartford Historical Society.

Although some decorative mast arms existed in the early days of traffic signals, as seen in the 1920s image below, the mast arm did not come into heavy use until the 1960s and 1970s. These mast arms were generally the very utilitarian designs we see today.

Decorative Mast Arms

Decorative Mast Arms: Image courtesy of the New York Police Department, 2006.

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