From Tech Transfer Newsletter, Fall 2007
From Waving Arms to LED's:
A Brief History of Traffic Signals
By Michele Regenold, Center for Transportation Research and Education, Iowa State University
Pierre Vivant, Traffic Light Tree, 1995-98
Can you imagine taking a drive anywhere without encountering a traffic signal? Traffic signals are so omnipresent today, and have been around for such a long time—since 1868—that it is hard to comprehend a time when they were not a part of the landscape.
In the London, England of 1868 there was so much horse-drawn traffic (motorized vehicles were still in the future), that a traffic signal using a colored light called a semaphore was installed at the intersection of George Street and Bridge Street near the Houses of Parliament. The purpose was to provide a safe crossing for pedestrians, especially members of Parliament.
A semaphore is a tall post with moveable arms. When the arms stuck straight out sideways, it meant stop. At night a gas light at the top was lit. The light had a red lens for stop and a green lens for go.
Traffic Police and Early Signals
Traffic police officers operated semaphores and early traffic lights by hand. City officials didn't think drivers would obey the signals otherwise. The traffic officers judged the traffic and decided when to change the signal. To alert traffic that the signal was about to change, they blew a whistle.
Besides cars and trucks, traffic included street cars (vehicles traveling on rails) and horse-drawn vehicles. With all this traffic, one problem that officers had was being able to see and be seen by drivers, especially at congested intersections.
To give traffic officers a wider view, many cities in the United States started using traffic towers in the late 1910s and 1920s. These towers were small booths several feet above street level on street corners or on concrete islands in the middle of a street or intersection. The officers inside the towers operated colored lights or semaphores or waved their arms.
A traffic tower in Detroit
Detroit was a hotbed of innovation for traffic signals. In 1917, Detroit installed the first traffic tower in the United States at the intersection of Woodward Avenue and Michigan Avenue. In 1920, Detroit became the first city to use red, green, and yellow lights to control traffic. And a Detroit police officer named William L. Potts invented the 4-way, 3-color traffic signal.
During the 1920s inventors came up with plenty of different designs for traffic signals. One thing these signals usually had in common was that someone had to push a button or flip a switch to change the signal. Imagine the thousands of police officers whose job it was to operate signals and enforce traffic laws at intersections. Once automated signals were invented, traffic officers were freed up for other duties.
Automated Traffic Signals
The first automated signals used timers to set the length of time the red, green, and yellow lights would be on.
Charles Adler, Jr. invented a signal that detected a vehicle's horn honk. A microphone was mounted on a pole at an intersection. The driver had to stop and honk. Sonic vibrations made the mechanism shift electrical circuits and change the light. Then the driver had 10 seconds to get through the intersection. In 1928 a horn-actuated signal was installed near Baltimore, Maryland.
Another type of traffic detector was invented about the same time as the sonic detector. Henry A. Haugh developed a detector that sensed the pressure of passing vehicles. The pressure caused two metal strips to touch, which sent electrical impulses to the signal controller.
Modern signals also still use the red, yellow, and green colors. These were standardized in 1935 in an early edition of the Manual on Uniform Traffic Control Devices. Making traffic signals look basically the same all across the country meant that drivers didn't have to figure out an unfamiliar signal. They could recognize a standard signal and react appropriately, which made driving safer.
Although the three colors in signals have stayed the same, the size of the lenses or heads has changed. Potts's original 4-way, 3-color signal used 4-inch lenses. Today heads are 8 inches or 12 inches in diameter. The larger size makes them much more visible in all kinds of weather and lighting.
At just 10-25 watts, today's LED traffic lights are far more energy-efficient than old-fashioned incandescent light bulbs which used 175 watts.
©  Go! Exploring the world of transportation, a magazine for teens published by Iowa State University. Used by permission.
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