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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 that it is hard to comprehend a time when they were not a part of the landscape.

It all started with the need for traffic control or the supervision of the movement of people, goods, or vehicles to ensure efficiency and safety. In London 1868 there was so much horse-drawn traffic that a traffic signal using a colored lanterns 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.

In the late 19th century, early 20th century 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 and horse-drawn vehicles. With all this traffic one problem was seeing and being seen in a at congested intersections. To improve problem across the pond, United States cities  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.

Detroit, the auto hub, installed the first traffic tower in the United States in 1917. 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. Forget fighting crime, thousands of police officers had the job operate signals and enforce traffic laws at intersections. Once this process became automated time was freed up.

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 a

s 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 we are familiar with today, using the red, yellow, and green colors came about in 1935. 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. For examples the lenses have increased from 4 inches to 8 inches or 12 inches in diameter. The larger size makes them much more visible in all kinds of weather and lighting. Traffic light have also evolved from gas lanterns to LED traffic lights of today which are more eco-friendly.

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