Historically speaking:

The stop and Go! of traffic signals and their sometimes weird designs

by Michele Regenold

If you had to guess, when would you say the first traffic signal using a colored light was installed? 1868, 1914, or 1947?

1914 is a reasonable guess. In the United States, cars were becoming popular, so traffic was increasing. In 1900 there were about 8,000 motor vehicles on the road. By 1910, there were 460,000 cars and trucks, and by 1920 more than 8 million.

But 1914 isn’t right. It was 1868. In London, England.

In 1868, London had lots of horse-drawn traffic (motorized vehicles were still in the future). There was so much traffic that a traffic signal 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. The one at right is similar to the one installed in London in 1868. The arms sticking straight out sideways 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 if traffic officers weren’t there to enforce them. 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. Some towers were on street corners. Some were 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.

The tower design in the top photo at right was used in Detroit, Michigan, in the 1920s. A traffic officer controlled signal lights at the top of the tower and at driver eye level.

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. Pre-timed signals are still used today (though they’re electronic now instead of mechanical). They’re usually in central business districts where traffic tends to be predictable (morning rush, afternoon rush).

Another kind of automated signal detects the presence of vehicles and changes the signal.

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.

Traffic detection is still in use today. One kind, similar to Haugh’s pressure-sensitive strips, is a loop detector. Wires embedded in the pavement (in a big loop) sense passing traffic.

Video detection, which started in 1992, uses a camera mounted near the traffic light to sense vehicles. Video of intersections can be viewed from a central office or on the web. Traffic engineers can also make it accessible to police and fire departments for emergencies.

Colored lights

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.

Traffic lights have shifted away from using old-fashioned incandescent light bulbs, which use a lot of electricity. Now they’re LED. Instead of 175 watts, they’re 11 watts, so they use dramatically less energy.

The future

What’s in the future? Imagine your car being able to talk to intersections to keep you moving and keep you safe. The equipment and the capability to do this already exist.