The Science Of Traffic Signal Colors

The concept that red signifies stop and green means go is deeply ingrained in our daily lives, extending beyond mere traffic light signals. From an early age, we are taught that red indicates danger, while green symbolizes safety.

But have you ever wondered why these specific colors were chosen for traffic lights? Why not opt for more visually appealing colors like magenta or turquoise?

The answer lies in history and practicality.

The Advent of Traffic Lights in the U.S.

The first traffic lights in the United States emerged in the 1920s due to the increasing number of vehicles on the road. To manage the growing traffic and reduce accidents, towns and cities installed traffic towers. Officers manned these towers, using whistles and red, green, and yellow lights to direct drivers.

In 1920, William Potts revolutionized traffic management by inventing the first tricolor, four-directional traffic signal, which was installed at Woodward Avenue and Fort Street in Detroit, Michigan.

Despite this innovation, various traffic light systems still existed across the country. To standardize these systems and avoid confusion, the Federal Highway Administration introduced the “Manual on Uniform Traffic Control Devices” in 1935.

This manual established uniform standards for all road signs, pavement markings, and traffic signals, mandating the use of red, yellow, and green lights.

The Historical Significance of the Colors

Before traffic lights were used for automobiles, trains had their own signaling systems. Originally, railroad companies used red to signify stop, white to indicate go, and green to denote caution. However, white signals led to problems, as bright stars could be mistaken for go signals at night.

To address this issue, green was adopted to signify go, and yellow was chosen to indicate caution due to its distinctiveness. This color scheme was later adopted for road traffic signals, except in Japan, where a different color is used to signal go.

The Choice of Red for Stop

Red was selected as the stop signal color for several reasons. It has the longest wavelength of all visible colors, meaning it is less likely to be scattered by air molecules and can be seen from a greater distance.

This is similar to how the sky turns red at sunset. Although the human eye is most sensitive to yellow-green, red's visibility over long distances makes it ideal for stop signals. Yellow, with a wavelength shorter than red but longer than green, serves as a mid-range warning color.

The adoption of red for stop signals originally came from train warning lights, which could have been chosen due to its wavelength, contrast with green, or its association with danger and blood.

The Evolution of Stop Signs

Interestingly, yellow was once used for stop signs in the early 1900s because red signs were difficult to see in low light conditions. With the advent of highly reflective materials, red stop signs became feasible and standard.

Today, yellow is still used for school zones, some traffic signs, and school buses due to its high visibility at all times of the day.

The next time you find yourself waiting at a traffic light, remember the history and logic behind the colors red, yellow, and green. These colors have been carefully chosen to ensure maximum safety and efficiency on the roads.

So, instead of getting frustrated, appreciate the thoughtful evolution of traffic signals that help keep us safe every day.