(WHTM) — We see traffic lights all the time. Sometimes we sail past green lights, and sometimes we hit every red light on our commute to work.

But while sitting at a traffic light, have you ever wondered why they are red, yellow and green?

Traffic lights in the US

The first traffic light came into existence in the United States in the early 1900s. Called semaphore traffic lights, they had retractable arms — similar to railroad crossing arms — with the words “stop” and “go” on them. They would also have only red and green lights. These types of traffic lights were popular in cities like Chicago, where traffic was on the rise.

The yellow light didn’t enter the scene until the 1920s. William Potts, who was a police officer in Detroit, is credited with designing the first three- and four-way colored traffic lights. The amber light was introduced during this time.

Why the traffic light colors were chosen

There are a few reasons why the colors red, yellow and green were chosen for traffic lights.

Red is an obvious choice. The color red means danger or to be alert for something occurring. It is the same reason why stop signs are red. The human eye is more apt to be attracted to something that is red, according to the National Library of Medicine.

Yellow lights were chosen to alert drivers that the light will change to red in a few seconds. You should proceed with caution if you’re already in the intersection, but if you’re not, you should stop if it’s safe to do so.

An interesting fact about the yellow light, according to idrivesafely.com, is that it should last longer if the speed limit is higher.

For example, if the speed limit is 45 mph, that yellow light will be on for a shorter time than if the speed limit was 55 mph. However, a yellow light does not mean to speed through so you don’t hit a red light!

Green was picked because of how the color is pleasant to the eye. Green has a shorter wavelength, which makes it easier to see. Before the color green was chosen, white was used. However, the color white was easily confused since street lamps used white lights. Since then, the universal color for “go” has been green.

International use of traffic lights

The Vienna Convention on Road Signs and Signals Treaty is the main reason why red, yellow, and green lights are used internationally. Ironically, the U.S and Japan have not signed the treaty, so deviations from these standards are found in those countries.

For the most part, traffic lights mean the same thing in every state throughout the U.S.