Why traffic lights are red, green and yellow

It’s a given that red means stop, green means go and yellow means slow down, right? But have you ever thought about why these colors are used to convey those meanings in traffic lights? It seems like they could have just as easily used blue, purple and brown.

It turns out these three colors weren’t chosen at random, and there’s actually some logic behind each color’s designation.

Red probably has the simplest explanation. Long before cars or even trains existed, the color red has been used to indicate danger, and scientists believe that reason is rooted in evolution. In a study published in “Psychological Science,” researchers found that rhesus macaques, a species of Old World monkeys, avoided humans offering them a snack when those people were wearing red clothing as opposed to blue or green.

“The similarity of our results with those in humans suggests that avoiding red or acting submissively in its presence may stem from an inherited psychological predisposition,” Dartmouth College neuroscientist and study author Jerald D. Kralik told Science Daily.

Because red has so long been associated with danger or as a directive to stop, it’s natural that it was chosen by railroad companies to tell trains to stop. At first, in addition to using red for stop, they used white to mean go and green to mean caution. However, this caused problems when conductors saw white light from other sources, such as stars, and mistook them for the signal to go ahead. For that reason, green was swapped in to mean go.

Although they used red to mean stop with trains, when cars first came on the scene, many stop signs were actually yellow, because the color was easier to see in poorly lit areas. Eventually, more highly reflective materials were developed, allowing red to once again mean stop for both traffic lights and stop signs.

However, yellow is still a highly-visible color (second only to red in terms of visible wavelength), making it the natural choice to mean “slow down” or “caution.” In addition to traffic lights, you’ll notice that other signs that serve as warnings are also yellow, such as signs indicating school zones, buses and crosswalks.

Well, there you have it! You learn something new every day.