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(WHTM) — When driving on an interstate or any other major highway, you may see a sign directing you to a different city or another roadway. Most of these signs are green.

But who chose green, and why? Well, there’s actually a science behind it.

As the Arizona Department of Transportation explains, green is a “cool” color that won’t be distracting. Being on an interstate or highway is already hazardous because of high-speed limits. Because of this, you have to keep your eyes on the road at all times.

“It blends in enough to be considered part of the scenery, but sticks out enough to notice when you need it,” says ADOT’s John LaBarbera.

If highway signs, mile marker signs, or any other non-emergency signs were a different color, like red, our eyes would be darting all over the place, making driving even more hazardous.

The effect green is believed to have on our emotions was also a factor.

Green, unlike red, actually calms humans down, researchers say. A study linked on the National Library of Medicine’s website, found that walking in a green environment resulted in a significant reduction in heart rate values as compared to red and white conditions.

So what about other highway sign colors?

Here are the various colors you’ll see and what they mean, according to the U.S. Department of Transportation Federal Highway Administration:

  • Red — regulatory (Stop, Yield, Do Not Enter)
  • Yellow — warning/caution (turns, deer crossing, signal ahead)
  • Orange — construction (road work, closed exits)
  • Brown — rest stops, parks, bathrooms
  • Blue — motorist services (food, hotels, hospitals)
  • Fluorescent yellow-green — pedestrian/bicycle signs (school crosswalk, pedestrian crossing)