Isn't bizarre that certain feelings or things are associated with particular colors? For instance, red is connected with anger and yellow is usually associated with happiness. Then there is green. That color surrounds the earth from trees, plants and the regular grass almost everyone steps on. It gives us the air we breathe and the food we eat so it’s not easily arguable that green embodies life. You know that phrase “go green”? It’s when you try to go for more natural things to suit the environment. To show you are saving the “life” of the earth.

  But why is this color hand-in-hand with go? You see it every time you drive on a busy street. To figure out this outlandish phenomenon we have to look back to when the first railroad signals were born. The 1830s.  Just like most of the traffic lights we know today, they had three lights meaning different things. Back then, red meant come to a halt, but green meant continue with the intention of stopping, and white meant go. Red was put with stop because as any rational human would know, blood is red and is associated with danger.

  Green and white on the other hand can’t really be elucidated. Some propose it is from the disparity these colors have with red. These color coded messages were put into action for years, until the true issues will make themselves prominent.  The issue was that when the red lens from the traffic signal fell out, it makes it give the appearance of being white-or go. So, when a train saw the signal, instead of coming to a stop like they were supposed to, an atrocious train crash befell. Despite the tragedy, we finally learned the problem needed to be fixed. So, the color was sooner or later changed to green. Then caution was changed to yellow since it contrasted well and was easily seen.

  Once the car was born and became a common way of transportation, the colors stayed the same and are still the ones we use till this day.

Source: http://wonderopolis.org/wonder/why-does-green-mean-go