Green, Yellow, Red

As already discussed, most people know that red means stop.  That is universal.  However, it is also universal that green means go and yellow means slow down.

If the green light is lit up, you subconscious know that you must continue forward. 
If the yellow light is lit up, you know that you must start to slow down. 
And if the red light is lit up, you know that you must stop until the green light is lit up again. 

How old are we when we are taught this?  Does anyone really remember being taught this? 
Most visual literate thing are picked up by watching others and learning as you go through life.