How will you know what a stale green light is if you do not know what it is? A stale green light is a green light that is about to turn yellow. You will know if you are approaching one when you see that the crosswalk signal has changed from a white figure to a flashing orange hand.
You can determine if you should try slowing down or if you should still go based on the freshness of the green light that you see. Remember to follow the rules in order to avoid getting into possible accidents.