Traditionally speaking, if you are driving, and you see a green light, you go. But that's not always the best policy. Sometimes, there is a green light, and you must stay stopped. Maybe there is something in your way? Maybe you won't clear the intersection? Maybe you can tell that there is room for you, but there's a huge jam up ahead.
Whatever the reason, sometimes the traditional signs tell you one thing, and common sense, general knowledge, or even just a hunch tells you other wise.