رضا: I like how Marfitalu has it. If it ain't a problem but something that needs doing eventually, I'd go with "have to" or "should" or even "ought to". If it's urgent and can't wait or be denied, I'd be more inclined to say "must".

So we get back to the traffic light and external authority. We must obey the traffic signals, but some people don't think that way and would say, "We should obey the traffic signals". To me the word "must" is often times an absolute and I avoid speaking in absolute unless it really is an absolute. It should be obvious that we should obey traffic signals, we don't have to obey them. There's no must to it. A law of physics is different. It is an absolute in almost all cases. You jump off the ground, you must come back down. It has nothing to do with having to do it or should do it. It will happen and it must happen. Must implies little or no choice in the matter and the other ones leave room for different actions.