Well, there are plenty of times when exceeding the posted speed limit is legal. Do we all know why x driver is going hell bent for leather in the passing lane? Maybe his kid was injured, maybe his wife is giving birth, maybe he's injured himself and is bleeding profusely and is on his way to the hospital? (this has happened to me btw).
But, as stated, if everyone on the highway is doing 120, then the police will NOT pull everyone over. I've passed many a speedtrap on the 401 to Montreal and never been pulled over (I was doing 120 easily).
As for causes of traffic jams: Everyone driving too slow IS one of the problems. Look at it this way: If you have a 10 gallon barrel that refills every 10 minutes that you have to empty, is it better to let it drip out or to tip the thing over and pour it out quickly? Just remember: every 15 minutes there are another 1000 cars coming onto the highway. the quicker you get off, the more room there is for the next batch coming along.
Another example: You have to get 10000 ppl into a building through 1 door. Is it faster to let them through 10 at a time, every 10 seconds or 10 at a time, every 5 seconds? (if you don't know already, time and volume is how they measure the usage of the 401, it used to be just west of keele at the express lane exit but I think they've moved the measuring zone).
IN addition, tell me, you've never seen a slow moving truck in the right lane, and some wingnut only doing 10 kph faster in the middle lane? Typically there are about 2000 cars lined up behind this rolling road block trying to get over to the far left to get around them. Then you have someone like mmouse who is only doing the speed limit, not one click more and you've now got gridlock.