I had the stop/start thing described to me by someone who studies these things as the concertina effect, so it fits right in with a folk music forum! It is so called because if you look at the bellows of a concertina, or any bellows instrument, they start off bunched together, then spread out, then start to bunch together again and so on.

In traffic spread across several miles this effect is magnified so you get the periods of stop/start that we all see. By keeping the traffic at a steady speed, usually lower if the traffic is very busy, you do lessen this effect. The other thing he mentioned, which makes sense, is if you get a single lane with a car doing, say, 55MPH and a car behind is doing 60MPH then the average speed of the traffic will still be 55MPH. The car behind has to keep to the average so he will slow down to less than 55MPH and then speed up with the resultant braking and acceleration. The longer the queue the more pronounced this becomes but, if they all keep at 55MPH then the concertina effect is removed.

Probably not explained it as well as he did. It does make sense to me anyway.