Ever drive one?
There are a million scenarios, but not all tractors are 550/600hp monsters and the (car) guy "loping" along at the speed limit or just under that just passed you because of a minor grade slowed you down, now just pulled back in front of you (too close). Then there's the next grade just coming up and the trucker try's to gain a little momentum to crest the hill at a reasonable speed so this cycle does not repeat get's a little close to him, again.
I am not a t/t driver but have driven heavier truck/trlr combinations (privately) and until you've spent time in a heavier vehicle where a little "speed strategy" is important to weather you actually start to impede the normal flow of traffic or not. If all vehicles had the same hp per pound of vehicle weight things would be simple(er).

Then there's the guy on the phone................:laughing: