You ever stop to think that maybe people slow down as they get up to you because they're wary of being cut off or run into the median, or even smashed into oblivion, by some jackhole that changes lanes at the last second, and it could be any trucker in that line that does it to them?
Maybe its just me, but I can tell you, I have literally been stuck in the lane, turn signal on for MILES because people driving 4 wheeled vehicles don't pay attention. Perhaps, J, the turn signal these people who are "wary of being run off the road" on the truck at "last second" has been on for quite some time.
From personal experience, I have made lane changes when there have been cars hundreds of yards behind me only to have them run up on my ass so closely, they could taste what I had for breakfast, then squirt into the right lane as soon as there is two inches of clearance. Only to find that there is something in their way, then crowd the lane making it impossible for me to complete a pass and return to the right lane. It sucks being the apparent cause of a back up in the left lane (people always blame the big truck) when its the jackwagons in the cars that forced the truck to stay there.
Just watch sometime, you'll start seeing it happen more and more. In general, drivers in cars fail to take into consideration that everyone on the road has the same rights, especially trucks. A 5 second drop in speed for a car is equally 3 to 4 minutes to a truck (depending on the drop) 80,000 lbs tends to create a lot of drag, ya know?