I know exactly why they do it, They don't want the chance of getting held up so they shift the problem and guarantee all other cars are held up. Car drivers do it too but not as effectively. and they have to change lanes into and out of the middle so... The non rush hour times I drive the trucks are in the middle the left lane is clogged up with other middle lane drivers merging with the fast lane and the right lane is empty because you're not supposed to pass on the right and the trucks are the slowest vehicle. I didn't know there was a provision excluding trucks from slower vehicles stay right.