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Other than the military, commercial vehicles are the priority users of the Interstate and US highway system. Cars are ancillary users. The two tiered speed limits and heavily governed and often under powered trucks are the main cause of this kind of problem. If 5 axle (80K pounds max) semi won't hold 60 or a bit more on a 5% grade, the problem is how it is specced. Some companies govern their trucks to 62 or so to save fuel (a .001 mpg improvement in a fleet is a lot of money) and sometimes for perceived safety. Annoying to car drivers, but in the grand scheme, an inanity.
What I hate is the people in cars who start passing a semi and slow down because they are too timid and fearful to just keep going. Sitting next to a semi for no good reason is really dangerous. Can't handle the close encounter? Pull over, have the car towed to a dealer to be sold, and call for a ride. Those people should not be on the road, ever.