IMO, the second largest irritant for sure and easily top-3 for creating dangerous situations on ND highways are people who don't use their cruise.
Last Sunday I was on Hwy 2, in the left lane @ 75MPH passing someone. This black sedan is coming up behind me all of 85MPH about 1/2Mi back, so, I get into the right lane when it's safe and courteous both to the person I just passed as well as the car going much faster. Black sedan ALMOST gets past me, like their rear bumper is even w/my front bumper, and, they hit the brakes. Drops back to my rear bumper and sits there for ~2Mi, then, takes off again going over 80. They gets about 2 car-lengths ahead of me and brake again to get behind. I can see the driver and passenger conversing. No texting, no phone calls. The car is going straight so the driver is obviously watching the road but paying 0 attention to their speed. The driver then merges behind me and varies from 1/4Mi, to less than a car length off my bumper for 20Mi. I bet their car gets REALLY crappy MPG! Finally I pull out to pass another car, the black car rides right up behind that one and stays there, decides to follow that person instead of me.
About 6wks ago, from Valley City to Fargo I passed, and was re-passed 4x by the same driver texting away every time I glanced @ them. Actually kept the vehicle surprisingly straight in their lane, and had this person set the cruise I would only have dealt with them once. Bouncing from 75-85MPH+ in a 4-cyl smallish SUV I'm fairly sure makes it downshift a lot even in our flat country and has to be hard on MPG.
The least courteous Semi-drivers are folks who are NOT running daily and/or commercially.