Everyone here is either totally wrong, or somewhat wrong. Please read this comment in full. It all comes down to an unholy mixture of speed governors, adaptive cruise control (which involves RADAR), control lockouts, weight differences + terrain + make/model/engine/final drive combo, pay per mile, strict timeframes, strict safety departments, insurance mandates, FMCSA scores, and plain ol' impatience. MPG has little to do with it, nor does impeding traffic just for the sake of being an asshole. Let's break it all down: Let's start with speed governors. Common governed speeds are 63, 65, and 68. Some companies have split governors with lower speeds on pedal and higher with cruise (E.G 58/62, 62/66, 64/68, etc). Now let's add in adaptive cruise control, which detects future grades both up and down, is sattelite driven via GPS for terrain, while the RADAR can detect distance, speed, and closing speed to the vehicle ahead. It can automatically slow down trucks up to 4.5s behind if the company's safety department is super anal. Some trucks can override the auto-decelerate by holding the pedal down, while others a press of the pedal brings the cruise speed down to the non and slows the truck down. A driver in this case must initiate a pass from upwards 500ft behind the proceeding vehicle. There may be only .5-2mph difference. A pass initiated must be committed to complete, or aborted (if the driver has an sense or regard to impedence). A truck iniating a pass at these distances at let's say, 400ft- would take 2.5minutes tk safely pass the truck in the way that is traveling 1mph slower. Then you have weight, terrain, and powertain differences. A truck in front may be faster, but heavier and have a weaker drive train. This can be a frustrsting experience to the driver closing the gap. They might initiate a pass with an advantage, then quickly lose it. Another tactic with drivers of slower fleets, such as Prime, is to "Hill Bomb". Those fuckers are SLOW, but will drop into neutral and coast overspeed at 20mph+ over the limit downhill and fight passers. Some company safety depsrtments closely monitor and punisg overspeeds, while others don't. The perfect storm is when slow companies don't care about overspeed. Then there's the impatience. A driver may have been stuck behind another truck for 3+Hrs and is finally saying "fuck it all, I'm passing this shithead if it's the last thing I do". That can take time, but it IS best to abort if traffic builds up behind you as not to impede. I'm not even going to cover make/model/engine/drivetrain differences, nor FMSCA and insurance impact, but they each factor into all of the above. Nor will I cover MPG; most truckers ARE NOT O/O, and most company drivers do not get efficiency incentives. Most O/O or L/Ps pass when they want, fuel be damned, while company drivers usually have zero reason to care. Please have patience with truckers; most hate impeding traffic but are doing what they must. The respectful ones abort when traffic builds up, but if they don't, remember that it might be their 7th attempt! There are a lot of niggers driving nig rigs, and I find they more often than not, are the ones causing the issues. Also, while I try not to disparage boomers as much as others, boomer drivers are often some of the most entitled, arrogant, disrespectful fuckers on the road- third only to niggers and street shitters. Edit: I'm not going to fix my spelling errors or formatting, but will admit that I'm a trucker. I have avoided admitting it on POAL even though most goats know this. I was just awarded driver of the year out of ~4,000 company drivers in my fleet. I know a thing or two about this (really, I only covered a third of a half of it). If you have any more questions, feel free to ask. Edit 2: True passing etiquette works both ways, including the person being passed. A good driver will let a frustrated passing driver do so by dropping cruise a few mph to let the other pass along with traffic, then make it up if need be at an opportune moment. Of course, equipment calibration can severely hamper this. Beancounters and safety theorists hinder real world situations that rarely reflect models, and make it all-the-more difficult. I am outspoken in my beliefs in the matter and my company's vice president has personlly admit that I'm right and that the board is reviewing adaptive cruise algorithms. You'd be scared to know how much modern drivers are merely steering wheel holders and how little control over the trucks they have- in some cases they can't even lock a gear in or downshift! My company will soon be "giving back more control to the drivers". How they fuck it up next is yet to be determined, and I work for a *good* company. Food for thought. Again, ask me anything.