The highway speed scatterpot in the US has a floor at the "speed limit" and the median speed is usually 10mph to 15mph higher than that. In this scenario, driving the "speed limit" means nearly all other users will overtake you at a significant differential, and some in a disorderly manner. Consistently doing so in heavy traffic provokes backups and the usual consequences to other drivers.
My experience on US highways is that it generally isn't quite that bad: if you're going roughly the speed limit, you will be in company with a significant subset of the commercial vehicles, whose owners mandate following the speed limit (and in some cases enforce it with GPS surveillance).
Your description sounds right but also pretty similar to GP’s characterization of a floor at the limit and 10-15 mph higher median with the result that more than half of traffic is passing you at a 20+% speed differential.
Quite a few people drive somewhat below speed limit. There is nothing unsafe about it, except wishful thinking to rationalize fast driving. Fast driving is fun, quite a few people equate "fast and nimble" with "good driver", but safety is not the thing they are achieving.
If anything, the Autobahn proves that the system works when everyone respects the rules and each other, all cars undergo regular roadworthy inspections, driver training is rigorous, and the road is designed for speed variation. In the US, none of that is the case.
Having rigorous enforcement of minimum radius in curves*, preserving more available cornering/braking traction and wide lanes, providing for better sightlines ahead (for faster traffic) and behind (for slower traffic contemplating pulling out to pass even slower traffic).
* - I did an internship for Mercedes in the early 90s and we had testing access to a section of ex-A8 near Stuttgart that was retired because it didn't meet the modern autobahn requirements and so had been replaced with a re-routed A8. To my American college-student mind that seemed incredibly wasteful, but it sure was convenient for our testing. I can't find it now on Google maps, but it's been 30 years so may have been demolished by now.
The Autobahn is much more challenging to drive on due to the extreme speed variation between trucks in the right lane going 80 kph and race car drivers in the left going 250.
IMO the only reason that it's acceptably safe is strict German driver's licensing requirements and fairly strong enforcement of traffic laws. If you had the same regime but American style speed variation, I think you'd see considerably safer roads in Germany.
