Autonomous vehicles following proper signalling before lane changes can be safe at arbitrary speeds (see Autobahns working at all). Humans, we should limit passing speed to roughly ~5 mph delta between adjacent lanes and leave it at that.
Humans with adequate following distance in the entire lane can probably manage 10 mph delta. I routinely travel dozens of miles very safely at ~80 with the flow of traffic (including the cops), and been stressed out at 55 in the carpool lane through stop and go traffic in the right-hand lanes due to on ramps/offramps.
I think 75 is memorable and roughly in the region where the tradeoff between increased kinetic energy and decreased time to arrival per additional unit of velocity becomes untenable.
Humans with adequate following distance in the entire lane can probably manage 10 mph delta. I routinely travel dozens of miles very safely at ~80 with the flow of traffic (including the cops), and been stressed out at 55 in the carpool lane through stop and go traffic in the right-hand lanes due to on ramps/offramps.