In Italy several cities lowered the maximum speed from 50 to 30 km/h.
There was a huge fight over it, car drivers in those cities were mad. Plenty of politicians opposed it.
One year later stats were super clear: streets got way safer and the number of fatal accidents dropped to near 0. Time to traverse cities didn't change much, as it was already limited mostly by traffic and lights.
I think this ignores the argument the high speed limit people make which basically boils down to "sure some people will die or get injured but its worth it because driving faster is fun"
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.
There was a huge fight over it, car drivers in those cities were mad. Plenty of politicians opposed it.
One year later stats were super clear: streets got way safer and the number of fatal accidents dropped to near 0. Time to traverse cities didn't change much, as it was already limited mostly by traffic and lights.