I've heard before about setting speed limits using percentile studies of people driving on the road, which in the absence of some specific safety concern (which then needs engineering like narrowing the road or adding turns) makes the most sense.
I also wish there was more of a culture of pulling over if you don't want to drive at the flow speed. If I want a leisurely drive and see someone rapidly coming up behind me, I'll happily pull over and let them pass. There seem to be these sociopaths or self-righteous jerks who will happily drive 5km/h under the speed limit with 20 cars behind them. This is way more dangerous than speeding and should be treated as such. If you just want to drive slowly, why would you want the stress or a bunch of angry drivers behind you.