Trouble with speed limits is that people unthinkingly tend to drive to them and never use their judgement, even when conditions (fog, traffic density, tight bends, bad surfaces, standing water, snow, ice or other hazards) make a much lower speed more appropriate.
I have a cautious, law-abiding friend who lives and who learned to drive entirely in an urban area, and who rarely drives in the countryside. I tend to drive fast, but they terrify me by the speed they drive along narrow single-track country lanes.
If I ever dare to comment on this, they immediately say "But I am only doing 27 (or whatever) mph". They always know exactly how many mph they are doing, but seem utterly incapable of judging whether that is actually a safe speed on a narrow lane with blind corners and no passing spaces.
However low you make a speed limit, there will always be some times, places and conditions where even that wil be too high. As others have said, better training and more stringent (and more regular) testing are more importnt than trying clumsily to impose unrealistic universal speed limits.
As a cylist, I don't want motorists to hit me more slowly. I want them to be looking out for me, to see me and to steer round me please.