Even with the caveats about limited data and untangling causation and correlation, the statistics are striking: the first year of a scheme in Wales where the speed limit on urban roads was lowered to 20mph resulted in about 100 fewer people killed or seriously injured.
A lot of cars I’ve been in with speed limiters, the minimum speed is 25mph. I’ve never understood why it’s not 1mph (or more reasonably 5mph, which is the lowest limit I see regularly).