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.
Costs money to rip and replace existing infrastructure. Counties and cities with less income can’t afford to make such drastic changes unless it’s a higher priority than the other 25 things that need funding