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.
We actually have pockets like this in the US. A small town nearby has nothing but roundabouts and max 30mph and it’s makes the area very walkable. Parts of Pasadena CA is similar. It does happen but not on a grand scale. The areas we have adopted this method are very pleasant.
Which always makes me wonder why, if it’s so nice, this isn’t done everywhere
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