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.
The reduced speed limit should come with traffic calming relative to the speed limit. If a through road has a 20 MPH zone that area should have lane narrowing and at least raised crossings.