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 thing about the lamp posts doesn’t really have any significance. Roads in the UK have a default speed limit that doesn’t need to be marked with the speed limit - it’s 30mph for “normal” streets with regular lamposts, 60mph for other roads, 70mph if there’s an embankment thing in between each side of the road. 180m spacing must just be the technical definition for what makes a road 30mph. In Wales, they made default 30mph roads 20mph.