As a councillor, I introduced 20mph limits on most local residential roads & shopping streets. I didn't do other main roads though. I thought people would be less likely to rat-run if the main roads were kept at 30mph. I now believe I was wrong.
A blanket urban 20mph (with exceptions when appropriate) puts drivers in a 20mph mindset.
As we drive around, there's less chopping and changing and 20mph becomes a habit. The Welsh approach bakes that in, making everyone safer. #20mph