I tried Googling to find out if 20 mph zones do actually reduce accidents, but came across many peer-reviewed papers going back many years from many different countires / regions & pretty well all cocnclude that accident rates were either the same or worse than 30 limits IIRC. The severity of the outcomes may be lessened, but the rates didn't change much or changed the wrong way.

While I have to point out that what I found rather ties up with my bias against these "sticking plaster" schemes that play heavily on the "perception of safety" over reality, it also ties up with my largely negative experiences of such schemes. Both pedestrians & other drivers in such areas appear to think that they don't have to think any more & are more reliant upon drivers who have the right of way avoiding their lack of attention to their surroundings - cars pulling out of junctions without looking or having any intetion of stopping or giving way, pedestrians just stepping in front of moving vehicles without looking up from their mobile phones etc.

Roads are inherently dangeraous spaces for all users & I feel it is wrong to try and give the impression that putting in place an unfeasably slow speed limit somehow makes them "safe".

Nigel B.