20 and 30mph speed limits are mostly antiquated speed limits...cars and bikes at 30mph stop on a dime, a car will basically stop in its own length...go out and try it...a bike in a few feet...okay there is thinking distance and other stuff but what it is really all about is the fact that people do stupid things in villages and walk across the road expecting you to see them and stop...its the drivers fault not the pedestrian....
Actually the stats and I am a bit rusty on them these days and cant be bothered to checdk, but almost all pedestrian deaths are the fault of the pedestrian...however the law decides that speed is the problem and therfore wants slower speed limits in towns and villages, ie 20mph so stupid people dont get killed when hit by the car or the car driver, who has insurance can be blamed and money extracted...or a jail sentence dolled out and the law seen to be good...
Its all about manipulation of the people...