Your post appears to suggest that the 20 mph speed limit, or indeed any speed limit in this country, was dediced upon by a body of impartial, unbiased, and non-political independent experts and boffins, rather than politicians.
This is blatantly bollocks of the highest order, course. Unless you actually tell me you believe that for almost the entire lifespan of speed limits in this country, every single government and local authority and their road safety experts in charge of speed limits were so absolutely fucking useless at their jobs, they grossly overestimated the maximum safe speed on urban roads by an incredible 50% for nearly a hundred years, until the current generation of experts luckily arrived and showed all previous generations the error of their ways.