Despite a study in 2012 showing average traffic speeds for the 12 hours between 7.00 am to 7.00 pm across central London being 8.8 mph many still rail at the suggestion of 20mph zones. There are many reasons for this but I saw in a Tweet by Kerena Fussell a quote by President of the AA, Edmund King;
So what he's saying is that many motorists will make their own judgement as to what the speed limit should be...or...that reducing the speed limit is dangerous to the public because many motorists will drive over the speed limit..."If you have [20mph] limit on a road that clearly doesn't need it then many motorists won't slow down and this just puts the public at risk"
Just spend a few moments letting that sink in.
It's almost like saying burglary shouldn't be illegal as people will burgle you anyway.