30mph was known to be a good residential speed for decades. If conditions made it unadvisable drivers would adjust as appropriate.
Compulsory 20mph is ridiculously slow. And will create issues re pollution, reduced opportunities to get into a junction and thus a longer wait, plus the continual change from 30, 20, 30 20 30, 20 can only ratitional be understood as an attack on decent drivers in the hope of issuing fines and blotting their record.
And it's clearly an attempt to further this idiotic push for citizens to not take responsibility for themselves, even when they are the more maneuverable, and to expect others to compensate for the pedestrian's irresponsibility by blaming drivers for everything.
One need not look far to see multiple such manipulations, and proof as to why our politicians, and others in position of power, prove themselves time & again utterly inadequate for the position they're in.