I live in a quiet street, which is not a through road, yet every day I see and hear drivers whizzing down it at totally mental speeds, often 40-50mph.
The advantage they gain by that is minimal, as there’s a main-road junction at the end for which they must (if halfway-sane) pause. Yet they do it nonetheless, no doubt because it’s a reasonably straight stretch.
Meanwhile, children and adults cycle along this street; people cross it on foot to visit friends or relatives; pet cats wander back and forth; in short, everything about it says, to the sensible motorist, that it’s an area fraught with hazard.
And yet drivers seem oblivious.
Obviously, they’re ignoring even the current 30mph rule, so the effectiveness of a 20mph one might be questionable.
But it might, at least, concentrate minds.
Even if all it did was to bring the current 40mph offenders down to a still-illegal 30mph, the damage done by any collision would be that much less.
And if, in an ideal world, drivers actually observed the 20mph speed limit, well, the lives and injuries saved would be incalculable.
20 is plenty for the side-streets of Worthing, or of any other town.
The good, clever driver adapts his/her speed to the conditions, which include the nature of the area he/she is driving through.
In inhabited areas, you show your skill best by driving with due caution.
Time enough for speed once you get to the main roads and motorways.