It seems unnecessary on some wide roads. But in areas with narrower streets, even speed bumps, bus stops, children passing to and from school, and several crossroads with no traffic lights, the 20mph signs are ignored.
Safety is the aim. But when traffic flows at 30 to 40mph, pedestrians use care and caution to cross.
When traffic is slowed down in a “low speed” zone, I’ve noticed more pedestrians being relaxed, casual, abandoning the “look right, left and right again” rule and putting themselves more at risk.
Sticking to 20mph can make it harder to pass parked vehicles smartly and efficiently to keep the road clear for other cars.
There are some roads it should work on, and others it doesn’t.
But the whole city? That doesn’t make sense.