In most cases it is yes I entirely agree. I can only comment on my own experience of the road near my house which seems to be targeted despite the council admitting that it doesn’t even really need to be a 20mph limit outside of school hours (it is a quite wide road and only has one corner – ie lots of long straight to see down).
Perhaps when they can afford the time-specific replacement signs the police will go back to doing something more productive with their time (like perhaps enforcing fair and acceptable speed limits elsewhere in the town)?
Or even just spend their time ticketing the bloody drivers who park on the pavement along that road – especially the ones dropping off little Tarquin in their big 4×4 right outside the school gate and on the pedestrian crossing which surely is more of a danger than someone doing 30mph along it at 2am…