I emailed councillor mordey about this. He head's up the team that made this decision, the reason is to bring it in line with other similar roads like the A690
Not that they are in anyway similar apart from being a dual carriageway though. Although it wont really matter when the other roundabouts are converted into light controlled junctions in stage 4 or 5 of the SSTC.
This is exactly my thoughts, during school times 20mph around schools makes perfect sense. At 10pm at night it makes zero sense. There has to be some way to have variable speed limits.
In Bishop Auckland, the road that runs by King James School and ASDA is 30mph or 20mph when the crossing lights by the side of the road are flashing.
However since many primary schools are in residential areas and have children playing nearby outside of school hours I think permanent 20 zones around schools are not a bad idea.