This is how the government conned everybody - allowed councils the freedom to decide to setup 20mph zones, means councils take the flak, not the government, and means the councils have to fund all these costs. Whereas the only true logical way of doing it would have been to singularly reduce all 30mph zones to 20mph and fund it by central funds. Much cheaper to simply replace the 30s with 20s - none of the stupid expense of zillions of 20mph repeaters in the back streets. Much simpler to understand - the speed limit in town is now 20mph, not 30, no wondering if it's been changed and you didn't notice.
Where there's a will .... there's a wake.