I think it they are a white elephant because of the way they are implemented - whole swathes of roads are blanketed as 20 - why not specific areas? Sometimes 20 is too fast and I could understand if the limit was lower at certain points - eg near a school as parents/children are leaving and arriving. It removes common sense (let’s face it sometimes it can be difficult to get to the dizzy heights of 20 at peak times in London). I also think it causes problems as you often get idiots do a dangerous overtake and brake test you if you do observe the limit. When you are not near a school etc, visibility is good and nothing is about, what is wrong with 30 really? Also, it creates the additional hazard of going near to the speed of cyclists. I think all they have achieved is to get most drivers to drive at 30 in built up areas.