As someone who lives in Wales and drives, cycles and walks around daily, I think this is a great policy, but poorly communicated and has been a target by populist politicians which has fed into the discontent.
Many people speed and break the limit anyway regardless of the speed limit. In my experience about 80% of cars will be sticking to 20 - either abiding by the limit or stuck behind someone keeping to it. The majority of people speeding will only speed up to ~30mph, which will be less than what they’ll be speeding if the limit was at 30 originally.
It may vary by council, but I personally can’t think of any roads near me that could do with reverting back up to 30 (that haven’t already). Some main roads will still be 20, but then there are schools and residential on these roads which doesn’t make sense to revert - but some people will still complain.
Personally I don’t think this policy will ever have national popular support. It is a policy that directly affects someone’s day to day rather than a stat which makes it a perfect candidate for people to complain about (potholes are similar). This is despite polling also saying that the vast majority support 20 outside their own house (but outside other people’s houses should be reverted?).
I am kinda sick of how much this policy is brought up. It seems like every month there is a new article critiquing the limit and it is getting quite repetitive.