I think this policy will fail to be honest; while I’m not in Wales any more Oxford where I live now is a supposed ‘cycling city’ with most roads at 20 mph and honestly I hate being on the road there either in a car or on a bike; I get the bus in even though it’s often flaky. Driving you’re sat between second and third gear the whole time so you’re either revving too hard and wasting petrol or the car naturally speeds up so you’re spending a lot of attention on not getting fined rather than keeping it on the road, and when you’re cycling the cars that do keep to 20 mph linger beside you for what feels subjectively a lot longer when they overtake which isn’t fun either and the tiny bike lines make you feel vulnerable to traffic. I had a lot of near misses when I did it regularly and Oxford is a very small city, the idea of cycling somewhere like Cardiff would be terrifying to me.