This was introduced where I live on practically all of the roads that were previously 30. With two kids I have to admit I was very much in favour.
After having a year of it I’m still in favour on side streets. For example ours where there is a school. However as an example the road that runs above and parallel to us is a ‘main road’ and when you’re going at 20mph I’ve even had idiots overtaking. I think it’s made the road more dangerous and, as much as I hate to say it, I think for the new limits to work they really must be enforced by average speed cameras. Sadly it will lead to people being caught out having a momentary lapse and also people being fixated on the Speedo. But either keep 30 and keep 20
For small side roads, or enforce 20 rigidly, because there is nothing worse than having some dickhead tailgating the whole journey. All in, or all out imo