Portsmouth has rolled out 20mph across the city. Sort of. Originally it was going to be across the entire Isle of Portsmouth, but after protest through routes were kept at 30. Which may benefit through drivers but not the street residents, cyclists or pedestrians.
It increases rollout costs of the 20 zone, as now you need entry signs on every side road, exit signs when leaving the zone, and reminder signs. Also, it gives you a better excuse when caught speeding.
What it does do is make it safer to walk on the road, which makes passing cars parked in demi-drives easier.
Which is handy, as the foreshortened gardens of Portsmouth are not big enough to park cars, so where someone has managed to get council clearance to knock down the wall, you can't get four cars in the garden except demi-drive style.