In principle a great idea. That said, I live in a 20 mph zone and in the five years I've lived here I've seen zero enforcement. I suspect I'd be more likely to a see aliens dancing the tango. The problem is that many of the roads are rat-runs between two A roads and attract the kind of drivers who hammer down them at forty because they think it'll save them several seconds at the lights on the main road. That's not really a criticism of the 20 mph limit and I'd rather have it than not, and I think it ought to cover all residential areas, there's really no good reason to be driving faster. Driving at 20mph is actually quite hard, it's amazing how used we have become to zooming around and I think that learning to drive slow is no bad thing. Modern cars make speeding along too easy.
So, yes, I'm totally in agreement, but I'd like to see some enforcement. Also, for my local roads, they ideally ought to be no-through-roads to stop the rat-running. I guess there needs to be an urban design for residential areas of which 20 mph is a key starting point, but residential streets should be proper mixed use amenities.