It's a great idea in my opinion. Such limits exist already in small areas and I really don't see why all urban areas shouldn't have the 30mph limit reduced to 20mph.
Of course, there are many who would completely disregard a 20mph limit in exactly the same way as they disregard every other speed limit and that's why we need enforcement to ensure people comply. I've mentioned it before but we really, really need a highway patrol in this island of ours. Only when people understand that breaking traffic laws is not very different to breaking other laws (AND WE CALL IT A CRIME!) will we ever see improved driving standards in the UK.
I know that flecc has a view on driving tuition and the driving test in the UK and I think I possibly find myself in agreement with him on this issue for the following reason. Having driven extensively through europe for many years, of the first world countries, (the western end of mainland europe) the most stringent driving test, according to what I've read, is the Spanish one. Incredibly, Spain is the country in which I have noted the most consistently bad driving. France can be a frightening experience if you haven't done it before as everyone seems to drive their Clios etc foot to the floor at all times. The French simply don't observe speed limits, end of!
All that said, the general driving skills I have encountered in France have been pretty good. In Spain, however, it's a different story. Many young Spaniards drive very fast, too fast but they drive STUPIDLY with it! It seems to be a trait picked up from the older generation who are simply useless behind the wheel of a car.
Standards in Germany, Holland, Belgium and the Scandanavian countries are all much better than in southern europe and I'm at a loss to explain why there are noticeable differences. Perhaps it's a cultural thing as when we watch football, there's definitely a marked difference between British refs and their mainland europe counterparts, well, the latin countries certainly!...I don't know.
Anyway, to get back to the point, an enforced, well policed 20mph limit in urban areas might just get some drivers thinking they'd be better off cycling so yes, I'm all for it.
Indalo