The problem with a lot of the 20 zones is that there is no common sense used with where they are put, I can understand it on roads outside schools, perhaps on no through roads (in housing estates, ect), and where it's difficult to get two cars side by side down a road, due to parking.
But some roads around Oxfordshire seem to have the most random speed limits, there's a village on my commute to work where the limit is 40, yet all the surrounding villages are 30, and a lot of main roads that were previously NSL have been changed to 50. Other main roads have been changed to 20 zones from 30/40 zones, which I could understand if there was development of new buildings that would make the roads a danger to pedestrians.
However there needs to be a lot more teaching in schools and from parents of the highway code, as so many children just seem to run out for no apparent reason without even looking!
Ultimately though it's all down to emissions, I tried doing a mock theory test yesterday to pass the time, and a lot of the questions asked were focused on emissions of vehicles, the most efficient way to drive, and one question that seemed a bit random was asking how much of the UK emissions is caused by vehicles on the road (Answer - 20%).