I have to agree with others, what we need is enforcement of the 30mph speed limits (and 40mph for that matter) not more 20mph zones. All the villages around where I live have recently been converted to 20mph, including mine, and when driving it literally sets your blood boiling crawling through villages of empty pavements at a frustratingly slow and pointless 20mph. I frequently cycle faster than many of the cars are allowed to drive, and often feel like the only person following the limit in the first place. So many people just ignore the sign saying 20mph, especially when the road is actually mostly used as a through-road, and continue to travel at the old speed limits of 30mph and 40mph. Unless the road is literally the market square or right on top of a school or something, these limits often seem to have been put in place for the sake of the 1 or 2 pedestrians visible on the street in an hour, in my town at least. More zebra crossings would be better for everyone! Or just pedestrianise town centres, put in speed camera, anything really.
I also disagree that it causes less noise and pollution to have people driving at 20mph... round here, you get huge worming trains of traffic going through, especially in places where the speed limit has suddenly gone from 60mph country roads down to 20mph and so you create a 100% constant flow of traffic for hours every morning and every evening. I don't know whether statistically this causes less pollution, but as a pedestrian or cyclist you have fumes in your face the whole time and a slow parade of wub-wub-wub boy racers creeping past.