On the lowering to 50 front, they needn't bother, I've noticed everyone seems to drive between 45 and 50 these days anyway, when I first past my test I was amazed to see everyone driving at 70 but then a year after onwards every car on the road seems to be about 50 MPH round here, overtake them and just get stuck behind another doing 45. (Then it really annoys me how as little of a hurry as they’re obviously in they then don't slow down for towns) Personally I'm under the opinion that if it's an open road out in the country with no pedestrians around, then speed limits are really not needed, I'm not sure why they're there on open roads and motorways, so limiting to 50 is pointless.
As for in towns and villages, I've always said that whilst I do tend to often speed on open roads, I wouldn't speed in towns and villages because there are people around, so I'm all for limits in those areas, but 30 to 20? 30 seems fine to me, it seems a problem with those idiots I described earlier who are doing more then that though. Isn't this an area where hazard perception should play a big part? Whilst you do occasionally get the drunk who jumps into the road out of nowhere, surely most of the in town accidents could be avoided if the driver is alert and paying attention to the world around them. Utilizing good driving.
Putting it at 20 will just add frustration and therefore make people less calm, thus less alert and aware, more likely to have an accident by not noticing the hazard, even if they aren't frustrated, they'll be daydreaming. 20 for a short stint outside a school fine, the whole town? nah.
The of course there's the traffic issues as you've mentioned, there will be more cars in the city at any time as they haven't managed to get out fast enough Lol, it will become more clogged up, people will be driving at 20MPH between traffic jams. It'll be quicker to walk at that rate. Plus has anyone thought about the years off your life that stress takes?
Ok I went a bit daft at the end there, but still, I think 50mph is a pointless waste of time. And 20 MPH is not much better. We need to focus on improving drivers’ hazard perception, alert/ awareness.