First of all, obviously 20 mph will slow down traffic - that's the point! As for the argument that it makes life harder in villages because it takes longer for traffic to pull out - what bunk! The reason why this happens currently (if it happens at all) is obvious to anyone who thinks about it: as people generally expect cars to be travelling at 30 mph (or faster) they're not confident about the speed of oncoming vehicles; if everyone travelled slower, the longer 'wait' for the cars to go past would be compensated for by the fact that the rate at which the vehicles arrive would be slower, meaning that the safe distance to pull out would be reduced. As for the argument that life on the M8 would be harder at 20 mph, grow up! Obviously, the rule doesn't refer to motorways! Although the question about whether it's right to have motorways in the centre of town is another matter.
Some of the comments show why it'll be difficult to change to speed limit. Even on a bicycle forum, some people can't accept that anything touch the sacrilegious motor vehicle. So it'll make it slower to get from A to B in a car; it'll maybe 'choke' traffic. But when you consider that according to
official statistics in the GB for 2010, 20% of all journeys in the car were less than 1 mile, that only 2% of trips of less than 5 miles were made by bicycle,
that 20% of people say they took walks of 20 minutes or less only once a year and that 60% of adults are overweight - I say SO WHAT if car journeys in town become harder? Maybe that way people will start to think beyond their bonnet? Maybe, just maybe, people will start to become a little more intelligent: use the bus, use the bike,
walk like they used to for God's sake. And when that happens not only will it be easier to walk and cycle in town, it'll also be easier and safer for those who really need their cars (the elderly, the handicapped ...) to drive.
The point is that when pedestrians and cars share the same space, 30 mph is too fast. On my residential road the speed limit is 30 mph. If a child - or an adult for that matter - is hit at that speed there's a very good chance they won't survive; reduce the speed limit by 10 mph, they probably will. And don't anyone dare tell me that children shouldn't be playing outside!