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> slow down cars in urban areas This is actually happening. Portland has recently fallen in line with other cities, like Seattle, and lowered thousands of miles of streets from 25 MPH to 20 MPH. http://www.oregonlive.com/commuting/index.ssf/2018/01/portla...
In the UK a lot of residential areas have introduced 20mph limits (down from 30mph), close to no-one (except learner drivers) obey this new limit! :(
I live in Edinburgh, which has been gradually introducing 20mph limits over the last couple of years, and I've actually taken to them much better than I'd expected to: they don't reduce my average speed all that much, especially as the arterial roads are still 30mph, and they're much more pleasant when I'm out of the car.