The m'way speed limits were set when we were driving Morris 1000's, Anglias and Minis.
Many urban limits are unrealistic, especially 20mph ones, [if you can't think of any, I would be more than happy to show you some in my area].
The only reason we have 20 mph speed limits, and we have some near us, is that arrogant, know it all drivers will not stick to the 30mph speed limit. Admittedly it is a chore to adhere to 20mph a BMW up my rear end helps me to comply.
Now you are using the same argument as my wife - set the speed limit 10 mph lower than you really think it should be, except, there are people like Grahams lost friend, who will not obey any limit, what would you do to help those people see the light?