Yes curlilox, we have blanket 20mph speed limits in the town where I live too. As you say, it is more or less impossible to keep to such a slow speed. Fair enough on play streets or roads with schools, but on main thoroughfares, it really is silly. The local police have said that they will not enforce the speed limits as they do not have the staff, so the result is that everyone totally ignores it.
It is a real problem for driving schools as they obviously have to comply with the legal limit and drivers aren't able to practice driving at higher speeds.
I suppose the only thing to be said for it is that people actually drive at 30mph rather than 40!