They are also set for other reasons, like "Bob complained a lot, because of the noise of cars", and the limit gets reduced. Or "this road was quiet 20+ years a

They are also set for other reasons, like "Bob complained a lot, because of the noise of cars", and the limit gets reduced. Or "this road was quiet 20+ years ago", but the city has grown, and now it is a major throughfare, so "Local residents campaigned to get a low speed limit", so that alternate routes are preferable. Both of the above have happened in my locale.