If they are mostly unenforced, how does the system work? Does everyone implicitly add 20% to the posted limit, or do they just go whatever speed feels right to them given the road and the conditions?
(In my part of Australia, speeding is fairly common, but not speed limits are not a joke. You can expect most people to stick within ~5km/h of the limit, and if you choose not to then there's a real chance you'll get done by a hidden camera. The idea of everyone ignoring the limits sounds terrible: of course they are a bit arbitrary, but that's unavoidable for any unambiguous law, and the alternatives of arbitrary enforcement or no enforcement seem much worse.)