While I'd like that approach in theory, I see two issues with this: 1) It doesn't answer my 'going from some limit to no (declared) limit' thought experiment.

While I'd like that approach in theory, I see two issues with this: 1) It doesn't answer my 'going from some limit to no (declared) limit' thought experiment. I still think that this is ~hard~. 2) Who are the local citizens to change the signs? Who's responsible for signs on highways/streets through parts of the country w/o a lot of people? On a sidenote: Advisory speed limits are exactly what DE is doing: If there's no sign indicating something otherwise, the Autobahn has an advised maximum speed limit of 130km/h. If you go faster, you might lose parts of your insurance protection (depending on the results in case of an accident). It's legal, but in that case you're just ignoring the friendly suggestion to stay at 130.