Theres no reason they have to be physical signs, just link it to the speed limit database companies like TomTom seem to have for their GPS units and use that.

Theres no reason they have to be physical signs, just link it to the speed limit database companies like TomTom seem to have for their GPS units and use that. There's always the problem of keeping that up to date, but its no harder than the problems that would have to be solved for physical signs.