That can't possibly work everywhere. Here in my state in Australia we have speed signs for the off-ramps where they are clearly visible from the freeway, so you

That can't possibly work everywhere. Here in my state in Australia we have speed signs for the off-ramps where they are clearly visible from the freeway, so you might be in a 100 km/h zone but see a 60 km/h sign off the side that could easily be mistaken for a speed sign for the freeway (it's actually not that great design, sometimes they have a sign saying 'ramp speed' under it but not always)