I would imagine collecting speed limit information in a SC overlay more like adding parts to a puzzle. You won’t be able to start with any arbitrary piece (at least, this will be extremely difficult), but it should be easy to add to what is already finished, as you got more context there and the number of missing parts gets smaller and smaller. So, the short answer to
is: Maybe just nothing, and wait for other puzzle pieces to give more context that allow for a more substantial answer. Why couldn’t a user say: I’m not sure, and the overlay does nothing? But keep in mind, I don’t know very much about US speed limits, and this perspective fits probably best for countries with comparatively straightforward speed limit rules. Perhaps in the US (and maybe other countries) a tagging the absence of speed limit signs could be helpful.