It would appear both are being used for the exact same purpose. If there's an interest in keeping street_sign around for road signs that actually provide street names, an implication could be set.
I've cleaned out the tag and left only those that would be (I'm assuming) street names. The tag only had ~40 posts, and since it would have been cleaned out over 4 years when it was previously brought up, I'd say that is pretty sparse usage overall even if people would using it for road_sign.
