I don't believe primary is the best classification for roadways leading to a supermarket. The roads themselves aren't significant on their own merits, as they link no major population centers, transportation hubs, or other highways as the tag is intended to represent. A rural Walmart itself can be regionally significant, but only in its own niche as a private business that just so happens to be located along that road, and not like a typical primary road destination such as a town or an airport is.