Decided to look into the yellow counties in Montana because I was shocked if they could even be 49%. Petroleum County (a truly American name) in Montana has 228 occupied households and seemingly no public transportation. However, 58.2% of the county is employed in mining.
So my best guess is a company bus that comes around and picks up everybody? It seems like the bulk of the remainder are farmers, which would mean they don't "need" to drive anywhere. Although they do drive constantly across their property as part of the work, which feels like cheating.