I have reviewed this OSM Wiki page in the past, but I (and, judging by the OSM US Slack server, many other people) try to avoid and highly recommend avoiding functional classification data. Functional classification from DOTs generally do not line up with OSM's guidelines regarding roads and are mainly intended to display how the roads are funded and maintained by the DOT rather than reflect the road's importance for civilian navigation. Rather, a basis on what major destinations (populated places, commercial areas, airports, etc.) and other major roadways a road links to is what should be used for classification rather than how the state describes it for what are primarily internal purposes. Additionally, AADT data is useful to determining how important a road is relative to nearby roads that serve a similar purpose, but should certainly not be relied upon always.

