See Road names/USA, Road types/USA
Most of the state's roads are typed according to WSDOT functional classification. Most of the state road types are set to WSDOT's map. Of course, there are always exceptions, so the Functional Class map isn't biblical truth, but if we map the roads according to WSDOT's FC map the routing choices will be a lot better!
If you want to know more about the Federal Functional Classification check the Functional Classification article.