The situation is that there is no publically available digital files for map authors to use which have this data (detail). Unlike the situation is Canada, the US (USGS) has not digitized most of the 24k scale paper maps. Census, the primary source of transportation data, is primarly interested in and funded to collect data on where people live and housing units - thus, no people, likely no data. Some State highway and/or GIS offices have more detailed info, but this of course is limited to their State. Try the NorthWest Trails overlay/transparent mapset.