The forum is five years old already!
Your link to the detailed highway history of Oregon doesn't work, though. Here's the correct link:http://egov.oregon.gov/ODOT/HWY/GEOMETRONICS/ROW_Eng/HistoryHighwaysOregon/HSHO.pdf
Here's Iowa: http://www.iowadot.gov/maps//msp/historical/transmaps.htm
The Virginia link seems to have died. Anyone have a new link?
I don't know if there's anything useful here that's not on the other maps, but Oklahoma also has traffic count maps going back to 1937: http://www.okladot.state.ok.us/aadtcnt/Default.aspx?list=staticMaps