On my way home from a trip to Crater Lake National Park I drove the entirety of OR 39 and much of it's continuation on CA 139 towards Adin. I thought CA 139 in particular was notable due to it following some really remote areas of Modoc County. For the most part CA 139 is signed with 65 MPH speed limits and the road definitely is equipped to handle it. Interestingly (Daniel pointed this out some time ago) Modoc County has it's own County Route program along with shields. I managed to capture all the County Route shields found on CA 139 from the State Line towards Adin:
https://flic.kr/s/aHsmWry6e8