Thanks everyone for the tips. Interesting discussion on mirrors. I'm glad I brought it up
My concern isn't merging across lanes; all of these roads are only 2-lane so there is no lane changing going on.
If I'm riding on sand roads, my bike -- and the wind -- makes enough noise that I may not hear vehicles approaching very early (especially if there is a headwind). Looking over my shoulder several times per mile will work fine some places, but not necessarily when it's hilly. Often times, on these sand roads, I found it takes quite some concentration to keep my bike out of the thick sandy areas left behind by road maintainers or cars, which could cause me to crash or suffer a lack of traction. Sometimes I've got about 6 inches to work with. I want to avoid looking over my shoulder to see if a car is coming, then promptly wipe out right before it arrives because I wasn't watching the road in front of me! Also, on those roads, there will often simply be no possible way for a vehicle -- whether approaching from the front or behind -- to get around me unless I move over.
Several of you suggested a forward-facing headlight of some sort. I'm curious about that. I don't feel particularly concerned about oncoming traffic. I figure I will see the cars before they see me, and I ought to be able to pull over in time, whether encountering a single vehicle on a sand road or someone passing a car on a paved road. Am I being over-confident somehow?
The other thing I'm quite interested in is thoughts on riding on the sand roads vs. the 2-lane paved road. The sand roads are slower traveling, but have far less traffic. On the other hand, there are some risks there -- like piles of sand, dirt, etc. -- that could cause more than a minor accident if I hit them right when there is a vehicle passing. What do you think -- is it safe to ride on a 2-lane paved road with some hills, no shoulders, and (automobile) commuters and semis? How far to the right should I be riding there? On my test ride (a Sunday afternoon, so less traffic than I'd normally see), I rode pretty much on the white line the whole time, and only one driver passed me uncomfortably close.