I am completely with you when it comes to trying to photograph good examples of old road design and identification. I find I resort to drawing lines and arrows post production, which is not altogether satisfactory.
I often find myself driving a modern two lane road, and saying to my wife, “Notice the old road over there, to which her reply is “Where?” What you or I “see” is not always self evident, and as you say, hard to capture in a 2D photo.
I am looking forward to your book. It is much needed.
Oh, I should add something. I am no beginner at road identification, but I have a friend who can smell the old routes. I have been at this for at least 50 years so I have learned a lot, but he has a natural talent. Kind of like old time water witchers, he can spot the target most every time. If you want contact info, send me a Forum message. Someone should pick his mind and get his insights for others to use.
Keep the Show on the Road