I had a personal anecdote about descending on MD route 77 just south of Camp David after climbing out of Thurmont. A single lane each way with ample shoulder. During the decent, I could have reached out and touched the fender of the cars pacing me. This was a decade or so ago. Now I'm so old that I can't remember the exact speed number. It may have been 50 mph + or - 2, but don't quote me. Again, I don't remember things as reliably as I used to.