I rode the carriage roads this past Sept for the first time, getting in a 20 miler, on what was one of the most enjoyable rides I've ever done.
It was totally random, as I had my touring bike with me on vacation, with 28mm tires and was planning to ride the paved roads around Mt. Desert Island. I detoured to a carriage road at a spur of the moment and had a wonderful ride. I also missed a turn and went "Around the mountain" on a long climb with some exceptional views.
FWIW, I found the riding surface appropriate for the bike and tires. Most folks were on hybrids or rented mt. bikes, which I was passing constantly. I had to be a bit more cautious around the downhill turns on gravel, but for the most part, the road surface was hard packed dirt, very smooth, with a bit of gravel.
Going back next fall.
Steve B.