Katahdin Woods and Waters is America's Newest National Monument (2016). It's still basically undeveloped, and the loop road is one of the few amenities. The Katahdin Loop Road is a semi-improved gravel road open to passenger vehicles, but for now, it sees very little traffic and makes for a very peaceful gravel grinder. You still have the chance to see bears (saw mama with three cubs), moose (one cow), deer, and maybe even the odd homo sapien (one male/female pair).
The area is heavily forested (logging trucks ply the roads), and the area is particularly beautiful during fall color season.
The "loop" road starts at the boundary of the National Monument at Sandbank Stream, which is 12.5 miles down the gravel Swift Brook Road from highway ME 11. At Sandbank Stream, there is parking, a pit toilet, an information kiosk, and a small pond. You'll travel 2.5 miles from the start of the loop road before you reach the loop proper.
You'll definitely want to take the short diversion on the Scenic Overlook Road
at about the 7.0 mile mark to get a good view of Mt. Katahdin itself. There is a picnic area and pit toilet at the overlook.