Popular with hunters during deer season. Check Michigan DNR site for exact dates and wear orange when appropriate. A Michigan "Recreation Passport" is required to park at the trailhead.
D Loop has some of the more technical running in the Highland State Recreation Area with lots of tight turns, some rocks, and longer climbs. Your time is spent navigating rocks and roots on this narrow singletrack. From the trailhead on Livingston Road, you can connect to the D Loop from the A Loop.