This is another old service road that the park service has allowed bikes to use in an effort to look like the park is more bike friendly. Simply goes from the Grand Loop to Tower Creek Campground.
Note: Some would like to pretend this is a loop trail. It most certainly is not a loop. There is not a trail paralleling the highway. During a small window in the fall, the Great Loop Road is open to bicyclists when vehicles are absent. Otherwise, plan on riding this an out-and-back.