Mich. Comp. Laws § 247.953
Certain portions of the state trunkline highway system are so uniquely endowed by natural aesthetic, ecological, environmental, and cultural amenities immediately adjacent to the roadside that their use by a larger percentage of the motoring public, particularly during the recreational season, is for the experience of traveling the road rather than as a route to a destination. Because of the immediate proximity of these features, roads may possess characteristics such as the following: pavement width of 16 to 20 feet, shoulders as narrow as 2 feet with trees immediately adjacent, curves that restrict maximum legal speeds, hills, steep side slopes, and narrow rights-of-way. The improvement philosophy for these roads is to maintain the essential elements of the road and the area immediately surrounding the road that create its unique character.
MCL 247.953