Mich. Comp. Laws § 247.953

Sec. 3. 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.