Michigan has a unique highway, M-185. It is only accessible by ferry, as it is on Mackinac Island.
It is also inaccessible to cars, as the island does not allow them, being the only state highway in the country in which cars are banned.
M-185 is only 8 miles long and circles the island. It is referred to as Main Street in town.
Michigan has two other island highways on Drummond Island and Harsens Island.