The State Trunkline Highway System consists of all the state highways in Michigan, including those designated as Interstate, United States Numbered, or State Trunkline highways. In their abbreviated format, these classifications are applied to highway numbers with an I-, US, or M- prefix, respectively. The system is maintained by the Michigan Department of Transportation (MDOT) and comprises 9,669 miles (15,561 km) of trunklines in all 83 counties of the state on both the Upper and Lower peninsulas, which are linked by the Mackinac Bridge. Components of the system range in scale from 10-lane urban freeways with local-express lanes to two-lane rural undivided highways to a non-motorized highway on Mackinac Island where cars are forbidden. The longest highway is nearly 400 miles (640 km) long, while the shortest is about three-quarters of a mile. Some roads are unsigned highways, lacking signage to indicate their maintenance by MDOT; these may be remnants of highways that are still under state control whose designations were decommissioned or roadway segments left over from realignment projects.

M-185 is a state trunkline highway in the U.S. state of Michigan that circles Mackinac Island, a popular tourist destination on the Lake Huron side of the Straits of Mackinac, along the island's shoreline. A narrow paved road of 8.004 miles (12.881 km), it offers scenic views of the straits that divide the Upper and the Lower peninsulas of Michigan and Lakes Huron and Michigan. It has no connection to any other Michigan state trunkline highways—as it is on an island—and is accessible only by passenger ferry. The City of Mackinac Island, which shares jurisdiction over the island with the Mackinac Island State Park Commission (MISPC), calls the highway Main Street within the built-up area on the island's southeast quadrant, and Lake Shore Road elsewhere. M-185 passes by several important sites within Mackinac Island State Park, including Fort Mackinac, Arch Rock, British Landing, and Devil's Kitchen. Lake Shore Road carries the highway next to the Lake Huron shoreline, running between the water's edge and woodlands outside the downtown area.