M-123 is a state trunkline highway in the eastern Upper Peninsula of the US state of Michigan. It is one of only a few highways in Michigan that curve around and form a U-shape. In fact, M-123 has three intersections with only two state trunklines; it meets M-28 twice as a result of its U-shaped routing. M-123 also has a rare signed concurrency with a County-Designated Highway in Michigan; in Trout Lake, there is a concurrency with H-40. All of M-123 north of M-28 is a Scenic Heritage Route within the Michigan Heritage Route system.

M-28 is an east–west state trunkline highway that traverses nearly all of the Upper Peninsula of the U.S. state of Michigan, from Wakefield to near Sault Ste. Marie in Bruce Township. Along with US Highway 2 (US 2), M-28 forms a pair of primary highways linking the Upper Peninsula from end to end, providing a major access route for traffic from Michigan and Canada along the southern shore of Lake Superior. M-28 is the longest state trunkline in Michigan numbered with the "M-" prefix at 290.373 miles (467.310 km). The entire highway is listed on the National Highway System, while three sections of M-28 are part of the Lake Superior Circle Tour. M-28 also carries two memorial highway designations along its route.