M-51 starts at the Indiana state line as a continuation of SR 933 into Michigan. The trunkline runs north through residential areas along 11th Street into Niles. On the south side of town, it meets US 12 before continuing northward through commercial areas. Between the intersections with Maple and Main streets, M-51 splits into a
one-way pair of streets to follow 12th Street northbound and 11th Street southbound. Oak Street marks the western end of
Bus. M-60, and M-51 turns westward on Main Street into downtown Niles. At 5th Street, the highway turns north again to exit downtown. before curving northeasterly near the Plym Park Golf Course. M-51 angles parallel to the
Dowagiac River and the Amtrak line as the highway crosses from
Berrien County into
Cass County. The landscape transitions to farmland along the river, and the highway turns to the east between
Sumnerville and
Pokagon. M-51 is maintained by MDOT like other state highways in Michigan. As a part of these maintenance responsibilities, the department tracks the volume of traffic that uses the roadways under its jurisdiction. These volumes are expressed using a metric called
annual average daily traffic, which is a statistical calculation of the average daily number of vehicles on a segment of roadway. MDOT's surveys in 2010 showed that the highest traffic levels along M-51 were the 20,298 vehicles daily south of US 12; the lowest counts were the 2,658 vehicles per day west of Decatur. M-51 between US 12 and the state line has been listed on the
National Highway System, a network of roads important to the country's economy, defense, and mobility. ==History==