the concurrency and leaving downtown. North of town in rural Cass County, M-51 intersects M-152 near location where the highway crosses the Dowagiac River. North of that intersection, the trunkline crosses into Van Buren County and turns to the east again. M-51 passes south of Knickerbocker Lake before turn northeasterly parallel to the rail line along Delaware Street in Decatur. after which is heads due north to I-94. 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 Previous designations When the state highway system was originally signed in 1919, M-51 was initially designated from Holland to Grand Rapids. The highway ran from Zeeland along a route that used Byron Road and 32nd Avenue to connect with Port Sheldon Street in Jenison before connecting with Chicago Drive. The highway's course was simplified by 1924 to use Chicago Drive between Zeeland and Jenison. When the US Highway System debuted in 1926, this original M-51 was renumbered as an extension of M-21, and the M-51 number was reused for the original M-27 in The Thumb. The highway ran from Port Huron to M-83 (now M-142) west of Harbor Beach. The designation of M-51 from Port Huron to M-46 was removed in late 1961, and the remainder to M-142 was removed in 1965. Current designation In 1971, all of M-40 south of I-94 to Niles was reassigned the designation M-51 while M-40 was shifted to the east to take over the routing of the contemporary M-119 between Paw Paw and its intersection with US 12 in Porter Township near Mottville. The routing of M-51 was extended down to the state line in April 1998 when US 33 was decommissioned out of the state of Michigan; the extension formed a concurrency