From there the trunkline runs north through an interchange with US Highway 12 (US 12) into Niles along a route that was once part of Business US 12 (Bus.

North of Niles, the highway runs parallel to a river and a rail line through rural areas.

The current highway dates back to 1971 when the southern end of M-40 was rerouted, and the previous alignment was given the M-51 moniker.

[4][5] The highway crosses a rail line owned by Amtrak[6] before curving northeasterly near the Plym Park Golf Course.

North of town in rural Cass County, M-51 intersects M-152 near the location where the highway crosses the Dowagiac River.

M-51 passes south of Knickerbocker Lake before turning northeasterly parallel to the rail line along Delaware Street in Decatur.

As a part of these maintenance responsibilities, the department tracks the volume of traffic that uses the roadways under its jurisdiction.

[7] M-51 between US 12 and the state line has been listed on the National Highway System,[8] a network of roads important to the country's economy, defense, and mobility.

[9] When the state highway system was originally signed in 1919,[10] M-51 was initially designated from Holland to Grand Rapids.