M-121 is maintained by the Michigan Department of Transportation (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 lowest traffic levels along M-121 were the 10,103 vehicles daily at the western terminus in Zeeland; the highest counts were the 20,140 vehicles per day at the eastern terminus. None of M-121 has been listed on the National Highway System, a network of roads important to the country's economy, defense, and mobility.