 M @-@ 81 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 highest traffic levels along M @-@ 81 were the 17 @,@ <unk> vehicles daily between Van <unk> Road and Frank Street in Caro ; the lowest counts were the 3 @,@ 527 vehicles per day east of Cass City to the M @-@ 53 intersection . No sections of M @-@ 81 have been listed on the National Highway System , a network of roads important to the country 's economy , defense , and mobility . 
