M-18 is maintained by the Michigan Department of Transportation (MDOT) like all other state trunkline highways. As a part of these maintenance responsibilities, the department tracks the traffic volumes along the roads under its jurisdiction using a measurement called annual average daily traffic (AADT). This is a metric for the level of traffic along a roadway segment for any average day of the year. In 2009, MDOT calculated that 14,017 vehicles used the section of M-18 along the M-61 concurrency, the highest traffic levels along M-18. The lowest AADT was found along the highway in Crawford County in the 2009 surveys. Commercial traffic varied between the 50 trucks daily in Crawford County and the 214 trucks a day north of Gladwin. No section of M-18 has been listed on the National Highway System, a network of roads important to the country's economy, defense, and mobility.