The original highways were approved on November 11, 1926, by AASHTO including 14 mainline highways. A handful of these original highway designations no longer run within Michigan, and a few numbers have been added since the 1930s. Since 1999, there have been 13 mainline highways, and with the creation of a business route for Constantine, there are a total of 30 special routes in the state. Several highways have been converted to freeways, some of which are now Interstate Highways and no longer part of the US Highway System. These freeway conversions started in the 1940s and 1950s and continued through to the turn of the 21st century.
MDOT is the agency responsible for the day-to-day maintenance and operations of the State Trunkline Highway System, which includes the U.S. Highways in Michigan. The numbering for these highways is coordinated through AASHTO, an organization composed of the various state departments of transportation in the United States. Under the scheme finalized in 1926, two-digit US Highways are numbered in a grid; east–west highways have even numbers while north–south routes have odd numbers. The lowest numbers are in the east and the north. The east–west highways in Michigan number in the single digits up to the low 20s. The north–south highways range from the low 20s into the 40s. There are also three three-digit highways numbered in the 100s and one in the 200s as branches of related two-digit highways.
In Michigan, the US Highway System covers about 2,300 miles (3,701 km) of mainline highways and another 160 miles (257 km) of special routes. There are 13 mainline highways and 30 special routes in total, including one business route that is unsigned. Two of the mainline highways in the Upper Peninsula exit the state and re-enter, resulting in two discontinuous segments of highway for each in Michigan. The longest US Highway in the state is US Highway 23 (US 23) at just over 362 miles (583 km) spanning from the Ohio state line north to the Straits of Mackinac in the Lower Peninsula, while the shortest is the 2.3-mile (3.7 km) segment of US 8 south of Norway in Dickinson County. Special routes range in length from about 11⁄4 miles (2.0 km) for Business US 2 (Bus. US 2) in Ironwood to 10 miles (16 km) for Bus. US 131 in Kalamazoo.