M-21 is an east–west state trunkline highway in the US state of Michigan connecting the cities of Grand Rapids and Flint. The highway passes through rural farming country and several small towns along its course through the Lower Peninsula. Following the course of a handful of rivers, M-21 also connects some of the state's freeways like Interstate 96 (I-96), US Highway 127 (US 127) and I-75. The highway is used by between 1,700 and 36,000 vehicles daily.
The Thumb is a region and a peninsula of the U.S. state of Michigan, so named because the Lower Peninsula is shaped like a mitten. The Thumb area is generally considered to be in the Central Michigan region, east of the Flint area and the Tri-Cities and north of Metro Detroit. The region is also branded as the Blue Water Area.
M-46 is an east–west state trunkline highway in the US state of Michigan between Muskegon and Port Sanilac, terminating near Lake Michigan and Lake Huron on each end. Except for the north–south segment that corresponds with the US Highway 131 (US 131) freeway between Cedar Springs and Howard City, M-46 is practically a due east–west surface highway. The road runs through rural sections of the Lower Peninsula connecting several freeways including US 31, US 131, US 127 and Interstate 75 (I-75).
M-13 is a 73.339-mile (118.028 km) north–south state trunkline highway that runs through the Saginaw Bay region of the US state of Michigan. It runs from Interstate 69 (I-69) south of Lennon to US Highway 23 (US 23) near Standish. The southern section of the trunkline runs along a pair of county lines in a rural area dominated by farm fields. The highway directly connects the downtown areas of both Saginaw and Bay City. North of the latter city, the Lake Huron Circle Tour follows M-13 along the Saginaw Bay.
M-53 is a north–south state trunkline highway in the US state of Michigan that connects Detroit to The Thumb region. The highway starts in Detroit at a connection with M-3 and ends in Port Austin, Michigan at M-25. In between, the trunkline passes through the northern suburbs of Metro Detroit, connects to freeways like Interstate 69 (I-69) and provides access to rural farmland. In Macomb County, M-53 follows the Christopher Columbus Freeway and POW/MIA Memorial Freeway, while the remainder of the highway is known as Van Dyke Avenue in the metro area or Van Dyke Road elsewhere. The highway has also been named the Earle Memorial Highway for one of the pioneers of the Good Roads Movement and Michigan's highway system.
M-142 is an east–west state trunkline highway in The Thumb region of the US state of Michigan. It is a "trans-peninsular" highway in that it travels through an agricultural area from Bay Port on Saginaw Bay to Harbor Beach on Lake Huron. The current trunkline in Huron County was originally parts of other state highways that date back to the initial 1919 signposting of the state highway system in the state. The designation was applied in 1939, and the road has remained unchanged since it was completely paved in the 1950s. One other highway, near Lake City, carried the number in the 1930s.
M-19 is a north–south state trunkline highway in the US state of Michigan. The trunkline begins northeast of Detroit at a junction with Interstate 94 (I-94) near New Haven and runs northward to a junction with M-142 just east of Bad Axe in The Thumb region of the Lower Peninsula. The highway runs through mostly rural and agricultural areas, connecting several small communities.
M-29 is a state trunkline highway in the US state of Michigan that runs in a south–north direction from Chesterfield Township to Marysville. It generally runs along the shore of Lake St. Clair and the St. Clair River.
M-31 was a state trunkline highway in the Lower Peninsula in the US state of Michigan. It generally ran north from Port Huron along the Lake Huron shoreline through The Thumb region before turning inland. The highway crossed The Thumb and then ran along the Saginaw Bay shoreline before running inland again, terminating at Saginaw. It was one of the original state highways signposted in 1919, but it was renumbered as other highways in 1926, decommissioning the designation in the process. Parts of its route are now M-24, M-25, M-81 and M-142.
M-146 was a state trunkline highway in the US state of Michigan that initially served as a bypass for traffic going around Port Huron and as a short cut for traffic between U.S. Highway 25 and M-21. Later it connected to the Blue Water Bridge before it was truncated to its final form. Segments of M-146 still exist today as state highways as part of Interstate 94 (I-94) and I-69 and the Lapeer Connector.
M-81 is an east–west state trunkline highway in the Lower Peninsula of the US state of Michigan. The trunkline travels from the city of Saginaw at the junction with M-13 to the junction with M-53 east of Cass City over the county line in Greenleaf Township in northwestern Sanilac County in The Thumb area of the state. Outside of the cities and villages along its route, M-81 passes through mostly rural farm country. Near Saginaw it intersects the freeway that carries both Interstate 75 (I-75) and US Highway 23 (US 23) in an industrial area.
M-83 is a north–south state trunkline highway in the Lower Peninsula of the US state of Michigan primarily serving as a link between Interstate 75/US Highway 23 (I-75/US 23) in Birch Run, including a short east–west section with M-54, and the Bavarian-themed town of Frankenmuth. M-83 is primarily a north–south trunkline that passes by such landmarks as Bronner's Christmas Wonderland, Zehnder's and the Bavarian Inn before leaving town. The landscape in the remainder of the area is composed of farm fields between Frankenmuth and the northern terminus at M-15 near Richville.
M-90 is a state trunkline highway in The Thumb region of the US state of Michigan. It runs from near North Branch eastward to Lexington situated on Lake Huron. The highway is a lightly traveled roadway that runs through rural farmlands. The trunkline runs mostly east–west with two short north–south segments where it turns to run concurrently with other state highways. Along the routing, there are two river crossings and one railroad crossing.
US Highway 23 (US 23) is a north–south United States Numbered Highway that runs from Jacksonville, Florida, to Mackinaw City, Michigan. In the US state of Michigan, it is a major, 362-mile-long (583 km), north–south state trunkline highway that runs through the Lower Peninsula. The trunkline is a freeway from the Michigan–Ohio state line near Lambertville to the city of Standish, and it follows the Lake Huron shoreline from there to its northern terminus. Serving the cities of Ann Arbor and Flint, US 23 acts as a freeway bypass of the Metro Detroit area. Overall, the highway runs through rural areas of the state dominated by farm fields or woodlands; some segments are urban in character in the Ann Arbor, Flint and Tri-Cities areas. The section from Flint north to Standish also carries Interstate 75 (I-75) along a concurrency that includes a segment that carries almost 70,000 vehicles on a daily basis.
Interstate 69 (I-69) is a part of the Interstate Highway System that will eventually run from the Mexican border in Texas to the Canadian border at Port Huron, Michigan. In Michigan, it is a state trunkline highway that enters the state south of Coldwater and passes the cities of Lansing and Flint in the Lower Peninsula. A north–south freeway from the Indiana–Michigan border to the Lansing area, it changes direction to east–west after running concurrently with I-96. The freeway continues to Port Huron before terminating in the middle of the twin-span Blue Water Bridge while running concurrently with I-94 at the border. There are four related business loops for I-69 in the state, connecting the freeway to adjacent cities.
There are currently eight business routes of Interstate 94 (I-94) in the US state of Michigan. These business routes connect I-94 to the downtown business districts of neighboring cities. Seven of the eight routes are business loops which bear the Business Loop I-94 designation while one is a business spur that bears the Business Spur I-94. These loops are former routings of I-94's two predecessors in Michigan: US Highway 12 (US 12) or US 25. The westernmost BL I-94 runs through the twin cities of Benton Harbor and St. Joseph along the former routing of US 12 and US 31/US 33 that now includes a section of the Lake Michigan Circle Tour in the state. The loops in Kalamazoo, Battle Creek, Marshall, Albion, and Jackson were also formerly segments of US 12 which were later designated as separate version of Business US Highway 12 through their respective cities before becoming BL I-94s in 1960. The BL I-94 in Kalamazoo was converted into BS I-94 in 2019. The route of the business loop through Ann Arbor was previously US 12 and then later M-14 before receiving its current moniker. The BL I-94 through Port Huron was previously US 25 and then Business US Highway 25.
There are currently four business routes of Interstate 69 (I-69) in the US state of Michigan. Designated Business Loop Interstate 69, they are all former routings of I-69's predecessor highways, US Highway 27 (US 27), M-78 or M-21, in whole or in part. The BL I-69 in Coldwater and the one in Charlotte were both parts of US 27 before the freeway bypassed those two cities in 1967 and the early 1970s, respectively. The BL I-69 through Lansing and East Lansing was previously part of M-78 and Temporary I-69 until it was redesignated in 1987. Before 1984, the loop in Port Huron was originally part of M-21 and was initially a business spur numbered Business Spur Interstate 69. It was later redesignated when it was extended to run concurrently with that city's BL I-94 which was originally part of I-94's predecessor, US 25. Each business loop follows streets through each city's downtown areas and connects to I-69 on both ends, giving traffic a route through the downtown and back to the freeway.
There have been nine business routes for Interstate 75 in the US state of Michigan. Numbered either Business Loop Interstate 75 or Business Spur Interstate 75 depending if they are a full business loop or a business spur, these highways are former routings of I-75's predecessor highways in the state. They were designated as I-75 was completed through the various areas of Michigan. The business loop in Pontiac runs through that city's downtown along a section of Woodward Avenue and a segment of roadway formerly used by M-24. The former Saginaw business loop was once a part of US Highway 23 (US 23), as was most of the original Bay City business loop. The roadways that make up the business loops in West Branch and Roscommon were previously part of M-76, I-75's predecessor through that part of the state. In Northern Michigan, the Grayling and Gaylord BL I-75s were part of US 27, and the two business routes in St. Ignace and Sault Ste. Marie in the Upper Peninsula of Michigan were part of US 2. A tenth business route, a loop through Indian River has been proposed. Each of the business loops connects to I-75 on both ends and runs through their respective cities' downtown areas. The two business spurs only connect to I-75 on one end and run into the appropriate downtown.
US Highway 25 (US 25) was a part of the United States Numbered Highway System in the state of Michigan that ran from the Ohio state line near Toledo and ended at the tip of The Thumb in Port Austin. The general routing of this state trunkline highway took it northeasterly from the state line through Monroe and Detroit to Port Huron. Along this southern half, it followed undivided highways and ran concurrently along two freeways, Interstate 75 (I-75) and I-94. Near the foot of the Blue Water Bridge in Port Huron, US 25 turned north and northwesterly along the Lake Huron shoreline to Port Austin.
There have been five different business routes of US Highway 23 in the state of Michigan. These business routes were designated along former sections of US Highway 23 (US 23) to provide signed access from the main highway to the downtowns of cities bypassed by new routings of US 23. Two are still extant, connecting through downtown Ann Arbor and Rogers City. Three others have been decommissioned. The former Business US 23 in Fenton was split in half during the 1970s and later completely turned back to local control in 2006. The former business loops through Saginaw and Bay City were renumbered as business loops of Interstate 75 in the 1960s.
Lake Huron Circle Tour 
Center Avenue Heritage Route
BL I-69 / BL I-94 at Port Huron 
I-75 / BS I-75 / US 10 / US 23 at Bay City
