The State Trunkline Highway System of the US state of Michigan is a network of roads owned and maintained by the Michigan Department of Transportation (MDOT). The most prominent of these roads are part of one of three numbered highway systems in Michigan: Interstates Highways, US Highways, and the other State Trunklines. Other systems of roads are operated by the 83 counties in the state as well as each city.
Interstate Highways and US Highways are assigned at the national level. Interstate Highways are numbered in a grid—even-numbered highways are east–west highways (with the lowest numbers along Mexico and the Gulf of Mexico), and odd-numbered highways are north–south highways (with the lowest numbers along the Pacific Ocean). US Highways are also numbered in a grid—even numbered for east–west highways (with the lowest numbers along Canada) and odd numbered for north–south highways (with the lowest numbers along the Atlantic Ocean). For this reason, mainline (two-digit) Interstate Highways in Michigan all have numbers above 69 and mainline US Highways all have numbers below 45. Three-digit Interstate and U.S. Highways, also known as "child routes," are branches off their main one- or two-digit "parents". The Interstate and US Highways are maintained by MDOT. Interstate 75 (I-75) and US Highway 23 (US 23) are the longest examples in the state.
State Trunklines are the other state highways maintained by MDOT. These highways are completely owned and maintained by the state. Michigan highways are properly referred to using the M and never as "Route 28" or "Highway 115", but as M-28 or M-115. The marker used for state trunklines is a diamond with a block-letter "M" at the top. Roads that are maintained by the state but not assigned a state trunkline designation carry an unsigned highway designation.
County-Designated Highways are assigned numbers in a zone system by MDOT, but maintained by the counties. Each zone is indicated by a letter A–H which is followed by a number based on a grid inside that zone. Each county also maintains any other roadway that is not a state trunkline or a city street. The numbering and signing practices vary from county to county, as does the size of each county's system. Numerical designations typically do not carry over from one county to the next; a CDH that crosses county lines keeps its designation in each county however. County road designations are typically abbreviated "CR" or "Co Rd" followed by the number, and CDHs are abbreviated to just their letter and number assignment.
Recognized content - 
Interstate 375 (I-375) is a north–south auxiliary Interstate Highway in Detroit, Michigan, United States. It is the southernmost leg of the Walter P. Chrysler Freeway and a spur of I-75 into Downtown Detroit, ending at the unsigned Business Spur I-375 (BS I-375), better known as Jefferson Avenue. The freeway opened on June 12, 1964. At only 1.062 miles (1.709 km) in length, it once had the distinction of being the shortest signed Interstate Highway in the country before I-110 in El Paso, Texas, was signed. The Michigan Department of Transportation (MDOT) announced in 2021 plans to convert the freeway to a boulevard. Details of that project were revealed in April 2023 with MDOT reaffirming that construction is scheduled to begin in 2025. (Full article...)
-
H-58 west of Grand Marais
-
M-39 bridge over the River Rouge in Dearborn
-
Map of the pre-statehood Indian trails
-
Milemarker used in 1922 for M-14
-
M-121 in Jenison
-
Grand River Avenue, once a part of M-16 and later US 16, was originally an Indian trail converted as a plank road before becoming a state highway.
-
Planning map for the Detroit area freeways from 1955
-
A-2 in Allegan County south of Holland
-
Welcome sign along US 8
-
Dead Man's Curve along the Marquette–Negaunee Road shown in 1917 with its hand-painted centerline, the first in the nation
-
Construction along I-96 for the interchange with M-231 in July 2013 (from Michigan State Trunkline Highway System)
-
Exit 254 on I-75 south of Grayling
-
M-553 approaching Glass' Curve south of Marquette
-
M-87 on November 1, 1956
-
Junction between M-28 and M-178 south of Munising before 1941
-
Map of the zone system
-
Interstate HighwaysOther freewaysMiscellaneous expressways
-
H-13 during winter
State Trunkline System, overview of the system
- Interstates Highways in the state
- US Highways in the state
- State Trunklines
- County-Designated Highways in the state
- Pure Michigan Byways, a system of scenic, historical or recreational highways in the state
- Great Lakes Circle Tour
Nominations and votes for selected articles and selected pictures are always needed. Anyone can nominate an article, and anyone can vote for an article. You can also recommend items for Did you know?. Improvements to the articles are always welcome as the project works on expanding and nominating the remaining ones for Good Article or Featured Article status.
