List of Interstate Highways
There are 70 primary Interstate Highways in the Interstate Highway System, a network of controlled-access freeways in the United States. They are assigned one- or two-digit route numbers, whereas their associated "auxiliary" Interstate Highways receive three-digit route numbers. Typically, odd-numbered Interstates run south-north, with lower numbers in the west and higher numbers in the east; even-numbered Interstates run west-east, with lower numbers in the south and higher numbers in the north. Highways whose route numbers are divisible by "5" usually represent major coast-to-coast or border-to-border routes (ex. I-10 travels from Santa Monica, California, to Jacksonville, Florida, traveling from the Pacific to Atlantic oceans). Additionally, auxiliary highways have their numbering system where a different number prefixes the number of its parent highway.
List of Interstate Highways in Michigan
Construction of the first expressways in Michigan predates the Interstate Highway System. During World War II, the Michigan State Highway Department (MSHD) built the Willow Run and Detroit Industrial expressways (now part of I-94) to carry workers from Detroit to the defense plants at Willow Run Airport. The state created the Michigan Turnpike Authority (MTA) in 1951, which proposed the construction of a toll freeway to run north–south in the state. The original termini for the Michigan Turnpike were Bridgeport and Rockwood. Interagency politics stalled progress on any proposed turnpikes while MSHD had three freeways under planning or construction.
The Interstate Highway System was authorized by the Federal Aid Highway Act of 1956, and the state had already designed several freeways for its portion of that system. Seizing the opportunity brought by a 1957 state law, the department sold $700 million in bonds (equivalent to $ in 2011) in the late 1950s and early 1960s to finance land purchases and construction of the new freeways. The goal was to connect every city with a population over 50,000 with four-lane freeways that could accommodate rural traffic traveling at . The MSHD delayed numbering these freeways as part of the Interstate Highway System until the federal government had finalized the designations to be assigned to Michigan's freeways. The first highway to be signed as an Interstate in Michigan was I-75, which received signage in late 1959, along a section near the Ohio state line that opened to traffic in October 1957. I-94 was the first of the Interstates to be completed border to border in a US state. In 1974, the state implemented mileage-based exit numbers along the Interstates in Michigan.