There have been nine business routes for Interstate 75 in the US state of Michigan. Numbered either Business Loop Interstate 75 (BL I-75) or Business Spur Interstate 75 (BS I-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 (that one has been converted into a business spur). 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. __TOC__

Pontiac

Business Loop I-75 (BL I-75) is a business loop serving Pontiac and Auburn Hills. It uses portions of former Business M-24 (Bus. M-24) through town. The highway starts as an eight-lane freeway at an interchange on I-75 in Bloomfield Township running westerly. The freeway continues past an interchange with Opdyke Road to end at Square Lake Road, dropping to six lanes. BL I-75 continues to M-1 at Woodward Avenue. Square Lake Road continues to the west as Bus. US 24, and BL I-75 turns northwesterly along the eight-lane divided Woodward Avenue, running concurrently with Bus. US 24 into downtown Pontiac. At the south side of downtown, Woodward drops to six lanes as the two directions of Woodward Avenue split and form a four-lane loop. The Woodward Avenue Loop encircles downtown Pontiac, and it is crossed by the eastbound direction of M-59 running on Huron Street. One block north, the westbound direction of M-59 runs along University Drive and turns to follow BL I-75/Bus. US 24 (Woodward Avenue Loop). The loop intersects Perry Street another block further north, and BL I-75 turns northeasterly onto the two-way Perry Street. (The southbound direction from Perry Street continues around the Woodward Avenue Loop back to Woodward Avenue.) BL I-75 passes through residential areas and a minor business area along the four-lane Perry Street. Outside of the downtown area, Perry Street widens to include a center turn lane. North of Walton Boulevard in Auburn Hills, the street name changes to Lapeer Road. BL I-75 intersects Opdyke Road a couple hundred feet west of I-75 before crossing the freeway. Lapeer Road continues as a six-lane, divided roadway to the ramps of a double trumpet interchange, where the BL I-75 designation ends. The roadway however, continues as M-24. On average each day in 2013, 9,829 vehicles use the business loop between the two M-59 junctions, and 73,795 vehicles do so on the freeway stub east of Opdyke Road, the lowest and highest traffic counts.

In 1919 when the state highway system was first numbered, the main north–south highway through Pontiac was numbered M-10, and it was renumbered to US 10 seven years later when the United States Numbered Highway System was created. By the next year, M-24 was designated from downtown Pontiac northward to Lapeer. M-24 was moved to an eastern bypass of town in 1936, and the former routing in town became M-24A. This was redesignated Bus M-24 in 1940.

A number of highway designation and routing changes in the Pontiac were made when US 10 was shifted out of downtown to replace M-58 along Telegraph Road west of downtown by the middle of 1961. Before the change, US 10 followed Dixie Highway and Oakland Avenue southeast into Pontiac to Perry Street and then Perry to Saginaw Street to Woodward while M-58 was routed along Telegraph and Square Lake roads. After the change, US 10 turned south from Dixie Highway onto Telegraph and then east onto Square Lake to connect back to Woodward. The old route through downtown was assigned the Bus. US 10 moniker and M-58 was decommissioned as a highway designation. Two years later, I-75 was completed to the east of Pontiac, and the M-24 bypass of the city was turned over to local control. The former Bus. M-24 through downtown was renumbered BL I-75 along with a connection along Square Lake Road out to the freeway. The next year, the streets downtown Pontiac were reconfigured and a loop called Wide Track Drive was created to route traffic around the downtown area. The former routing of BL I-75 on Perry Street in the downtown core was replaced by routing the business loops on Wide Track Drive. Then in 1966, an interchange was built to replace the intersection at Opdyke Road and Square Lake Road.

In 1985, the Michigan Department of Transportation (MDOT) received permission from the American Association of State Highway and Transportation Officials to truncate US 10 to Bay City, and when the change was made the following year, US 24 replaced US 10 on Telegraph Road north of Square Lake Road, and Bus. US 10 through downtown was redesignated Bus. US 24, including the segment concurrent with BL I-75. Saginaw Street south of downtown was renamed as a northern extension of Woodward Avenue, while Wide Track Drive through downtown was renamed as the Woodward Avenue Loop, both in 2000.

The section of BL I-75 that follows Woodward Avenue has a pair of special designations attached to it. In 1999, it was designated by MDOT as what is now called a Pure Michigan Byway. Three years later, it was named a National Scenic Byway by the Federal Highway Administration National Scenic Byways Program on June 13, 2002, the only urban road at the time with that classification. It was later upgraded to All-American Road status on October 16, 2009. In 2017, MDOT reconstructed the southern terminus, by replacing the left exit and entrance ramps by right exit and entrance ramps from northbound I-75.

Major intersections

Saginaw

Business Loop I-75 (BL I-75) was a business loop in Saginaw that was previously Business US Highway 23 (Bus. US 23). It started at an interchange between I-75/US 23 and M-46 (Holland Road) in Buena Vista Township. From there, it ran concurrently with M-46 westward into Saginaw. East of 17th Street, the highway split to form a one-way pairing with Remington Street, which carried westbound traffic. About three blocks farther west, BL I-75 split from M-46 to follow Genesee Avenue northwesterly into the downtown area. At an intersection between Genesee Avenue, Janes Street and 2nd Avenue, the northbound BL I-75 followed 2nd Avenue while southbound continued on Genesee Avenue. Northbound traffic traveled along 2nd Avenue and Johnson Street to Washington Avenue near the Saginaw River, where the two directions of traffic reunited. Washington Avenue also carried M-13/M-81, and the three designations ran concurrently together northward and parallel to the river. BL I-75/M-13/M-81 curved northeasterly north of downtown, and M-81 left the business loop at an intersection with Veterans Memorial Parkway; BL I-75/M-13 turned north on the parkway and M-81 continued easterly on Washington Avenue. The business loop proceeded northward running parallel to the river and ended at an interchange with I-75/US 23 in Zilwaukee Township; M-13 continued along Bay City Road north of the interchange.

When the state highway system was signposted in 1919, the north–south highway through Saginaw was part of the original M-10. This highway was later redesignated as part of US 23 when the United States Numbered Highway System was created in 1926. US 23 was initially routed on the western side of the Saginaw River through the city, but it was moved in 1929 to run along the eastern side. In 1953, the initial eastern bypass of Saginaw was built as a two-lane highway, and the former routing through downtown was redesignated Bus. US 23. This bypass was upgraded in 1961 to a full freeway as part of I-75/US 23, and the business loop through downtown was redesignated BL I-75. In 1971, I-675 was completed, and BL I-75 was decommissioned through Saginaw. The segments of the business loop that were concurrent with M-13, M-46 or M-81 remained part of those state highways, but the rest of BL I-75 was returned to local control.

Major intersections

Bay City

Business Spur Interstate 75 (BS I-75) is a business spur running through Bay City following a section of the current routing of M-25 through town. Formerly a full business loop numbered Business Loop Interstate 75 (BL I-75), it followed what is now M-84 back to end at I-75/US 23 at exit 160 south of downtown. The business route also follows streets that previously were numbered Business US Highway 23 (Bus. US 23). The spur starts at exit 162 on I-75/US 23 at the same interchange where M-25 and US 10 end. BS I-75 runs concurrently along M-25, and for about the first , the spur is a four-lane freeway bounded by residential subdivisions on either side. In Bangor Township, the freeway ends and BS I-75/M-25 splits into a one-way pairing of Thomas Street (eastbound) and Jenny Street (westbound). These two three-lane streets continue along a residential area on the west side of Bay City. East of intersections with Henry Street, the opposing sides of traffic merge back together near Veterans Memorial Park to cross the Saginaw River on the four-lane Veterans Memorial Bridge. On the eastern side of the river, BS I-75/M-25 splits again into the one-way pairing of the three-lane McKinley Street (westbound) and 7th Street (eastbound). Just three blocks east of the river, BS I-75 ends at the intersections with M-84 (Washington Avenue) in downtown Bay City. On average each day in 2013, 11,678 vehicles use the business loop east of the M-13 junction, and 29,391 vehicles do so west of the M-84 junction, the lowest and highest traffic counts.

When the state highway system was first signposted in 1919, the north–south highway through Bay City was part of the original M-10, and the east–west highway was numbered as part of M-20. When the United States Numbered Highway System was created in 1926, M-10 became part of US 23, although it was routed on the western side of the Saginaw River. The highway was rerouted to the eastern side of the river in 1929. By early 1941, US 23 was rerouted to cross the Saginaw River on the southern side of Bay City, and the former routing along Washington Avenue and Midland Street through downtown was renumbered Bus. US 23. When the I-75/US 10/US 23 freeway bypass west of Bay City opened in late 1961, the former routing of US 23 was replaced by M-13 and the former Bus. US 23, including connections along the former routing of M-47 southwest and M-20 west of Bay City, were redesignated as BL I-75. The BL I-75 designation lasted until 1971 when the southern half of BL I-75 was removed, converting the business loop into a business spur; the southern section was renumbered as part of an M-84 extension, and BS I-75 would then run along the section concurrent with M-25 only.

Major Intersections

West Branch