Michigan Highways: Since 1997.

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M-22 & M-109 junction route signage in Glen Arbor, Michigan
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M-24 Route Marker On to Next Route:
Former US-25
Southern Terminus:    I-75 at Exit 81 (at jct I-75 & BL I-75) in Auburn Hills northeast of Pontiac
Northern Terminus:    M-25 in downtown Unionville (cnr Center St & Bay St)
Length: 73.779 miles
Map: Route Map of M-24
Updated 2024-03 M-24: Thumb Extension Saga map
Notes: A northerly extension of M-24 from Caro to Unionville was completed in 1997. Part of the southern portion of this new extension was built on new alignment north of Caro, while the remainder runs along Unionville Rd, partially replacing the M-138 designation between the junction with that highway and M-25 in Unionville. M-24 was extended, in part, to provide an all-weather route between M-81 and M-25. All-weather highways are important in Michigan's Thumb region, as the production of sugar from sugar beets is a leading industry in the area. Plans for this northerly extension of M-24 date back to 1966 and an Environmental Impact Statement was completed in 1973, although another 25 years would need to pass before the extension itself would be completed and opened to traffic. See M-24: Thumb Extension Saga and accompanying map for details on what went into this extension project.
  Before the extension from Caro to Unionville was complete, M-24's length from Auburn Hills to Caro was 59.133 miles.
  While M-24 and US-24 seem to pass by rather closely (and at one time, US-24's northern terminus was also M-24's southern terminus), M-24 was never a part of US-24, nor was it intended to be. The two similar and nearby designations is a remnant from an era when the State Highway Department created similar situations elsewhere, including US-25 meeting M-25 at Port Austin, US-131 meeting M-131 (now M-119) at Fife Lake and later Petoskey, and US-112 and M-112 co-existing between Ypsilanti and Detroit.
  In "State Trunkline Needs, 1960–1980," a set of maps prepared by the State Highway Dept's Office of Planning, Programming Division in 1960 showing possible additions, upgrades and improvements to the state trunkline system over the ensuing twenty years, MSHD staff largely left M-24 as-is along its 1963–1997 alingment, with the three primary exceptions being:
  • The planning maps showed the segment of M-24 along Opdyke Rd from M-59 north to (then-proposed BL I-75) Perry St on the east side of Pontiac as being turned back to local control, as it would be in 1963.
  • The maps interestingly showed M-24 along Opdyke Rd from M-59 south to Square Lake Rd as remaining on the state trunkline highway system, although it can be easily assumed the route designation would have changed to something other than M-24 (otherwise that route would've been discontinuous). What that designation would have been is unclear, however. In 1963, though, all of Opdyke Rd would be transferred to local control in 1963.
  • In Tuscola Co, no northerly extension of M-24 from Caro northerly to Unionville is not shown on the 1960 planning maps as such an extension was not conceived of for a few more years. Indeed, all of M-138 was shown as a trunback candidate, including the portion which would be supplanted by the M-24 extension in the 1990s. The Caro–Unionville extension, however, was shown on statewide trunkline planning maps by 1968, with the remainder of M-138 still shown as a turnback candidate.
History: 1920 – The "original" M-24 in Michigan runs along a route considered to be the "classic" routing of M-20 from Muskegon to Midland via Fremont, Big Rapids and Mount Pleasant, then southeasterly along what later became US-10 then M-47 into Saginaw. In a sense, the first iteration of M-24 is somewhat of a "northern alternate" route to M-46, which also runs between Muskegon and Saginaw.
  1922 – M-24 is realigned in eastern Isabella and western Midland Counties. From Shepherd Rd, M-24 now turns northerly from Broadway Rd to Pickard Rd, then continues easterly on Pickard Rd (Isabella Co) and Isabella Rd (Midland Co) into Midland. The former route along Remus and East County Line Rds (Isabella Co), and along Salt River, Miller and Chippewa River Rds (Midland Co) are turned back to local control.
  1926 – With the debut of the US Highway system in Michigan, many route designation changes occur across the state. One such change is the redesignation of all of M-24 from Muskegon through to Midland as a new routing of M-20. Much of what had been M-20 is now part of US-10. From Midland to Saginaw, the remainder of the "original" M-24 becomes part of US-10. The M-24 designation is immediately reapplied, though in what seems to be two disconnected segments at first. From the northern terminus of the new US-24 at Pontiac, M-24 now runs along what had been M-36 northerly through Oxford to M-21 at Lapeer. M-24 then picks back up at M-38 in downtown Vassar and runs northwesterly to end at M-29 (present-day M-25) in Bay City.
  c.1927–30 – While a more permanent routing is under contemplation, M-24 is temporarily routed west from the northern end of the southern segment in Lapeer along M-21 to Davison, then northerly with M-15 to Vassar and the southern end of the northern segment. This solves the discontinuous problem noted above.
  1930 – M-24 is realigned to its own routing north of Lapeer, running northerly from M-21 at Lapeer to M-38 at Mayville, then turning westerly, follows along M-38 into Vassar.
  1933 – A new alignment of M-24 is completed from Dryden Rd at Metamora to Lapeer. The old route along Metamora, Hunters Creek and Clark Rds is turned back to local control.
  19___ – A new alignment of M-24 is completed from Metamora Rd north of Oxford to Dryden Rd at Metamora. The old route along Metamora Rd is turned back to local control.
  1935 (May) Updated 2026-06 – From Vassar to its northern terminus at Bay City, the M-15 designation is added to the route of M-24 resulting in a M-15/M-24 concurrency between those cities. At M-25/Center Ave in Bay City, M-15 is extended westerly in a rather cumbersome triple-concurrency as M-15/M-24/M-25 through downtown Bay City and further west as a bizarre four-way concurrent US-23/M-15/M-24/M-25 to a terminus at Euclid Ave on the west side of the city. The Bay City Chamber of Commerce Roads Committee has been requesting the change to "prevent confusion at Vassar" and at a conference earlier in the year, State Highway Commissioner Murray D. Van Wagoner agreed and the State Highway Department implements the change and adds the concurrent designation to its May 15, 1935 edition of the official highway map.
  1936 – A new M-24 eastern bypass of Pontiac is completed when the M-24 designation is transferred to Opdyke and Square Lake Rds along Pontiac's eastern and southern boundaries, continuing westerly along Square Lake Rd to its terminus at jct M-58 & US-24/Telegraph Rd. The former route of M-24 through downtown Pontiac along Perry St, Parke St and Woodward Ave is designated M-24A.
  1940 – Around this time, the State Highway Department begins embracing a new type of route designation: the Business Connection (e.g. Business Routes, Loops and Spurs). In late 1940, M-24A through downtown Pontiac is, accordingly, redesignated as BUS M-24.
  1941 (Mar 14) Updated 2023-08 – As the northernmost 34 miles of M-24—or 40% of the entire route mileage—are concurrently designated with other trunkline routes (M-38 from Mayville to Vassar and M-15 from Vassar to Bay City), the State Highway Dept decides to replace the entire M-85 designation from M-38 at Mayville northerly to M-81 in downtown Caro with a rerouted M-24. The change will take over a year to actually implement in the field, however.
  1942 (Apr 18) New! 2023-08 – Although announced over a year prior, State Highway Dept crews finish removing M-24 route markers from the routes of M-38 (Mayville–Vassar) and M-15 (Vassar–Bay City) and move them to replace M-85 route markers along the Mayville–Caro trunkline route. The second iteration of M-85 thus officially fades into history.
  1949 – In mid-1949, a new alignment at Mayville simplifies the routing of M-24, which now enters the town from the east instead of the south.
  1950 – M-24 is realigned on the north side of Lapeer, from Saginaw Rd onto a newly constructed portion of Lapeer Rd.
  1953 – M-24 is realigned onto a new highway bypassing the downtown area of Mayville and shaving more than ½-mile from the route.
  1957 – The final 5 miles of gravel-surfaced M-24, south of Caro, are paved.
  1961US-10 is realigned to bypass Pontiac to the west replacing the M-58 designation in its entirety. Due to this, the former M-24/M-58 segment of Square Lake Rd becomes US-10/M-24, while the former US-10/BUS M-24 segment of Woodward Ave becomes BUS US-10/BUS M-24.
  1963 – With the completion of I-75 around the east side of Pontiac, M-24 is scaled back to end at jct BL I-75 at the I-75 connector at Exit 81 on the northeast side of Pontiac. The former M-24 along Opdyke Rd on the east side of Pontiac is turned back to local control, while the former route along Square Lake Rd between Opdyke and Woodward Ave becomes a part of the newly-designated BL I-75. The remaining portion of the former BUS US-10/M-24 along Square Lake between Woodward and Telegraph Rd becomes just a portion of BUS US-10. All of BUS M-24 through downtown Pontiac becomes a part of the new BL I-75. This is the first time since the mid-1920s that US-24 and M-24 do not touch.
  1971 (Jan 13) Updated 2024-03 – The State Highway Commission approves an 8.3-mile northerly extension for M-24 from Caro to M-138 east of Akron, where M-24 is then proposed to take over the routing of the existing M-138 to a new northern terminus at M-25 in Unionville. Two other alternatives, one running from Caro to M-25 northeast of Sebewaing and the other running northeasterly from Caro (concurrently with M-81) at a point west of Cass City, then turning northerly to M-25 near Caseville or a point to the east, are rejected in favor of the Caro–Unionville route, as it involves less construction, lower costs and less disruption to homes and businesses. (The Cass City–Caseville route is also rejected as the Dept of State Highways states such a route would be too close to and somewhat duplicate the function of the parallel M-53 to the east.) The Commission calls for construction of the M-24 extension to M-138 east of Akron to take place in 1974. (See M-24: The Thumb Extension Saga and accompanying map.)
  1989 (July–Aug) New! 2024-03 – The first two phases of a project to extend M-24 from Caro to M-25 at Unionville are completed and opened to traffic—nearly nearly 30 years after the extension was first discussed and nearly two decades after the State Highway Commission first approved the project. At Unionville, the portion of M-138 from M-25 in downtown to the south village limit is widened to four lanes and resurfaced for $610,000, which is completed and opened to traffic in July. The second phase involves the widening of the existing M-24 bridge spanning the Cass River at Caro and widening and reconstructing existing M-24 along Mertz Rd from Gun Club Rd to Frank St (where M-24 currently turns west toward downtown Caro), then continuing northerly along Ellington St to M-81/State St and an additional 500 feet north along Cleaver Rd. This 1.1-mile phase cost $1.2 million in addition to $836,000 for the Cass River bridge widening and is opened to traffic August 23. (See M-24: Thumb Extension Saga and accompanying map.)
  1990 (Apr 20) Updated 2024-01 – The 0.46-mile segment of Ellington St from the point where existing M-24 makes a 90° turn to the west into downtown Caro, northerly to M-81 on the northeast side of Caro is officially transferred to state control and established as a state trunkline route in preparation for the completion of the M-24 extension northerly from Caro to Unionvile. The former route of M-24 into downtown Caro along E Frank St is retained as an unsigned state trunkline route, internally designated within MDOT as OLD M-24, for the time being.
  1990 (Oct 27) New! 2024-03 – The north-south portion of M-138 along Unionville Rd from the Unionville south village limit and then southerly for 5½ miles to Akron Rd (where M-138 turns westerly toward Akron) is reconstructed and modernized, widened from two nine-foot lanes to two 12-foot lanes with complete shoulders and All-Season status. While this $1.1-million project is on M-138, it's actually the third phase of the overall M-24 northerly extension from Caro to Unionville. (See M-24: Thumb Extension Saga and accompanying map.)
  1997 (Oct 6) Updated 2024-03 – After numerous delays and setbacks, the M-24 extension north of Caro is finally completed and opened to traffic. (The extension is completed 23 years after originally intended). From M-81/State St in Caro, the extension runs northerly for 1½ miles via Cleaver Rd, then curves westerly for 4 miles on a combination of newly-built highway and upgrading of existing Biebel Rd, then curving northerly again along Unionville Rd to M-138 east of Akron. From there, M-24 replaces the M-138 designation northerly into Unionville. Oddly enough, jurisdiction of the ½ segment of Cleaver Rd (part of the M-24 extension) from M-81 northerly to the Caro village limits is not (yet) transferred from local to state jurisdiction! (See M-24: Thumb Extension Saga and accompanying map.)
  2001 (Aug 31) – Four years after becoming part of the M-24 northerly extension from Caro to Unionville, the 0.48-mile segment of Cleaver Rd from M-81 in Caro northerly to the northern village limit is officially transferred from village to state control. This segment had been signed as part of M-24 since the extension was completed and opened to traffic in 1997, but for reasons currently unclear, the transfer did not take place until now.
Controlled Access: No portion of M-24 is freeway or expressway.
NHS: M-24 from I-75 at Exit 81 northeast of downtown Pontiac to I-69 at Exit 155 south of Lapeer is on the National Highway System (NHS).
Memorial Highways:  The following Memorial Highway designation has been officially assigned to a partof M-24 by the Michigan Legislature:
  • William Davidson Memorial Highway – "Mr. Davidson was the president and chairman of Guardian Industries, one of the world's largest glass manufacturers, and he owned several professional sports teams, including the Detroit Pistons, Detroit Shock, and Tampa Bay Lightning, as well as Palace Sports and Entertainment. Bill Davidson also donated millions of dollars to various charitable and educational causes, including more than $55 million to his alma mater, the University of Michigan. He was recognized by former Israeli Prime Minister Golda Meir for outstanding philanthropic deeds toward Israel and honored by the Council of Michigan Foundations for his local, national, and international contributions."
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