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| Southern Terminus: | Indiana state line (connection w/IN SR-933) five miles south of downtown Niles |
| Northern Terminus: | I-94 at Exit 56 west of Paw Paw |
| Length: | 40.406 miles |
| Maps: | Route Map of M-51 Niles Area Trunklines Map, 2010– |
| Notes: | In April 1998, M-51 was extended by five miles to head southerly from Niles via the former routes of BUS US-31 (S Eleventh St) and US-33 to the Indiana state line in order to fill the void left when US-33 was decomissioned in Michigan. The South Bend Tribune featured a photo of the crew changing some former US-33 sign assemblies on Tuesday, April 7, 1998, as well as an article on the "confusion" the change has caused on Thursday, September 17, 1998. Interesting to note, however, the "confusion" involves not only the general public, but also the Tribune itself! In articles, M-51's previous designation between Niles and the Indiana state line, US-33, is never mentioned and they maintain Eleventh St south of US-12 (or as they call it, "Bypass 12"... ahem!) was designated as BUS US-31. Since 1926, this route was first US-31, US-31/US-33, then just US-33, and now M-51. |
| The entire route of M-51 north of Niles was formerly designated as a part of M-40 before M-40 was transferred to the former route of M-119 between Paw Paw and US-12. | |
 By 2018, the twin overpasses on US-12 at the M-51 interchange south of Niles along the "Niles Bypass" were 63 years old, having been built in 1955, and the interchange itself had been constructed using older specifications which no longer met current standards. With the overpasses needing replacement, MDOT kicked off a Planning & Environmental Linkages (PEL) study to consider alternatives for both the outmoded interchange as well as the entire M-51 corridor from the Indiana state line northerly to BUS M-60 near downtown Niles. Alternatives for the junction included replacing the overpasses and ramps with updated versions using somewhat more modern specifications, rebuilding the interchange as a simpler "diamond" interchange, and several at-grade alternatives including a roundabout, signalized intersection with direct left turn movements and a signalized intersection with indirect left turns (e.g. Michigan Lefts). Although the updated interchange was deemed the best alternative in terms of traffic operations (travel delay, pedestrian accommodations, motorist safety and geometry), MDOT decided in 2019 to remove the interchanges—both US-12 overpasses and connecting ramps—and replace it with an at-grade signalized intersection with Michigan Lefts as well as making similar improvements to the existing US-12 & Third St signalized intersection just to the west. Reasons for choosing the lower-scoring alternative included lower initial construction cost—up to $9 million cheaper—and lower future maintenance costs. Another reason cited by MDOT was the existing interchange being "out-of-character" with the overall US-12 bypass route... even though the US-12 & M-60 interchange sits just 1½ miles to the east. Work to remove the 1955-era US-12 & M-51 interchange began in September 2023 with a new intersection being constructed during 2024. (See US-12 over M-51 Interchange website from MDOT.) |
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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 recommended many changes to the route of M-51—which, at that point, ran from Port Huron northerly through Sanilac Co, ending at M-142 between Harbor Beach and Verona—during that timeframe, including the following major change:
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| The Dept of State Highways had planned on transferring the Eleventh St alignment of US-31/US-33 (later BUS US-31, then BUS US-12, now M-51) from BUS US-12/Oak St (now BUS M-60) southerly to local control as far back as 1960 (once the proposed US-31 freeway was completed west of Niles). By the 1970s, not only had the department improved US-31/US-33 along Eleventh St and a newly-built northbound-only segment along Twelfth St immediately south of BUS US-12/Oak St (now BUS M-60), the Dept of State Highways also proposed extending the trunkline routing, designated as part of M-51, northerly in the Twelfth St corridor back to the existing route of M-51 north of Niles. The new alignment would have featured a one-way pair mirroring the Eleventh St-Twelfth St configuration south of BUS US-12/Oak St (now BUS M-60), before merging back together as a five lane street along the Twelfth St corridor to approximately Wayne St. From there, M-51 would have angled northeasterly on a new alignment before turning due northerly as it crossed Lake St east of Thirteenth St, returning to the existing M-51 north of Monroe St. While the project was never implemented, the Dept of State Highways went as far as producing a Noise Study & Analysis document (archived) in 1974. |
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| History: | 1918 – Even prior to M-51's existence in the Thumb region, M-51 exists as the primary connection between Grand Rapids and M-11/West Michigan Pike at Holland on the Lakeshore. The original routing of M-51 begins in downtown Holland at M-11 and continues easterly via 8th St and Holland-Zeeland Rd (present-day Paw Paw Dr) and Main Ave into Zeeland, then easterly via Byron Rd through Vriesland to 32nd Ave, northerly through Hudsonville via 32nd Ave & Balsam Dr to Port Sheldon St, easterly again via Port Sheldon St to Jenison where it connects with Chicago Dr through Grandville and into downtown Grand Rapids. |
| 1922 (Apr 6) – The route of M-51 is realigned to follow Chicago Dr, a direct routing from Zeeland to northeast of Hudsonville, parallelling the Pere Marquette Railroad to the south. The former route via Byron Rd, 32nd Ave, Balsam Dr and Port Sheldon St is turned back to local control. At Zeeland, M-51 departs Chicago Dr/Washington Ave near the eastern city limit and continued southerly via Carlton St to Main Ave where the route of M-51 turns westerly again via Main, although the portion of Main Ave through Zeeland is not yet a state-maintained trunkline (and would not be for another 9 years until the Dykstra Act allowed state-maintained trunklines through cities). | |
| 1926 – Coincident with the renumbering of many of Michigan's state highways at the advent of the US Highway system, all of M-51 between Holland and Grand Rapids becomes part of an extended M-21 while the former M-27 in the Thumb region becomes M-51. The route now begins at M-29 (newly re-designated from M-31) in Port Huron and runs north-northwesterly through Croswell, Applegate, Carsonville and Deckerville before ending at M-83 (newly re-designated from M-31, presently M-142) five miles west of Harbor Beach. | |
| 1939 – With the commissioning of US-25A in the Port Huron area along Pine Grove Ave and Lyburner Rd, M-51 and US-25A run concurrently from downtown Port Huron to the intersection of Pine Grove & Lyburner. | |
| c.1944 – M-51 is rerouted at Carsonville to run westerly with M-46 for three miles, then northerly toward Deckerville on Ruth Rd back to its original alignment. The former alignment via Maple Grove & Forester Rds is turned back to local control. | |
| 1959 – The final 6 miles of gravel surface on M-51 are paved, from Applegate to Carsonville. | |
| 1961 – The M-51 designation is scaled back from Port Huron to end at M-46 three miles west of Carsonville, a loss of 41 miles. The former M-51/US-25A between Port Huron and Gardendale becomes just US-25A, while the portion from US-25A to jct M-136 becomes an easterly extension of M-136. The remainder is turned back to local control. The 27 miles of M-51 from M-46 to M-142 are left as-is. | |
| 1965 – The rest of M-51 from M-46 west of Carsonville to M-142 west of Harbor Beach is turned back to county control and the M-51 designation disappears from the state trunkline system. | |
| 1971 (Sept 8) – The State Highway Dept announces a change in highway route designations involving M-40 and M-119 in Van Buren, Cass and Berrien Cos to go into effect later in the fall. From Paw Paw, M-119 will be supplanted in its entirety by a rerouted M-40, which will now continue south from Paw Paw through Lawton and Marcellus, terminating at US-12 west of Mottville. The former route of M-40 between I-94 west of Paw Paw southerly through Decatur and Dowagiac to Niles will be redesignated as M-51. The concurrent I-94/M-40 segment between Exits 56 and 60 becomes just I-94. Reasons given for the change by MSHD officials include motorist confusion with having M-40 routed along I-94 for four miles and the M-119 route from Paw Paw southerly being a much more logical routing for M-40 to follow. | |
| 1971 (Nov) – Following completion of a widening project along M-40 in downtown Dowagiac, the route designation changes for M-40 and M-119 announced earlier in the year take effect. All M-119 route markers are changed out for M-40 signs, while all M-40 signs between I-94 at Exit 56 and downtown Niles are swapped out for new M-51 markers. The M-119 route designation is retired at this point and the length of M-40 is shortened by 9½ miles. The newly created M-51 now begins at I-94 Exit 56 west of Paw Paw and continues through Decatur and Dowagiac, terminating at BUS US-12/US-31/US-33/Main St in downtown Niles. | |
| 1990 (Oct 8) – The route of M-51/M-62 through downtown Dowagiac is realigned when the portion of Division St between Main St and Front St is established as a state trunkline route, while the former route along Main St (from Division St to Front St) and Front St (from Main St to Division St) is cancelled as a state trunkline route and turned back to city control. The realignment results in a shorter and more direct route for both trunklines. The intersection of Main St & Division St is also reconfigured to favor the through trunkline route, requiring eastbound Main St traffic at Division St to make a 90° turn to continue into downtown via Main St. | |
| 1998 (Apr 5) – On April 5th, US-33 is 'decommissioned' in Michigan and M-51 is extended southerly from downtown Niles via BUS US-12 to US-12/Pulaski Hwy (replacing the BUS US-31 designation along that segment), then southerly via the former US-33 to the Indiana state line at a connection with IN SR-933. | |
| 2010 (Mar 5, Apr) – On March 5, the BUS US-12 route designation is "decommissioned" in the Niles area due to the transfer of four blocks of Main St in downtown Niles to city control. While the remaining western portion of BUS US-12 is supplanted by an extended M-139 designation, the eastern portion retains the M-51 designation that had been co-signed with since 1998. (See Niles Area Trunklines Map, 2010–.) | |
| Controlled Access: | No portion of M-51 exists as freeway or expressway. |
| NHS: | One segment of M-51 is on the National Highway System (NHS), from the southern terminus at the Indiana state line northerly to the intersection of M-51/N 5th St & Lake St on the north side of Niles. (5.9 miles) (The segment from the US-12 interchange south of Niles northerly to Lake St north of downtown was added in 2012 with the passage of the MAP-21 funding and authorization bill.) |
| Continue on: | US-33 and SR-933 into Indiana – via the Indiana Highway Ends website |
| Memorial Highway: | At present, no portion of M-51 has been designated as part of a Memorial Highway. |
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