M-66 (Michigan highway)

M-66
M-66 highlighted in red
Route information
Maintained by MDOT
Length	266.399 mi[1] (428.728 km)
Existed	c. July 1, 1919[2]–present
Major junctions
South end	SR 9 near Sturgis
Major intersections	US 12 at Sturgis I-94 / I-194 at Battle Creek I-96 near Ionia US 10 near Sears US 131 / M-72 at Kalkaska M-32 at East Jordan
North end	US 31 at Charlevoix
Location
Country	United States
State	Michigan
Counties	St. Joseph, Branch, Calhoun, Barry, Eaton, Ionia, Montcalm, Mecosta, Osceola, Missaukee, Kalkaska, Antrim, Charlevoix
Highway system
Michigan State Trunkline Highway System Interstate US State Byways
← M-65 → M-67

M-66 is a north–south state trunkline highway on the Lower Peninsula of the US state of Michigan. It runs from the Indiana state line in the south to Charlevoix in the north. M-66 is the only state highway to run the north–south distance of the Lower Peninsula. It starts as a continuation of State Road 9 (SR 9), which provides access to the Indiana Toll Road. The total length is approximately 272.9 miles (439.2 km), which includes almost 3.4 miles (5.5 km) of freeway between Interstate 94 (I-94) and downtown Battle Creek designated as I-194. A section of the highway immediately south of I-94 is an expressway, a type of divided limited-access highway, while the section along I-194 is a full freeway; otherwise M-66 is a two-lane rural highway. Two sections are listed on the National Highway System.

The first usage of the M-66 designation dates back to around July 1, 1919 with the rest of the original state highway system. At the time, the highway only extended between Lowell and Lakeview, a route now covered by M-91. The highway has been lengthened in a series of extensions north and south starting in 1925. A rerouting in 1944–45 removed M-66 from its original 1919 routing to replace another highway south of Six Lakes, the change that spawned M-91. The last big extension in 1965 resulted in the modern trans-peninsular highway route. The last modifications were shorter reroutings in the 1970s.