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80.65 miles ; from I-94 in Benton Harbor to I-96 in Grand Rapids. The northern portion of Interstate 196 (in Kent, Ottawa, and Allegan counties) is known as the Gerald R. Ford Freeway.  (Old Saturday Night Live fans would rather see this name applied to a highway in Chevy Chase, Maryland.)
Changes in Proposed Numbering
Originally (1957-58), the Benton Harbor - Grand Rapids - Detroit highway (now I-96 and I-196) was to be part of Interstate 94. A short spur to Muskegon from Grand Rapids was to be I-94N. (Today's I-94 between Benton Harbor and Detroit was to be Interstate 92.)  
In April 1958, meanwhile, the Michigan Highway Department recommended: 
Changes between I-96 and I-196
By the early 1960s, I-96 followed the 1957 I-94 route, from Benton Harbor to Grand Rapids to Detroit. The 37-mile spur from Grand Rapids to Muskegon (old I-94N) was called I-196.
Many Michiganders were unhappy with the designation. The Detroit to Muskegon route, which was the predominant traffic pattern and had enjoyed a single number since 1920, now changed suddenly from I-96 to I-196 as drivers traveled west. It was difficult to provide clear, concise guide signage in the area. 
On May 1, 1963, Highway Commissioner John Mackie announced the State Highway Department would petition AASHO to redesignate area routes to provide a continuous route number to Muskegon.  His proposal:
On Aug. 24, 1963, the State Highway Department submitted the request to AASHO. Its proposal revived the old I-67 idea, much like the 1958 plan: 
Highway Department spokesmen said the AASHO review would take several months. Not long afterward, AASHO approved the request in part. I-196 became part of a unified I-96 as desired. However, the same wish to keep I-67 in reserve prevailed, and Michigan's proposed I-67 became the new I-196.
I-196: I-67? redux
What was old could be new again. US 31 is being upgraded between Benton Harbor and South Bend, Indiana, and many lay people at least are talking about designating I-67 in the area. If that ever happened, it would probably incorporate all of I-196 except the Holland - Grand Rapids portion, which diverges from US 31.
I-296 (not signed as interstate) Michigan
3.43 miles ; an unposted ("secret") designation for part of US 131 in Grand Rapids.
The I-296 section of US 131 was completed (and numbered I-296/US 131) in 1963.  Although it appears not to have ever been signposted, I-296 used to appear on maps regularly until the late 1970s.
In an email to fellow roadgeek Chris Bessert, MDOT official Ari Adler, head of communications for the Grand Region, confirmed the I-296 designation "is still officially there, although rarely used, except internally." 
29.39 miles ; from I-96 east to I-94 in north Detroit Metro area. It's called the Walter Reuther Freeway.  Interstate 696 is eight lanes throughout, except for a six-lane section near Telegraph and Southfield. The oldest segment (1965 or earlier) is between I-96 and US 24; the section between I-75 and I-94 opened in 1979; the final 9.1 mile section, between M-10 and I-75, opened on December 14, 1989. 
The northern bypass of Detroit has been planned since at least the 1950s. In 1958, Michigan's preferred number for the highway was, believe it or not, I-98. (Other short Michigan-only interstate proposals were 67, 73, and 77.) 
Disputed in 1970
In 1970, proposed I-696 in Oakland gained federal attention as one of the "Major Interstate System Route Controversy in Urban Areas" -- a select group of 21 highly contested freeways including Boston's Inner Belt (I-695), Hartford's I-291, and Washington D.C.'s Potomac River Freeway (I-266). Here is the assessment of I-696:
"Prior to 1967, Michigan law required the approval of local communities for highway design and locations. Since the State was not able to obtain approval of any alternate location for I-696 by the eight communities involved, an arbitration law was passed in 1967 to settle Interstate highway disputes. The arbitration board selected a location generally proposed by the highway department. Two of the communities, Pleasant Ridge and Lathrup Village, contested the constitutionality of the arbitration board. The State Supreme Court has heard the case and found the an arbitration to be constitutional."