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11.04 miles ; north from I-75 for those motorists who want to intercept Atlanta's I-285 about 30° counterclockwise. Proposed in the 1960s; opened fall 1987. I-675 was planned to extend north to I-420, and then continue as GA 400 to the top of I-285. However, I-420 was never built. Jim K. Georges writes that I-675 does cut out about 10 miles, and avoids some airport traffic, for travelers from eastern Atlanta Metro to Florida. It's six lanes, except for a 3-mile four-lane section at the extreme south end.
26.53 miles ; from I-75 to I-70, serving as an eastern bypass of Dayton.
Rand McNally atlases from the 1960s show proposed I-675 starting south of Dayton (as it does now), continuing northeast and north, but then turning back to meet I-75 near Northridge instead of heading toward I-70 and Springfield.
In the early 1970s, construction began on the northernmost part of I-675, whose alignment had been moved to the Fairborn location it has now. The first segment terminated at N. Fairfield Road (exit 18). 
No further construction was done for over a decade. Dayton Mayor James H. McGee opposed the highway, contending it would draw economic development out of the city into the suburbs. 
Beltway considered on west side
Since the late 1990s (or even earlier), some local planners have advocated a western bypass for Dayton. The proposed number is OH 892. The plan does not have universal support.
See Transportation Planning (Miami Valley Regional Planning Commission) for more information.
7.72 miles ; west loop of I-75 serving Saginaw. Originally, Michigan wanted to call this Interstate 275.  Bill Cohen writes: "The main reason for its existence is the former drawbridge on I-75 in Zilwaukee (since replaced) back in the 1970s. The Saginaw river was navigable and used by GM's Malleable Iron Factory, [causing] frequent bridge openings causing severe traffic especially northbound on Fridays and southbound on Sundays as most of the better off people of Detroit vacationed 'up north.'
"As the Zilwaukee Bridge was finally replaced with an elevated bridge (you may remember a well-publicized 1975ish construction accident), GM closed its plant and the river was no longer dredged, eliminating any drawbridge concerns.
"None the less, I-675 was routed through Downtown Saginaw and enabled travelers to cross the Saginaw river at a narrower non-navigable point bypassing the drawbridge. There were electronic signs to advise people to take I-675 and avoid the drawbridge but for some reason most travelers never heeded them."