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I-585 South Carolina
An inadequate I-85 becomes BR 85
I-585 is unique in that it connects to no other "proper" interstate -- just a business route. Originally, however, the mainline Interstate 85 was located where BR 85 is today. That section, built before the interstate system was created in 1956, had substandard design leading to safety and capacity problems: "at grade intersections on ramps, too many interchanges, extremely poor geometrics at interchanges and on the mainline, and lots of adjacent development that made improvements difficult and expensive."  The best choice was to relocate the freeway.
Funding the new freeway would be a problem, since South Carolina's portion of I-85 was considered complete, and no 90-10 matching money would be available. Fortunately, lawmakers in North Carolina were formulating a provision in the Surface Transportation Assistance Act of 1982 that would inadvertently help. The provision -- Section 139 -- was intended for a specific stretch of I-40 in the Tarheel State, and couched in criteria only that highway should satisfy: a way of covertly directing funds to a single project.
As it turned out, Section 139's exacting criteria described not just one highway but two: I-40 in N.C. and I-85 in Spartanburg. Construction of the 8.75-mile bypass began in 1988 and finished in June 1995. 
South Carolina officials thought about leaving I-85 on the old route and calling the bypass I-285. However, Section 139 called for relocating the interstate, not creating a new one. So old I-85 became I-85 Business. 
Alex Nitzman writes:
[I-585 is] substandard at best. The US 221 exit ramp (from wb) is about 10 feet from a car dealership entrance. The interstate is signed on an at-grade portion for about 1 mile (just like all of I-180 in WY), in both directions no less.
I-585 may be extended to new I-85
South Carolina is retaining the 585 designation, and will in fact improve the segment of US 176 leading north from BL 85 to the new I-85. A new interchange will serve the University of South Carolina-Spartanburg campus, and the diamond interchange at 176/85 will be upgraded for a freeway connection.
SCDOT plans to sign this route as Business 585 / US 176 until funding is available for the I-85 interchange; when all is complete, the I-585 designation will be extended to I-85. 
BR 85 to I-685? Probably not
In 2000, some Spartanburg residents and businessmen lobbied SCDOT to rename Business 85 to Interstate 685, to counter reluctance drivers may have to taking a business loop. (The reason: to increase, or at least stem the decrease, of business along Bus. 85). However, SCDOT has not been receptive to the idea (in May 2000, an official reportedly said "people just need to read signs better") and the I-685 drive appears stalled.