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I-640 (numbered as another interstate) North Carolina
The conventional wisdom circa 1997 was that when the I-540 beltway around Raleigh was finished, its I-540 numbering (for "spur") would be changed to I-640 ("loop"). However, in 2002 there appears to be little advantage to the motorist for this, and thus no plans for a number change. 
7.03 miles ; bypasses Knoxville to the north.
Work on the $115M six-lane highway started in October 1977, and completed in April 1982. One immediate benefit was to ease transportation congestion in downtown Knoxville during the World's Fair. The choke point was the downtown cloverleaf where Interstates 75 and 40 met; frequent congestion earned the underpowered interchange the nickname "Malfunction Junction."
I-640 was built to route thru traffic on I-75 away from the city center, and I-75 itself was rerouted onto I-640 in 1982. The remainder of I-75 leading downtown was renamed I-275. Malfunction Junction was rebuilt with higher-capacity flyover ramps, and I-40 was also significantly widened and reconfigured between I-275 and US 129. 
A new outer bypass, I-475, is proposed for the Oak Ridge area to the northwest.