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I-990 New York
6.35 miles ; Interstate 990 is the Lockport Expressway, a spur off I-290 to the Millersport Hwy (NY 263) in the Buffalo suburbs. The first segment, leading to North French Rd, opened in the late 1980s. The newest portion, extending to New Road (NY 263), opened Dec. 21, 1990 (probably). 
AASHTO approved extending the designation on Oct. 22, 1993. 
I-990 is the highest numbered interstate In The World.
There were bigger plans for this highway in the 1960s: it would have started from I-190, crossed Buffalo's East Side, crossed I-290 and gone to Lockport; and from there, loop west and north of the city before turning east and heading toward Rochester (seems a bit contortional).
Dan Tasman wrote:
"Most transportation planners doubt that the road will be extended past Millersport, but there are 'ghost ramps' to take the I-990 over it if it ever gets extended to Lockport. A mile-long stretch of one side of what was going to be part of I-990 was built on the west side of Lockport, and is called the 'Lockport Bypass,' but it's essentially a road that goes from nowhere to nowhere - there's also some other neat I-990 artifacts in the city like evidence of displaced dirt where the road was supposed to be placed."