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I-266 (cancelled) Virginia; Washington, D. C. (link)
Proposed 1.79-mile freeway . Interstate 266 would have served as an alternate route into Washington D.C. from Interstate 66 in Arlington. The highway would have branched north from I-66 near exit 72 (US 29), crossing the George Washington Memorial Parkway and then the Potomac River, over a proposed Three Sisters Bridge (named after islands there). 
Across the Potomac, I-266 would have connected with the Whitehurst Freeway in Georgetown before reconnecting with I-66 at the M Street/Virginia Ave./Rock Creek stack, directly across the street from the Watergate complex. Numerous ramps into the dirt exist at this location. 
Peter Harnik writes that one I-266 design called for a double-decked, 10-story-high interstate, in retaliation against Arlington County and the National Park Service's decree that no land beyond the edges of the current Spout Run Parkway could be used for the interstate. 
The 1968 Federal Aid Highway act included the following passage:
"(b) not later than 30 days after the date of enactment of this section the government
of the District of Columbia shall commence work on the following projects:
That didn't happen, but I-266 was still officially part of the interstate system until at least 1978. 
One last bit of I-266 history: In 1958, D. C. officials submitted the number I-66N for the highway, a proposal that AASHTO rejected.  At that time, I-66N (266) would have gone north from I-66 to end at the proposed I-70S (270).