Virginia/Washington, DC
This is the only 2di that enters Washington, DC. I-66 used to intersect with I-95, before the latter interstate was rerouted onto the eastern half of the Capital Beltway.
I-66 has been written into law in Kentucky. It will be routed along the Cumberland and Daniel Boone Parkways. This routing is planned to become part of a much larger project, the proposed Transamerica Corridor that will run to California. Parts of US 50, US 60 and US 400 may one day become part of I-66 as well.
There could also be an eastern extension of I-66 through Washington, DC and into Maryland along US 50/US 301/Secret I-595. It would either end at Ocean City, MD, Dover, DE or the Jersey Shore.