In Washington D.C., I-66 was planned to extend east of its current terminus along the North Leg of the Inner Loop freeway.
I-66 would have also met the eastern terminus of a planned Interstate 266 at US 29, and the western terminus of the South Leg Freeway (I-695) at US 50; I-266 would have been a parallel route to I-66, providing more direct access to the North Leg from points west, while I-695 would have been an inner-city connector between I-66 and I-95.

The final plans for the North Leg Freeway, as published in 1971, outlined a 1.5-mile (2.4 km) six-lane tunnel beneath K Street, between I-266/US 29 and New York Avenue, where the North Leg would emerge from the tunnel and join with the Center Leg Freeway (formerly I-95, now I-395); the two routes would run concurrently for three-fourths of a mile before reaching the Union Station interchange, where I-66 was planned to terminate.

Despite the plan to route the North Leg in a tunnel beneath K Street, the intense opposition to previous, scrapped alignments for the D.C. freeway network, which included previous alignments for the North Leg Freeway, led to the mass cancellation of all unbuilt D.C. freeways in 1977, resulting in the truncation of I-66 at US 29.
In Washington D.C., I-66 was planned to extend east of its current terminus along the North Leg of the Inner Loop freeway. I-66 would have also met the eastern terminus of a planned Interstate 266 at US 29, and the western terminus of the South Leg Freeway (I-695) at US 50; I-266 would have been a parallel route to I-66, providing more direct access to the North Leg from points west, while I-695 would have been an inner-city connector between I-66 and I-95.
The final plans for the North Leg Freeway, as published in 1971, outlined a 1.5-mile (2.4 km) six-lane tunnel beneath K Street, between I-266/US 29 and New York Avenue, where the North Leg would emerge from the tunnel and join with the Center Leg Freeway (formerly I-95, now I-395); the two routes would run concurrently for three-fourths of a mile before reaching the Union Station interchange, where I-66 was planned to terminate.
Despite the plan to route the North Leg in a tunnel beneath K Street, the intense opposition to previous, scrapped alignments for the D.C. freeway network, which included previous alignments for the North Leg Freeway, led to the mass cancellation of all unbuilt D.C. freeways in 1977, resulting in the truncation of I-66 at US 29.
Interstate 66