Interstate 684 is a suburban freeway stretching 28 miles north from I-287 at White Plains to I-84 and NY 22 in the town of Southeast. The south end of the route partitions with separate branches to the Cross Westchester Expressway (I-287) and Hutchinson River Parkway. The New York State Department of Transportation (NYSDOT) inventories the connection with I-287 as Route I684 and the link with Hutchinson River Parkway as Route 984J.
I-684 parallels NY 120 north from Harrison between Rye Lake and Westchester County Airport (HPN) into the western panhandle of Connecticut. While there are no ingress or eggress points through the Constitution State, the Connecticut Department of Transportation (ConnDOT) maintains part of the roadway.1
Northward into Armonk, New York, I-684 and NY 22 cross paths at the first of three meetings. NY 22 shifts east from the freeway through the town of North Castle while Interstate 684 parallels the Byram River north to Byram Lake Reservoir. Traversing hilly terrain through the town of Bedford, I-684 converges with the north end of Saw Mill River Parkway at the community of Katonah.
Entering the town of Lewisboro, I-684 and NY 22 run along side each other east of Muscoot Reservoir to Goldens Bridge. The two again parallel each other directly east of the Croton River at Purdys Station in North Salem. NY 22 combines with U.S. 202 west of I-684 as both highways turn northeast across the Putnam County line to Interstate 84 at Brewster.
The corridor for I-684 was originally apart of Interstate 87 extending north from the Cross Westchester Expressway to I-84 at Brewster. This alignment included a concurrency along Interstate 84 west across the Newburgh-Beacon Bridge over the Hudson River to the New York Thruway at the town of Newburgh. The 1968 Highway Act redefined I-87 along the New York Thruway mainline from the Tappan Zee Bridge north to Interstate 84. Interstate 684 was established in 1969 with Federal Highway Administration (FHWA) approval on September 11 and American Association of State Highway and Transportation Officials (AASHTO) approval on October 26.