I-390 begins as westbound traffic on the concurrent routes of I-86 and NY 17 proceeds below the overpass carrying those two routes to the west, at an interchange just east of the village of Avoca in Steuben County. The expressway heads northwest through the narrow valley of the Cohocton River to Cohocton, where the river leaves the expressway and heads north toward the hamlet of Atlanta. I-390 climbs gradually as it continues past Cohocton, eventually turning due west as it descends to an interchange with NY 21 south of Wayland. This exit is also the southern terminus of NY 15, which eventually parallels the interstate further north.