The east–west highway extends for 1.67 miles (2.69 km) from an intersection with NY 151 east of the Rensselaer city limits to a junction with U.S. Route 4 (US 4) south of the hamlet of Defreestville.

NY 151 enters the intersection from the west on Third Avenue and exits to the south on Barracks Road while NY 915E heads to the northeast, paralleling a minor tributary of Mill Creek as it provides access to New Rural Cemetery and several residential housing tracts that lead away from the north side of the road.

The highway continues eastward amidst the warehouses to US 4, where both Third Avenue Extension and NY 915E come to an end at a junction about 0.7 miles (1.1 km) south of the hamlet of Defreestville.

[4][5] Reference markers on Third Avenue and its extension still bear markings for NY 43,[1] the street's former designation.

[10][11] NY 43 remained on Third Avenue until the late 1990s, when it was rerouted to meet I-90 at the new exit 8 west of Defreestville.