From there, the freeway follows the narrow valley of Devil Creek between the Bannock Range to the east and Elkhorn Mountain to the west.

At a diamond interchange at the hamlet of Virginia, the freeway begins to run concurrently with U.S. Route 91 (US 91) and parallel Union Pacific's Pocatello Subdivision rail line and Old Highway 91.

I-15 begins to curve north at its four-ramp partial cloverleaf interchange with Old Highway 91 at Portneuf shortly before entering the city of Pocatello.

I-15 veers away from the rail line and the Portneuf River, has a diamond interchange with Clark Street east of downtown Pocatello, and collects the other end of I-15 Bus.

The highway accesses the reservation's main settlement, Fort Hall, via a diamond interchange with Ross Fork Road on the Bannock–Bingham county line.

I-15 has a four-ramp partial cloverleaf interchange with US 91, at which the highway crosses over Union Pacific's Montana Subdivision rail line, near the northern edge of the reservation; I-15 Bus.

I-15 and US 20 run concurrently to a four-ramp partial cloverleaf interchange where US 20 heads northeast as a freeway towards Rexburg and Yellowstone National Park.

The freeway heads north out of Idaho Falls parallel to the Montana Subdivision rail line, Old Highway 91, and the Snake River.

[2] I-15 passes Market Lake Wildlife Management Area ahead of its diamond interchange with SH 33 at Sage Junction.

The freeway enters Targhee National Forest and has diamond interchanges with Stoddard Creek Road and at Humphrey.

I-15 reaches its northern end in Idaho at Monida Pass, where the highway crosses the Continental Divide into Beaverhead County, Montana.

[6] The new design eliminates left-side ramps from I-15 and allows traffic from the Pocatello Creek Road interchange to merge onto either freeway.