It was generally built along the corridor of U.S. Route 30 (US 30) and US 30S, which themselves largely followed the Oregon Trail; the US 30S designation was decommissioned in the 1970s after the freeway replacement was mostly complete.

This grade is also well known because of the distance between eastbound and westbound lanes, nearly 2 miles (3.2 km) between the opposite directions of travel at some points.

From Boise, I-84 continues southeast passing near several small cities (Mountain Home, Glenns Ferry, and Jerome) on its way to Twin Falls.

Access to Twin Falls is afforded by an intersection with US 93 at Exit 173; US 93 southbound crosses the Snake River via the Perrine Bridge.

[6] From Idaho, I-84 enters Utah at a point approximately 7 miles (11.3 km) from Snowville in Box Elder County.

It proceeds southeast through Rattle Snake Pass towards Brigham City where I-84 joins I-15 (just west of Tremonton) for its next 40 miles (64 km).

Just north of Brigham City, at Corinne, Utah, I-84 joins the route of the First transcontinental railroad which the highway follows to its terminus.

I-15/I-84 heads south to the cities of the Wasatch Front (Or the Ogden-Clearfield Utah Metropolitan part) passing through several smaller communities and then the west side of Ogden before I-84 separates towards Cheyenne, Wyoming, while I-15 heads to Salt Lake City, I-84 follows the Weber River east.

[9] The Portland to Utah corridor was proposed as one of the national "toll superhighways" in a 1939 report by the Bureau of Public Roads.

The motion was seconded by officials in Utah (who initially proposed this as I-82), but opposed by Oregon and Washington.