Interstate 15 (I‑15) is a major north‑south highway of the Dwight D. Eisenhower National System of Interstate and Defense Highways that traverses the western part of the U.S. state of Idaho. Within Idaho, the interstate runs for approximately 196 mi (315 km) from the Utah state line near Malad City to the Washington state line near Lewiston.
Route description
- Southern terminus: I‑15 enters Idaho at the Utah–Idaho border near the town of Malad City, continuing northward along the Bear River valley.
- Key communities: The highway passes through or near the cities of Malad City, Pocatello (where it intersects Interstate 86), Blackfoot, Twin Falls (near the southern terminus of Interstate 84), and the metropolitan area of Boise, the state capital.
- Boise corridor: In the Boise metropolitan area, I‑15 serves as a principal commuter route, intersecting Interstate 84 at the “Pioneer Bridge” interchange and providing access to the city’s northwestern suburbs. The interstate follows the Boise River and the Boise Valley through this segment.
- Northern stretch: After Boise, I‑15 continues north through the Snake River Plain, passing near the towns of Menlo and Aberdeen before turning northwest toward the Lewiston–Clarkston metropolitan area.
- Northern terminus: The highway reaches its northern terminus at the Washington state line just north of Lewiston, where it becomes Washington State Route 7.
Major interchanges
- I‑15 / I‑86 at Pocatello (provides connections to the Idaho‑Washington border via I‑90)
- I‑15 / I‑84 in Boise (major east‑west corridor across southern Idaho)
- I‑15 / U.S. Route 20 near McCammon (provides east‑west access across the state)
- I‑15 / U.S. Route 12 near Lewiston (connects to the Palouse region)
History
Construction of I‑15 in Idaho began in the late 1950s as part of the nationwide Interstate Highway System authorized by the Federal‑Aid Highway Act of 1956. The first completed segment in Idaho was the stretch between Pocatello and the Utah border, opened in 1960. Subsequent sections through the Treasure Valley and the northern Snake River Plain were completed during the 1960s and early 1970s. The final segment, linking the Lewiston area to the Washington state line, opened in 1977, completing the continuous interstate corridor.
Traffic and usage
The Idaho portion of I‑15 carries a mix of local, regional, and long‑distance traffic, including freight movements between the Intermountain West and the Pacific Northwest. The Boise–Twin Falls segment experiences the highest average daily traffic (ADT) volumes, exceeding 80,000 vehicles per day according to Idaho Transportation Department (ITD) counts (2022). Rural sections in the southern and northern extremities have lower ADT, typically ranging from 5,000 to 15,000 vehicles per day.
Maintenance and improvements
The Idaho Transportation Department is responsible for routine maintenance, resurfacing, and safety upgrades on I‑15 within the state. Recent projects (2020‑2024) have included:
- Reconstruction of the I‑15/I‑84 interchange in Boise to improve traffic flow and reduce congestion.
- Installation of a wildlife overpass near Twin Falls to reduce collisions with native fauna.
- Repaving of the south‑bound lane between Pocatello and Malad City to address pavement deterioration.
Significance
Interstate 15 serves as a critical transportation corridor for Idaho, linking major urban centers, agricultural regions, and tourism destinations (e.g., the Sawtooth National Forest near Pocatello). It also forms part of the broader I‑15 corridor that extends from San Diego, California, to the Canadian border in Montana, facilitating interstate commerce and travel across the western United States.
