Interstate 84 in Idaho partially coincident with U.S. Route 30 in Idaho
Interstate 84 (I-84) in the U.S. state of Idaho is a major Interstate Highway that traverses the state from the Oregon state line in the northwest to Utah state line in the southeast. It primarily follows the Snake River across a plain that includes the cities of Boise, Mountain Home, and Twin Falls. The highway is one of the busiest in Idaho and is designated as the Vietnam Veterans Memorial Highway.
I-84 runs for 276 miles (444 km) within Idaho, beginning near Ontario, Oregon, and traveling concurrent with several U.S. routes through the Boise metropolitan area and Mountain Home towards Twin Falls. I-84 splits away from US 30 and the Snake River at a junction with I-86 near Declo, where it turns southeast to cross the Sublett Range into northern Utah. The highway has an auxiliary route, I-184, which serves downtown Boise.Boise and Treasure Valley
I-84 enters the state of Idaho after crossing the Snake River from Ontario, Oregon, west of Fruitland, Idaho. The four-lane freeway travels southeast along the Snake River and passes an eastbound-only rest area and visitors center, which mirrors a westbound facility on the Oregon side of the river. Its first interchange is with US 95, the primary north–south highway in Idaho, at Palisades Corner south of Fruitland. I-84 continues southeast across farmland in the Snake River Plain and straddles the edge of a hill. South of New Plymouth the freeway intersects and becomes concurrent with US 30. The paired highways continue south along the side of a hill and cross into Canyon County northeast of Notus.The freeway enters the Treasure Valley, which also constitutes much of the Boise metropolitan area, near the outskirts of Caldwell and intersects State Highway 44 (SH-44), which continues east to Middleton and Eagle. Shortly before crossing the Boise River into Downtown Caldwell, I-84 and US 30 begin a brief concurrency with US 20 and US 26, with both continue west towards Parma and the Oregon state border. The freeway skirts the north side of Downtown Caldwell, which is served by a designated business route, and passes through residential neighborhoods before US 20 and US 26 split off and travel east onto Franklin Road. I-84 and US 30 continue southeast through an industrial area and pass the Caldwell Industrial Airport and Karcher Mall before reaching the city of Nampa. The freeway travels through an industrial area on the northwestern side of Nampa, parallel to a set of railroad tracks, but turns east to bypass the city center. SH-55 joins the concurrency at an interchange with Karcher Road and I-84 Business returns to the freeway at an interchange located near Nampa Gateway Center, the Nampa campus of the College of Western Idaho, and the Ford Idaho Center.I-84 expands from four lanes to eight lanes as it leaves Nampa and enters Ada County, passing through suburban neighborhoods in Meridian. The freeway crosses under two single-point urban interchanges at Ten Mile Road and SH-69, along with a partial cloverleaf interchange where SH-55 splits from the concurrency to head north. I-84 enters the city of Boise and expands to ten lanes before an interchange with I-184, an auxiliary route that travels into Downtown Boise. I-84 turns southeast and travels around the city's outer residential neighborhoods, passing the main terminal at Boise Airport and a nearby industrial area. The freeway is rejoined by US 20 and US 26 before leaving Boise, climbing out of Treasure Valley by following Fivemile Creek over a barren plateau.Southeastern Idaho
I-84 passes north of Burley, intersecting SH-27 near Paul. At a partial cloverleaf interchange in northeastern Heyburn, the freeway intersects SH-24 and is rejoined by US 30, beginning a new concurrency as the highway leaves the Magic Valley. I-84 crosses over the Snake River into Cassia County and intersects SH-25 and SH-25 at an interchange on the east bank of the river near Declo. Near the Minidoka National Wildlife Refuge, I-84 reaches the western terminus of I-86, which carries US 30 and follows the Snake River upstream to American Falls and Pocatello. I-84 turns southeast from the interchange and continues into the Raft River Valley, running parallel to SH-81 near Horse Butte in view of the Cotterel Mountains.After crossing the rural valley, I-84 begins following Meadow Creek and climbs into Mortenson Canyon, which marks the northeastern extent of the Black Pine Mountains. The highway crests at Sweetzer Summit, located 5,522 feet (1,683 m) above sea level, and descends into the Juniper Valley on the south side of the Sublett Range in Sawtooth National Forest. I-84 continues southeast across the valley, passing a set of rest areas in Oneida County, and reaches the Utah state border northwest of Snowville. The freeway continues into Utah and passes through Ogden before terminating at a junction with I-80 in the Wasatch Range.History
Prior to the construction of I-84, the corridor was served by Idaho State Highway 2 and Route A of the Sampson Trails system. U.S. Route 30 was created in 1926 under the national numbered highway system, traveling through southern Idaho along the historic Oregon Trail. Near Declo, US 30 split into two routes: U.S. Route 30N, following the Snake River to Pocatello, and U.S. Route 30S, providing a shorter connection to the Salt Lake City area.The Interstate highway was initially designated in 1957 as Interstate 82, but was renumbered as Interstate 80N in 1958 to correspond with US 30.The route of I-80N between Nampa and Boise was selected in 1958 as one of eight proposals and completed in stages between 1965 and 1968. Rural sections in southwestern Idaho had already been completed by the mid-1960s, connecting Ontario, Oregon, to Caldwell, and Boise to Mountain Home. The 6.6-mile (10.6 km) Nampa–Meridian section opened to traffic on September 29, 1965. Another 4.5 miles (7.2 km) from SH-69 in Meridian to western Boise opened on December 12, 1968, at a cost of $9.5 million. The freeway was extended west from Nampa to Caldwell on December 21, 1966, filling a gap in Interstate 80N and connecting to an already-completed rural section. The 11.1-mile (17.9 km) bypass around southeastern Boise was opened on December 1, 1969. It cost $7.13 million to construct and included five interchanges. Another section was opened around the same time in Sublett in eastern Cassia County, which caused the killing of eighteen mule deer from a local herd from collisions with drivers within the first six weeks.
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