U.S. Highway 20 (US 20) is the portion of an east–west United States Numbered Highway in the state of Idaho. It begins northwest of Parma at the Oregon state line and enters Montana 9.6 miles (15.4 km) away from the Yellowstone National Park west entrance.

Quick Facts Route information, Length ...

U.S. Highway 20
Idaho Medal of Honor Highway
US 20 highlighted in red
Route information
Maintained by ITD
Length	406.30 mi[1] (653.88 km)
Existed	1940[2]–present
Major junctions
West end	US 20 / US 26 at the Oregon state line
Major intersections	US 95 in Parma I-84 / US 30 in Caldwell I-84 / US 30 in Garden City I-84 / US 26 / US 30 in Mountain Home US 26 in Carey US 26 / US 93 in Atomic City I-15 in Idaho Falls
East end	US 20 at the Montana state line
Location
Country	United States
State	Idaho
Counties	Payette, Canyon, Ada, Elmore, Camas, Blaine, Butte, Bingham, Bonneville, Jefferson, Madison, Fremont
Highway system
United States Numbered Highway System List Special Divided Idaho State Highway System Interstate US State
← SH-19 → SH-21

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Since 2019, US 20 has been designated as the Idaho Medal of Honor Highway.^[3]

At the western border, adjacent to Nyssa, Oregon, US 20 (running concurrently with US 26) crosses the Snake River into Idaho at an approximate elevation of 2,200 feet (670 m) above sea level. It joins US 95 and runs southbound to Parma. US 20/US 26 leaves US 95 southeast of Parma and runs to Caldwell, where it briefly joins Interstate 84 (I-84; and US 30) from exit 26 until exit 29.

These four highways parallel each other east (on two roadways) to Boise where US 20/US 26 runs through downtown, then southbound on Broadway Avenue to rejoin with I-84/US 30 at exit 54. The four concurrent routes head southeast to Mountain Home, where US 20 departs at exit 95 to head east, past Rattlesnake Station, Anderson Ranch Dam road, and crests at Cat Creek summit at 5,527 feet (1,685 m). This portion of the route is near Goodale's Cutoff of the Oregon Trail.

US 20 continues east, into, and across Camas County through Fairfield to Timmerman Junction, the intersection in Blaine County with State Highway 75 (SH-75), the northbound route to Sun Valley, Galena Summit, and Stanley. US 20/US 26 goes through Picabo and Carey, joins with US 93, and on to Craters of the Moon National Monument and Preserve and Arco, where US 93 splits off and turns north-northwest to climb the Big Lost River valley.

US 20/US 26 continues on through Idaho National Laboratory, where the highways split just west of Atomic City; US 26 heads southeast to Blackfoot and US 20 to Idaho Falls, where it turns north-northeast to pass near Rexburg as a freeway. US 20 then climbs through the communities of St. Anthony, Ashton, and Island Park, and crosses the Continental Divide at Targhee Pass at 7,072 feet (2,156 m), entering Montana west of West Yellowstone.

This section needs expansion. You can help by adding to it. (March 2017)

US 20 was expanded past Yellowstone National Park in 1940, replacing the original US 191 and several state highways.^[2] Its original alignment through Boise included Main Street and Fairview Avenue for westbound and eastbound traffic, respectively, and Capitol Boulevard. US 20 and US 26 were realigned onto Myrtle and Front streets as well as Broadway following the completion of the Boise Connector extension on August 7, 1992.^[4][5]

The Rigby Freeway section of US 20, extending from Idaho Falls to St. Anthony, was constructed in the 1970s. Its northernmost section still featured at-grade intersections until the 2000s, when they were replaced with full interchanges,^[6] though a few at grade crossings still exist between Sugar City and St. Anthony as of September 2023.