U.S. Route 20 (US 20) is the portion of an east–west

U.S. Highway The United States Numbered Highway System (often called U.S. Routes or U.S. Highways) is an integrated network of roads and highways numbered within a nationwide grid in the contiguous United States. As the designation and numbering of these hi ...

in the state of

Idaho Idaho ( ) is a state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Montana and Wyomi ...

. It begins northwest of Parma at the Oregon state line and enters Montana away from the Yellowstone National Park west entrance. Since 2019, US 20 has been designated as the Idaho Medal of Honor Highway.

Route description

At the western border, adjacent to

Nyssa, Oregon Nyssa is a city in Malheur County, Oregon, United States. The population was 3,267 at the 2010 census. The city is located along the Snake River on the Idaho border, in the region of far eastern Oregon known as the "Treasure Valley". It is par ...

, US 20 ( running concurrently with US 26) crosses the Snake River into Idaho at an approximate elevation of 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 Caldwell may refer to: People * Caldwell (surname) * Caldwell (given name) * Caldwell First Nation, a federally recognized Indian band in southern Ontario, Canada Places Great Britain * Caldwell, Derbyshire, a hamlet * Caldwell, Eas ...

, 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 Boise (, , ) is the capital and most populous city of the U.S. state of Idaho and is the county seat of Ada County. On the Boise River in southwestern Idaho, it is east of the Oregon border and north of the Nevada border. The downtown area' ...

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/US 26 departs at exit 95 to head east, past

Rattlesnake Station Rattlesnake Station was a stagecoach station northeast of Mountain Home, Idaho, and the original site of the Mountain Home post office. Approximately seven miles from exit 95 on Interstate 84 in present-day Elmore County, a historical marker lo ...

Anderson Ranch Dam Anderson Ranch Dam is an earth rockfill type dam in the western United States, on the South Fork of the Boise River in southwestern Idaho. In Elmore County northeast of Mountain Home, it is several miles north of U.S. Route 20 ...

road, and crests at Cat Creek summit at . This portion of the route is near

Goodale's Cutoff Goodale's Cutoff formed a spur of the Oregon Trail beginning in Idaho, United States. The cutoff left the trail near Fort Hall, Idaho, Fort Hall, crossed the Snake River Plain to the Lost River (Idaho), Lost River, and then turned west to the area ...

of the Oregon Trail. US 20/US 26 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 Galena Summit is a high mountain pass in the western United States in central Idaho, at an elevation of above sea level. The pass is located in the Boulder Mountains, in the northwest corner of Blaine County, within the Sawtooth National Recre ...

, and Stanley. US 20/US 26 goes through Picabo and Carey, joins with US 93, and on to Craters of the Moon and

Arco ARCO ( ) is a brand of gasoline stations currently owned by Marathon Petroleum after BP sold its rights. BP commercializes the brand in Northern California, Oregon and Washington, while Marathon has rights for the rest of the United States and ...

, where US 93 splits off and turns north-northwest to climb the

Big Lost River The Big Lost River is a major river in the U.S. state of Idaho, about long.U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map, accessed May 4, 2011 Description The river starts in the Rocky Mountai ...

valley. US 20/US 26 continues on through the

Idaho National Laboratory Idaho National Laboratory (INL) is one of the national laboratories of the United States Department of Energy and is managed by the Battelle Energy Alliance. While the laboratory does other research, historically it has been involved with nu ...

, where the highways split just west of Atomic City; US 26 heads southeast to

Blackfoot The Blackfoot Confederacy, ''Niitsitapi'' or ''Siksikaitsitapi'' (ᖹᐟᒧᐧᒣᑯ, meaning "the people" or " Blackfoot-speaking real people"), is a historic collective name for linguistically related groups that make up the Blackfoot or Bla ...

and US 20 to

Idaho Falls Idaho Falls ( Shoshoni: Dembimbosaage) is a city in and the county seat of Bonneville County, Idaho, United States. It is the state's largest city outside the Boise metropolitan area. As of the 2020 census, the population of Idaho Falls was 6 ...

, where it turns north-northeast to pass near

Rexburg Rexburg is a city in Madison County, Idaho, United States. The population was 39,409 at the 2020 census. The city is the county seat of Madison County and its largest city. Rexburg is the principal city of the Rexburg, ID Micropolitan Statis ...

as a freeway. US 20 then climbs through the communities of St. Anthony, Ashton, and Island Park, and crosses the

Continental Divide A continental divide is a drainage divide on a continent such that the drainage basin on one side of the divide feeds into one ocean or sea, and the basin on the other side either feeds into a different ocean or sea, or else is endorheic, not c ...

Targhee Pass Targhee Pass is a mountain pass in the western United States on the Continental Divide. It is located along the border between southeastern Idaho and southwestern Montana, in the Henrys Lake Mountains at an elevation of above sea level. The ...

at , entering Montana west of West Yellowstone.

History

US 20 was expanded past Yellowstone National Park in 1940, replacing the original US 191 and several state highways. 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. 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.

Junction list

References

External links