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 h ...

in the state of Idaho. 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, US 20 ( running concurrently with US 26) crosses the

Snake River The Snake River is a major river of the greater Pacific Northwest region in the United States. At long, it is the largest tributary of the Columbia River, in turn, the largest North American river that empties into the Pacific Ocean. The Snake ...

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, East ...

, 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 A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually higher th ...

, 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 l ...

, Anderson Ranch Dam 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, crossed the Snake River Plain to the Lost River, and then turned west to the area of Boise, crossing Camas Prairie. I ...

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 Sun Valley may refer to: Places Australia * Sun Valley, New South Wales * Sun Valley, Queensland, a suburb of Gladstone United States * Valley of the Sun, a region that covers the Phoenix metropolitan area *Sun Valley, Arizona * Sun Valley, Los A ...

, Galena Summit, and Stanley. US 20/US 26 goes through Picabo and Carey, joins with US 93, and on to

Craters of the Moon Craters of the Moon may refer to: * Lunar craters, craters on the Earth's Moon * Craters of the Moon National Monument and Preserve, a volcanic preserve in Idaho * Craters of the Moon (geothermal site) Craters of the Moon Thermal Area (or ''Kara ...

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 an ...

, where US 93 splits off and turns north-northwest to climb the Big Lost River 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, where it turns north-northeast to pass near Rexburg as a freeway. US 20 then climbs through the communities of St. Anthony,

Ashton Ashton may refer to: Names *Ashton (given name) *Ashton (surname) Places Australia * Ashton, Elizabeth Bay, a heritage-listed house in Sydney, New South Wales *Ashton, South Australia Canada *Ashton, Ontario New Zealand * Ashton, New Zealand ...

, 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 ...

at Targhee Pass at , entering Montana west of

West Yellowstone West Yellowstone is a town in Gallatin County, Montana, United States, adjacent to Yellowstone National Park. The population was 1,272 at the 2020 census. West Yellowstone is served by Yellowstone Airport. It is part of the Bozeman, MT Micropoli ...

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