U.S. Route 20 in Idaho

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U.S. Highway 20

Idaho Medal of Honor Highway
US 20 highlighted in red
Route information
Maintained by ITD
Length406.30 mi[1] (653.88 km)
Major junctions
West end US 20 / US 26 at the Oregon state line
Major intersections
East end US 20 at the Montana state line
CountryUnited States
CountiesPayette, Canyon, Ada, Elmore, Camas, Blaine, Butte, Bingham, Bonneville, Jefferson, Madison, Fremont
Highway system
  • Idaho State Highway System
SH-19 SH-21

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.

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

Route description

I-15/US 20 junction in Idaho Falls

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.


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]

Junction list

Snake River0.0000.000

US 20 west / US 26 west – Nyssa
Continuation into Oregon
Snake River Bridge; Oregon–Idaho state line
US 95 north – Fruitland, Payette
Western end of US 95 concurrency
US 95 south – Jordan Valley
Eastern end of US 95 concurrency

I-84 / US 30 west – Ontario
Western end of I-84/US 30 concurrency; I-84 exit 26

I-84 BL east to SH-19 – Caldwell, Wilder, Homedale
Exit numbers follow I-84
2810th Avenue – City Center

I-84 / US 30 east – Nampa, Boise
Eastern end of I-84/US 30 concurrency; I-84 exit 29
Ada34.26755.147 SH-16 – Emmett
BoiseEagle line40.22964.742 SH-55 – Caldwell, Eagle
Garden City44.16671.078 SH-44 – Eagle
BoiseMain StreetWestern end of freeway; westbound entrance only

I-184 west to I-84
Eastern terminus of I-184; westbound left exit and eastbound left entrance
48.05077.329River StreetEastbound exit only
Front Street westEastern end of freeway; westbound exit only
51.97583.646Federal WayInterchange

I-84 / US 30 west / Commerce Avenue – Nampa
Western end of I-84/US 30 concurrency; I-84 exit 54
57 SH-21 (Gowen Road) – Idaho CityExit numbers follow I-84
59S. Eisenman Road, Memory RoadSigned as exits 59A (S. Eisenman Road) and 59B (Memory Road) eastbound
64Blacks Creek Road – Kuna
Regina71Mayfield, Orchard
Elmore74Simco Road

I-84 BL east to SH-51 / SH-67 – Mountain Home, Bruneau
Eastbound signage
West Mountain HomeWestbound signage
Mountain Home95.066–

I-84 / US 26 / US 30 east / I-84 BL west (SH-51) – Mountain Home, Twin Falls, Air Force Base, Duck Valley
Eastern end of I-84/US 26/US 30 concurrency; I-84 exit 95
CamasFairfield SH-46 – Gooding, Wendell
Blaine SH-75 – Shoshone, Challis
Carey US 26 / US 93 – Bliss, Twin FallsWestern end of US 26/US 93 concurrency
ButteArco US 93 – ChallisEastern end of US 93 concurrency
Butte City SH-33 – Howe
Atomic City US 26 – Idaho FallsEastern end of US 26 concurrency
BinghamNo major intersections
BonnevilleIdaho Falls

I-15 south / I-15 BL / US 20 Bus. (Broadway Street east) to US 26 – Blackfoot, Pocatello, Jackson, Idaho Falls
Western end of I-15 concurrency; I-15 exit 118

I-15 north – Roberts, Butte, Airport
Eastern end of I-15 concurrency; I-15 exit 119; access to airport via Grandview Avenue; signalized interchange
Western end of Rigby Freeway
307Lindsay Boulevard
308ARiverside Drive – City Center
308BScience Center DriveEastbound exit and westbound entrance
US 20 Bus. – Lewisville
Orvin311Saint Leon Road, 15th East
313Hitt Road, 25th East
Ucon315 SH-43 – Ucon
county line
318County Line Road

US 20 Bus. to SH-48 – Rigby
Eastbound exit and westbound entrance
US 20 Bus. – North Rigby
Eastbound signage

US 20 Bus. to SH-48 – Rigby
Westbound signage
Lorenzo325Menan, Roberts
Snake RiverBridge
Rexburg332BYU-Idaho, South Rexburg
333 SH-33 – Salmon, Rexburg
337North Rexburg
Sugar City338Sugar City, SalemEastbound exit and westbound entrance

To SH-33 east – Driggs, Jackson, Sugar City
Eastern end of Rigby Freeway
Fremont344South St. AnthonyAt-grade intersection; no exit number southbound
St. Anthony346

US 20 Bus. east – St. Anthony

US 20 Bus. west – St. Anthony
SH-47 to SH-32 – Driggs
Island Park SH-87 – Ennis
Continental DivideTarghee Pass; Idaho–Montana state line

US 20 east – West Yellowstone, Bozeman
Continuation into Montana
1.000 mi = 1.609 km; 1.000 km = 0.621 mi


  1. ^ a b "Milepoint Log — State Highway System: US 20" (PDF). Idaho Transportation Department. January 25, 2016. Retrieved February 10, 2023.
  2. ^ a b Weingroff, Richard (October 17, 2013). "Ask the Rambler: What Is the Longest Road in the United States?". Federal Highway Administration. Retrieved December 4, 2014.
  3. ^ Blanchard, Nicole (March 25, 2019). "This Idaho road is now the Medal of Honor Highway. Meet the honorees who crossed its path". Idaho Statesman. Retrieved August 27, 2021.
  4. ^ "Drivers ease on down Boise's new roadway". The Idaho Statesman. August 8, 1992. p. 1A. Retrieved February 10, 2023 – via Newspapers.com.
  5. ^ "Regular Meeting of the Idaho Transportation Board, August 27, 1992" (PDF). Idaho Transportation Department. August 27, 1992. p. 35. Retrieved February 10, 2023.
  6. ^ "Thornton Interchange opens, U.S. 20 improvements now complete". East Idaho News. November 18, 2016. Retrieved July 16, 2018.

External links

U.S. Route 20
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