U.S. Route 20 in Idaho

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U.S. Highway 20 marker

U.S. Highway 20
US 20 highlighted in red
Route information
Maintained by ITD
Length411 mi[1] (661 km)
Major junctions
West end US 20 / US 26 at the Oregon state line
East end US 20 at the Montana state line
CountiesPayette, Canyon, Ada, Elmore, Camas, Blaine, Butte, Bingham, Bonneville, Jefferson, Madison, Fremont
Highway system
Idaho 19.svg SH-19Idaho 21.svg SH-21

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

Route description[edit]

I-15/US-20 Jct. in Idaho Falls, ID

US 20 crosses into Idaho northwest of Parma, running concurrently with US 26 and joins US 95 through Parma. US 20/US 26 leaves US 95 southeast of Parma and runs to Caldwell where US 20/US 26 joins with I-84 and US 30 for a short time. These four highways parallel each other (on two roadways) to Boise where US 20/US 26 runs through downtown before joining with I-84 and US 30 again to Mountain Home, where it departs at exit 95 to head east, past Rattlesnake Station, Anderson Ranch Dam road, and cresting at Cat Creek summit at 5,527 feet (1,685 m) above mean sea level. It continues into and across Camas County through Fairfield to Timmerman Junction, the intersection in Blaine County with State Highway 75, the route to Sun Valley, Galena Summit, and Stanley. US 20 continues east through Picabo and Carey, joined with US 26 and US 93, to Craters of the Moon 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 the Idaho National Laboratory, where the highways split just west of Atomic City; US 26 heads 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.[2]

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.[3]

Junction list[edit]

Snake River0.0000.000 US 20 / US 26 west – NyssaContinuation into Oregon
Snake River Bridge; Oregon–Idaho state line
Payette1.5782.540 US 95 north – Fruitland, PayetteWestern end of US-95 concurrency
Canyon9.64715.525 US 95 south – Jordan ValleyEastern end of US-95 concurrency
I-84 / US 30 west – OntarioWestern end of I-84/US-30 concurrency; I-84 exit 26
27 I-84 BL east to SH-19 – Caldwell, Wilder, HomedaleExit numbers follow I-84
2810th Avenue – City Center
I-84 / US 30 east – Nampa, BoiseEastern 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 StreetWest end of freeway; westbound entrance only
47.30076.122 I-184 west to I-84Eastern terminus of I-184; westbound left exit and eastbound left entrance
48.05077.329River StreetEastbound exit only
Front Street westEast end of freeway; westbound exit only
51.97583.646Federal WayInterchange
I-84 / US 30 west / Commerce Avenue – NampaWestern 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 Rd.) and 59B (Memory Rd.) eastbound
64Blacks Creek Road – Kuna
Regina71Mayfield, Orchard
Elmore74Simco Road
90 I-84 BL east to SH-51 / SH-67 – Mountain Home, BruneauEastbound signage
West Mountain HomeWestbound signage
Mountain Home95.066–
I-84 / US 26 / US 30 west / I-84 BL west (SH-51) – Mountain Home, Twin Falls, Air Force Base, Duck ValleyEastern 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
No major junctions
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, AirportSignalized interchange; eastern end of I-15 concurrency; I-15 exit 119; access to airport via Grandview Avenue
West end of Rigby Freeway
Lindsay Boulevard
Riverside Drive – City Center
Science 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

US 20 Bus. to SH-48 – Rigby

Eastbound signage

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
339 To SH-33 east – Driggs, Jackson, Sugar CityEast 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
Ashton SH-47 to SH-32 – Driggs
Island Park SH-87 – Ennis
Continental DivideTarghee Pass; Idaho–Montana state line
US 20 east – West Yellowstone, BozemanContinuation into Montana
1.000 mi = 1.609 km; 1.000 km = 0.621 mi


  1. ^ Google (February 29, 2016). "Snake River to Yellowstone National Park" (Map). Google Maps. Google. Retrieved February 29, 2016.
  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. ^ "Thornton Interchange opens, U.S. 20 improvements now complete". East Idaho News. November 18, 2016. Retrieved July 16, 2018.
  4. ^ "Milepoint Log — State Highway System" (PDF). Idaho Transportation Department. Retrieved April 15, 2016.

External links[edit]

Route map:

KML is not from Wikidata

U.S. Route 20
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