U.S. Route 20 in Idaho

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U.S. Route 20 marker

U.S. Route 20
Route information
Maintained by ITD
Length411 mi[2] (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

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.[1]

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 west / US 26 west – BendContinuation into Oregon
Snake River Bridge; Oregon–Idaho state line
Payette1.5782.540 US-95 – FruitlandWestern end of US 95 concurrency
Canyon9.64715.525 US-95 – Jordan ValleyEastern end of US 95 concurrency
26 I-84 / US-30 – Baker CityWestern end of I-84 / US 30 concurrency
27 I-84 Business Loop east to SH-19 – Caldwell, Wilder, Homedale
2810th Avenue – Caldwell City Center
29 I-84 / US-30 – Boise, Twin FallsEastern end of I-84 / US 30 concurrency
Ada34.26755.147 SH-16 – Emmett
Boise40.22964.742 SH-55 – Caldwell, Eagle
44.16671.078 SH-44 – Eagle
47.30076.122 I-184 west to I-84I-184 exit 3
48.05077.329River StreetEastbound exit only
51.97583.646South Federal WayInterchange
54 I-84 / US-30 – CaldwellWestern end of I-84 / US 30 concurrency
57 SH-21 (Gowen Road) – Idaho CityExit for South Federal Way
59South Eisenman Road InterchangeSigned as exits 59A (South Eisenman IC) and 59B (Memory Road) eastbound
64Blacks Creek Road – Kuna
71Orchard, Mayfield
Elmore74Simco Road
90 I-84 Business Loop east to SH-51 / SH-67 – Mountain Home, Bruneau
Mountain Home95.066–
95 I-84 / US-26 / US-30 / SH-51 – Bliss, Twin FallsEastern end of I-84 / US 26 / US 30 concurrency
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 Falls118
I-15 / I-15 Business Loop / US-20 Business – Pocatello
Western end of I-15 concurrency
119 I-15 – ButteEastern end of I-15 concurrency; access to Idaho Falls Regional Airport; west end of freeway via partial Interstate-standard interchange
Lindsay Boulevard
Bridge over the Snake River
Fremont Avenue
Science Center DriveEastbound exit and westbound entrance

US-20 Business (Holmes Avenue) – Idaho Falls
OrvinSaint Leon Road
North 25th East
Ucon SH-43 (105th North) – Beachs Corner
county line
County Line Road
JeffersonRigby SH-48 – Menan, RirieEastbound exit and westbound entrance
Farnsworth Way / Yellowstone Highway
LorenzoYellowstone Highway / East 700 NorthInterchange; east end of freeway
MadisonRexburgUniversity BoulevardInterchange
SH-33 – DriggsInterchange
North 2nd EastInterchange
Sugar CityCenter StreetInterchange; eastbound exit and westbound entrance
9th NorthInterchange
FremontSaint AnthonyBridge StreetInterchange

US-20 Business – Saint Anthony
Ashton SH-47 – Marysville
Henrys Lake SH-87Access to Henrys Lake Airport
Targhee PassIdaho–Montana state line
US 20 east – West YellowstoneContinuation into Montana
1.000 mi = 1.609 km; 1.000 km = 0.621 mi


  1. ^ 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.
  2. ^ Google (February 29, 2016). "Snake River to Yellowstone National Park" (Map). Google Maps. Google. Retrieved February 29, 2016.
  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.

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