|Maintained by ITD|
|Length||411 mi (661 km)|
|West end||US 20 / US 26 at the Oregon state line|
|East end||US 20 at the Montana state line|
|Counties||Payette, Canyon, Ada, Elmore, Camas, Blaine, Butte, Bingham, Bonneville, Jefferson, Madison, Fremont|
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.
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.
This section needs expansion. You can help by adding to it. (March 2017)
US 20 was expanded past Yellowstone National Park in 1940.
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.
|Snake River||0.000||0.000||US 20 west / US 26 west – Bend||Continuation into Oregon|
|Snake River Bridge; Oregon–Idaho state line|
|Payette||||1.578||2.540||US-95 – Fruitland||Western end of US 95 concurrency|
|Canyon||||9.647||15.525||US-95 – Jordan Valley||Eastern end of US 95 concurrency|
|26||I-84 / US-30 – Baker City||Western end of I-84 / US 30 concurrency|
|27||I-84 Business Loop east to SH-19 – Caldwell, Wilder, Homedale|
|28||10th Avenue – Caldwell City Center|
|29||I-84 / US-30 – Boise, Twin Falls||Eastern end of I-84 / US 30 concurrency|
|Ada||||34.267||55.147||SH-16 – Emmett|
|Boise||40.229||64.742||SH-55 – Caldwell, Eagle|
|44.166||71.078||SH-44 – Eagle|
|47.300||76.122||—||I-184 west to I-84||I-184 exit 3|
|48.050||77.329||—||River Street||Eastbound exit only|
|51.975||83.646||—||South Federal Way||Interchange|
|54||I-84 / US-30 – Caldwell||Western end of I-84 / US 30 concurrency|
|57||SH-21 (Gowen Road) – Idaho City||Exit for South Federal Way|
|||59||South Eisenman Road Interchange||Signed as exits 59A (South Eisenman IC) and 59B (Memory Road) eastbound|
|||64||Blacks Creek Road – Kuna|
|||90||I-84 Business Loop east to SH-51 / SH-67 – Mountain Home, Bruneau|
|95||I-84 / US-26 / US-30 / SH-51 – Bliss, Twin Falls||Eastern end of I-84 / US 26 / US 30 concurrency|
|Camas||Fairfield||SH-46 – Gooding, Wendell|
|Blaine||||SH-75 – Shoshone, Challis|
|Carey||US-26 / US-93 – Bliss, Twin Falls||Western end of US 26 / US 93 concurrency|
|Butte||Arco||US-93 – Challis||Eastern end of US 93 concurrency|
|Butte City||SH-33 – Howe|
|Atomic City||US-26 – Idaho Falls||Eastern end of US 26 concurrency|
||No major junctions|
I-15 / I-15 Business Loop / US-20 Business – Pocatello
|Western end of I-15 concurrency|
|119||I-15 – Butte||Eastern end of I-15 concurrency; access to Idaho Falls Regional Airport; west end of freeway via partial Interstate-standard interchange|
|Bridge over the Snake River|
|Science Center Drive||Eastbound exit and westbound entrance|
US-20 Business (Holmes Avenue) – Idaho Falls
|Orvin||Saint Leon Road|
|||North 25th East|
|Ucon||SH-43 (105th North) – Beachs Corner|
|||County Line Road|
|Jefferson||Rigby||SH-48 – Menan, Ririe||Eastbound exit and westbound entrance|
|Farnsworth Way / Yellowstone Highway|
|Lorenzo||Yellowstone Highway / East 700 North||Interchange; east end of freeway|
|SH-33 – Driggs||Interchange|
|North 2nd East||Interchange|
|Sugar City||Center Street||Interchange; eastbound exit and westbound entrance|
|Fremont||Saint Anthony||Bridge Street||Interchange|
US-20 Business – Saint Anthony
|Ashton||SH-47 – Marysville|
|Henrys Lake||SH-87||Access to Henrys Lake Airport|
|Targhee Pass||Idaho–Montana state line|
|US 20 east – West Yellowstone||Continuation into Montana|
|1.000 mi = 1.609 km; 1.000 km = 0.621 mi|
- Weingroff, Richard (October 17, 2013). "Ask the Rambler: What Is the Longest Road in the United States?". Federal Highway Administration. Retrieved December 4, 2014.
- Google (February 29, 2016). "Snake River to Yellowstone National Park" (Map). Google Maps. Google. Retrieved February 29, 2016.
- "Thornton Interchange opens, U.S. 20 improvements now complete". East Idaho News. November 18, 2016. Retrieved July 16, 2018.
- "Milepoint Log — State Highway System" (PDF). Idaho Transportation Department. Retrieved April 15, 2016.
|U.S. Route 20|