U.S. Route 20 in Idaho
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 highlighted in red | ||||
Route information | ||||
Maintained by ITD | ||||
Length | 411 mi[1] (661 km) | |||
Existed | 1940[2]–present | |||
Major junctions | ||||
West end | ![]() ![]() | |||
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East end | ![]() | |||
Location | ||||
Counties | Payette, Canyon, Ada, Elmore, Camas, Blaine, Butte, Bingham, Bonneville, Jefferson, Madison, Fremont | |||
Highway system | ||||
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Route description

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.
History
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
County | Location | mi[4] | km | Exit | Destinations | Notes | ||
---|---|---|---|---|---|---|---|---|
Snake River | 0.000 | 0.000 | ![]() ![]() | Continuation into Oregon | ||||
Snake River Bridge; Oregon–Idaho state line | ||||||||
Payette | | 1.578 | 2.540 | ![]() | Western end of US-95 concurrency | |||
Canyon | | 9.647 | 15.525 | ![]() | Eastern end of US-95 concurrency | |||
Caldwell | 21.995– 22.129 | 35.398– 35.613 | ![]() ![]() | Western end of I-84/US-30 concurrency; I-84 exit 26 | ||||
27 | ![]() ![]() | Exit numbers follow I-84 | ||||||
28 | 10th Avenue – City Center | |||||||
24.840– 24.994 | 39.976– 40.224 | ![]() ![]() | Eastern end of I-84/US-30 concurrency; I-84 exit 29 | |||||
Ada | | 34.267 | 55.147 | ![]() | ||||
Boise–Eagle line | 40.229 | 64.742 | ![]() | |||||
Garden City | 44.166 | 71.078 | ![]() | |||||
Boise | Main Street | West end of freeway; westbound entrance only | ||||||
47.300 | 76.122 | — | ![]() ![]() | Eastern terminus of I-184; westbound left exit and eastbound left entrance | ||||
48.050 | 77.329 | — | River Street | Eastbound exit only | ||||
— | Front Street west | East end of freeway; westbound exit only | ||||||
51.975 | 83.646 | Federal Way | Interchange | |||||
52.560– 52.664 | 84.587– 84.754 | ![]() ![]() | Western end of I-84/US-30 concurrency; I-84 exit 54 | |||||
57 | ![]() | Exit numbers follow I-84 | ||||||
| 59 | S. Eisenman Road / Memory Road | Signed as exits 59A (S. Eisenman Rd.) and 59B (Memory Rd.) eastbound | |||||
| 64 | Blacks Creek Road – Kuna | ||||||
Regina | 71 | Mayfield, Orchard | ||||||
Elmore | | 74 | Simco Road | |||||
| 90 | ![]() ![]() ![]() | Eastbound signage | |||||
West Mountain Home | Westbound signage | |||||||
Mountain Home | 95.066– 95.307 | 152.994– 153.382 | ![]() ![]() ![]() ![]() | Eastern end of I-84/US-26/US-30 concurrency; I-84 exit 95 | ||||
Camas | Fairfield | ![]() | ||||||
Blaine | | ![]() | ||||||
Carey | ![]() ![]() | Western end of US-26/US-93 concurrency | ||||||
Butte | Arco | ![]() | Eastern end of US-93 concurrency | |||||
Butte City | ![]() | |||||||
Atomic City | ![]() | Eastern end of US-26 concurrency | ||||||
Bingham |
No major junctions | |||||||
Bonneville | Idaho Falls | ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() | Western end of I-15 concurrency; I-15 exit 118 | |||||
![]() ![]() | Signalized 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 Drive | Eastbound exit and westbound entrance | ||||||
| 310 | ![]() ![]() | ||||||
Orvin | 311 | Saint Leon Road / 15th East | ||||||
| 313 | Hitt Road / 25th East | ||||||
Ucon | 315 | ![]() | ||||||
Bonneville–Jefferson county line | | 318 | County Line Road | |||||
Jefferson | Rigby | 320 | ![]() ![]() ![]() ![]() | Eastbound exit and westbound entrance | ||||
322 | ![]() ![]() ![]() ![]() ![]() ![]() | Eastbound signage Westbound signage | ||||||
Lorenzo | 325 | Menan, Roberts | ||||||
Snake River | Bridge | |||||||
Madison | | 328 | Thornton | |||||
Rexburg | 332 | BYU-Idaho, South Rexburg | ||||||
333 | ![]() | |||||||
337 | North Rexburg | |||||||
Sugar City | 338 | Sugar City, Salem | Eastbound exit and westbound entrance | |||||
339 | ![]() | East end of Rigby Freeway | ||||||
Fremont | | 344 | South St. Anthony | At-grade intersection; no exit number southbound | ||||
St. Anthony | 346 | ![]() ![]() | Interchange | |||||
| ![]() ![]() | |||||||
Ashton | ![]() ![]() | |||||||
Island Park | ![]() | |||||||
Continental Divide | Targhee Pass; Idaho–Montana state line | |||||||
![]() | Continuation into Montana | |||||||
1.000 mi = 1.609 km; 1.000 km = 0.621 mi
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References
- Google (February 29, 2016). "Snake River to Yellowstone National Park" (Map). Google Maps. Google. Retrieved February 29, 2016.
- Weingroff, Richard (October 17, 2013). "Ask the Rambler: What Is the Longest Road in the United States?". Federal Highway Administration. Retrieved December 4, 2014.
- "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.