# U.S. Route 20 in Idaho

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.

## Route description

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.

## History

US 20 was expanded past Yellowstone National Park in 1940, replacing the original US 191 and several state highways. 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.
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, though a few at grade crossings still exist between Sugar City and St. Anthony as of September 2023.

## Major intersections

| County | Location | mi | km | Exit | Destinations | Notes |
| Snake River | Snake River | 0.000 | 0.000 | | US 20 west / US 26 west – Nyssa | Continuation into Oregon |
| Snake River Bridge; Oregon–Idaho state line ||
| Payette–Canyon county line | ​ | 1.578 | 2.540 | | US 95 north – Fruitland, Payette | Western end of US 95 concurrency |
| Canyon | ​ | 9.647 | 15.525 | | US 95 south – Jordan Valley | Eastern end of US 95 concurrency |
| Canyon | Caldwell | 21.995– 22.129 | 35.398– 35.613 | | I-84 / US 30 west – Ontario | Western end of I-84/US 30 concurrency; I-84 exit 26 |
| Canyon | Caldwell | 22.6– 23.1 | 36.4– 37.2 | 27 | I-84 BL east to SH-19 – Caldwell, Wilder, Homedale | Exit numbers follow I-84 |
| Canyon | Caldwell | 23.5– 24.1 | 37.8– 38.8 | 28 | 10th Avenue – City Center | Exit numbers follow I-84 |
| Canyon | Caldwell | 24.840– 24.994 | 39.976– 40.224 | | I-84 / US 30 east – Nampa, Boise | Eastern end of I-84/US 30 concurrency; I-84 exit 29 |
| Ada | ​ | 34.267 | 55.147 | | SH-16 – Emmett | |
| Ada | Boise–Eagle line | 40.229 | 64.742 | | SH-55 – Caldwell, Eagle | |
| Ada | Garden City | 44.166 | 71.078 | | SH-44 – Eagle | |
| Ada | Boise | 47.205 | 75.969 | — | Main Street | Western end of freeway; westbound entrance only |
| Ada | Boise | 47.300 | 76.122 | — | I-184 west to I-84 | Eastern terminus of I-184; westbound left exit and eastbound left entrance |
| Ada | Boise | 48.050 | 77.329 | — | River Street | Eastbound exit only |
| Ada | Boise | 48.512 | 78.072 | — | Front Street west | Eastern end of freeway; westbound exit only |
| Ada | Boise | 51.960 | 83.622 | | Federal Way | Interchange |
| Ada | Boise | 52.681– 52.824 | 84.782– 85.012 | | I-84 / US 30 west / Commerce Avenue – Nampa | Western end of I-84/US 30 concurrency; I-84 exit 54 |
| Ada | Boise | 55.3 | 89.0 | 57 | SH-21 (Gowen Road) – Idaho City | Exit numbers follow I-84 |
| Ada | ​ | 57.9 | 93.2 | 59 | S. Eisenman Road, Memory Road | Signed as exits 59A (S. Eisenman Road) and 59B (Memory Road) eastbound |
| Ada | ​ | 61.9 | 99.6 | 64 | Blacks Creek Road – Kuna | |
| Ada | Regina | 69.2 | 111.4 | 71 | Mayfield, Orchard | |
| Elmore | ​ | 72.8 | 117.2 | 74 | Simco Road | |
| Elmore | ​ | 88.7 | 142.7 | 90 | I-84 BL east to SH-51 / SH-67 – Mountain Home, Bruneau | Eastbound signage |
| Elmore | ​ | 88.7 | 142.7 | 90 | West Mountain Home | Westbound signage |
| Elmore | Mountain Home | 95.308– 95.467 | 153.383– 153.639 | | 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 |
| Camas | Fairfield | 156.157 | 251.310 | | SH-46 – Gooding, Wendell | |
| Blaine | ​ | 178.095 | 286.616 | | SH-75 – Shoshone, Challis | |
| Blaine | Carey | 196.039 | 315.494 | | US 26 / US 93 – Bliss, Twin Falls | Western end of US 26/US 93 concurrency |
| Butte | Arco | 248.555 | 400.010 | | US 93 – Challis | Eastern end of US 93 concurrency |
| Butte | Butte City | 256.073 | 412.110 | | SH-33 – Howe | |
| Butte | Atomic City | 263.77 | 424.50 | | US 26 – Idaho Falls | Eastern end of US 26 concurrency |
| Bingham | No major junctions | No major junctions | No major junctions | No major junctions | No major junctions | No major junctions |
| Bonneville | Idaho Falls | 306.9 | 493.9 | | 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 |
| Bonneville | Idaho Falls | 307.617 | 495.062 | | 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 ||
| Bonneville | Idaho Falls | 307.72– 307.87 | 495.23– 495.47 | 307 | Lindsay Boulevard | |
| Bonneville | Idaho Falls | 307.967– 308.318 | 495.625– 496.190 | 308A | Riverside Drive – City Center | Signed as Exit 308 eastbound |
| Bonneville | Idaho Falls | 308.504 | 496.489 | 308B | Science Center Drive | Eastbound exit and westbound entrance |
| Bonneville | ​ | 309.585– 310.125 | 498.229– 499.098 | 310 | US 20 Bus. – Lewisville | |
| Bonneville | Orvin | 310.99– 311.739 | 500.49– 501.695 | 311 | Saint Leon Road, 15th East | |
| Bonneville | ​ | 313.063– 313.887 | 503.826– 505.152 | 313 | Hitt Road, 25th East | |
| Bonneville | Ucon | 314.92– 315.57 | 506.81– 507.86 | 315 | SH-43 – Ucon | |
| Bonneville–Jefferson county line | ​ | 317.559– 318.367 | 511.062– 512.362 | 318 | County Line Road | |
| Jefferson | Rigby | 320.375 | 515.594 | 320 | US 20 Bus. to SH-48 – Rigby | Eastbound exit and westbound entrance |
| Jefferson | Rigby | 322.083– 322.586 | 518.342– 519.152 | 322 | US 20 Bus. – North Rigby | Eastbound signage |
| Jefferson | Rigby | 322.083– 322.586 | 518.342– 519.152 | 322 | US 20 Bus. to SH-48 – Rigby | Westbound signage |
| Jefferson | Lorenzo | 325.202– 325.898 | 523.362– 524.482 | 325 | Menan, Roberts | |
| Snake River | Snake River | 326.2 | 525.0 | Bridge | Bridge | Bridge |
| Madison | ​ | 328.232 | 528.238 | 328 | Thornton | |
| Madison | Rexburg | 331.63– 332.27 | 533.71– 534.74 | 332 | BYU-Idaho, South Rexburg | |
| Madison | Rexburg | 333.198– 333.77 | 536.230– 537.15 | 333 | SH-33 – Salmon, Rexburg | |
| Madison | Rexburg | 336.53– 337.098 | 541.59– 542.507 | 337 | North Rexburg | |
| Madison | Sugar City | 338.018 | 543.987 | 338 | Sugar City, Salem | Eastbound exit and westbound entrance |
| Madison | Sugar City | 338.522– 339.295 | 544.798– 546.042 | 339 | To SH-33 east – Driggs, Jackson, Sugar City | Eastern end of Rigby Freeway |
| Fremont | ​ | 343.64 | 553.03 | 344 | South St. Anthony | New interchange under construction |
| Fremont | St. Anthony | 345.2– 345.91 | 555.5– 556.69 | 346 | US 20 Bus. east – St. Anthony | Interchange |
| Fremont | ​ | 347.851 | 559.812 | | US 20 Bus. west – St. Anthony | |
| Fremont | Ashton | 360.572 | 580.284 | | SH-47 to SH-32 – Driggs | |
| Fremont | Island Park | 402.27 | 647.39 | | SH-87 – Ennis | |
| Continental Divide | Continental Divide | 406.3 | 653.9 | | Targhee Pass; Idaho–Montana state line | Targhee Pass; Idaho–Montana state line |
| Continental Divide | Continental Divide | 406.3 | 653.9 | | US 20 east – West Yellowstone, Bozeman | Continuation into Montana |
| 1.000 mi = 1.609 km; 1.000 km = 0.621 mi Concurrency terminus Incomplete access Route transition |||||||