U.S. Route 20 in Idaho

U.S. Route 20
Route information
	Maintained by ITD
Length	411 mi[1] (661 km)
Existed	1940[2]–present
Major junctions
West end	US 20 / US 26 at the Oregon state line
	US 95 in Parma; I-84 in Caldwell; I-84 in Garden City; I-84 / US 26 in Mountain Home; US 26 in Carey; US 26 / US 93 in Atomic City; I-15 in Idaho Falls;
East end	US 20 at the Montana state line
Location
Counties	Payette, 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

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		US 20 west / US 26 west – Bend	Continuation into Oregon
Snake River		0.000	0.000	Snake River Bridge; Oregon–Idaho state line
Payette		1.578	2.540		US 95 – Fruitland	Western terminus of US 95 concurrency
Canyon		9.647	15.525		US 95 – Jordan Valley	Eastern terminus of US 95 concurrency
	Caldwell	21.995– 22.129	35.398– 35.613	26	I-84 / US 30 – Baker City	Western terminus of I-84 / US 30 concurrency
				27	I-84 Business Loop east to SH 19 – Caldwell, Wilder, Homedale
				28	10th Avenue – Caldwell City Center
		24.840– 24.994	39.976– 40.224	29	I-84 / US 30 – Boise, Twin Falls	Eastern terminus 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
		52.560– 52.664	84.587– 84.754	54	I-84 / US 30 – Caldwell	Western terminus 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
				71	Orchard, Mayfield
Elmore				74	Simco Road
				90	I-84 Business Loop east to SH 51 / SH 67 – Mountain Home, Bruneau
	Mountain Home	95.066– 95.307	152.994– 153.382	95	I-84 / US 26 / US 30 / SH 51 – Bliss, Twin Falls	Eastern terminus of I-84 / US 26 / US 30 concurrency
Camas	Fairfield				SH 46 – Gooding, Wendell
Blaine					SH 75 – Shoshone, Challis
Blaine	Carey				US 26 / US 93 – Bliss, Twin Falls	Western terminus of US 26 / US 93 concurrency
Butte	Arco				US 93 – Challis	Eastern terminus of US 93 concurrency
	Butte City				SH 33 – Howe
	Atomic City				US 26 – Idaho Falls	Eastern terminus of US 26 concurrency
Bingham	No major junctions
Bonneville	Idaho Falls			118	I-15 / I-15 Business Loop / US 20 Business – Pocatello	Western terminus of I-15 concurrency
				119	I-15 – Butte	Eastern terminus 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 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
Bonneville–Jefferson county line					County Line Road
Jefferson	Rigby				SH 48 – Menan, Ririe	Eastbound exit and westbound entrance
	Rigby				Farnsworth Way / Yellowstone Highway
	Lorenzo				Yellowstone Highway / East 700 North	Interchange; east end of freeway
Madison	Rexburg				University Boulevard	Interchange
					SH 33 – Driggs	Interchange
					North 2nd East	Interchange
	Sugar City				Center Street	Interchange; eastbound exit and westbound entrance
	Sugar City				9th North	Interchange
Fremont	Saint Anthony				Bridge Street	Interchange
	Saint Anthony				US 20 Business – Saint Anthony
	Ashton				SH 47 – Marysville
	Henrys Lake				SH 87	Access to Henrys Lake Airport
Targhee Pass				Idaho–Montana state line
Targhee Pass					US 20 east – West Yellowstone	Continuation into Montana
1.000 mi = 1.609 km; 1.000 km = 0.621 mi Concurrency terminus Incomplete access

References

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

U.S. Route 20
Previous state: Oregon	Idaho	Next state: Montana

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

[fhwa-2] 1 2 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.

[milepoint-4] "Milepoint Log — State Highway System" (PDF). Idaho Transportation Department. Retrieved April 15, 2016.