US 20 in Idaho


US 20
Get started Nyssa
End Targhee Pass
Length 409 mi
Length 658 km






Mountain Home



Idaho Falls





US 20 is a US Highway in the US state of Idaho. The road forms an east-west route through the south of the state, from the Oregon border at Nyssa through the Boise metropolitan area to Idaho Falls and then to Targhee Pass on the border with Montana. The route is double-numbered with several roads, especially Interstate 84 and US 26. The road is 658 kilometers long.

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Travel directions

US 20 in central south Idaho.

US 20/26 in southern Idaho.

US 20 crosses the Snake River at Nyssa, which forms the Oregon – Idaho border. US 20 in Oregon comes from Bend, a huge distance through desolate terrain. Directly from the Oregon border, double-numbering begins with US 26. The land in Idaho is more cultivated, with agricultural areas and small villages. Shortly after the state line, a double numbering follows with the US 95, which lasts beyond the town of Parma. The road leads south here, later southeast. One then reaches the fairly large urban area around the capital Boise. US 20 runs through Caldwell and crosses Interstate 84. US 20 then forms an east-west route between Caldwell and Boise.

In Boise, US 20 is a major urban arterial with traffic lights and passes through the center of Idaho’s capital. Interstate 184 also connects here. Through downtown Boise, US 20 is two one-way streets, with 5 lanes in each direction. The road then curves south and joins I-84 here, along with US 26.

About 40 miles to the southeast, US 20 branches off from I-84 again and then leads through a barren mountain range with peaks up to about 2,200 meters. The land around US 20 is barren and consists of desolate steppe. There is only afforestation on the higher parts of the mountains. The first place worth mentioning is the village of Carey, more than 150 kilometers to the east. The road here has long straight stretches and usually runs at an altitude of 1,500 meters.

From Carey, US 20 and US 26 are again double-numbered and US 93 will be added. The road runs along the special Craters of the Moon National Monument, which consists of very extensive lava fields. The road here forms the dividing line between the lava plains to the south and the mountains to the north. About 70 kilometers away in Arco, US 93 turns north, then US 20 and US 26 split 40 kilometers east, with US 20 running straight east to Idaho Falls and US 26 southeast to Blackfoot.

After about 70 kilometers you reach Idaho Falls, a somewhat larger town and intersection of roads. It connects to Interstate 15 and crosses US 91. They also cross the US 26 for the last time. The road will then be a 2×2 lane freeway from Idaho Falls past Sugar City, heading northeast past Rexburg. This is a flat agricultural area, the US 20 runs here at about 1,500 meters above sea level.

You then reach the foothills of the Rocky Mountains, the road starts to rise, although the area does not become very mountainous at first, but mainly becomes wooded. The road is fairly flat until about 2,000 meters above sea level, although there are higher mountains around it, just west of Yellowstone National Park. One then reaches the 2,156 meter high Targhee Pass, located on the border with Montana. Then US 20 continues in Montana to West Yellowstone.


US 20 was created in 1926, but at the time it ran no more westerly than Yellowstone National Park in Wyoming. In 1940, the route was extended west to Albany, Oregon, creating the current route through Idaho. US 20 is the northernmost major east-west route in this part of Idaho, but there is no Interstate Highway built parallel to it, it runs further south. The US 20 between Boise and Mountain Home does hitch a ride on I-84, just like US 26. The part in the east of Idaho runs through a somewhat more densely populated area, and from I-15 to Ashton is the US 20 is a four-lane, partly freeway section.

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Idaho Falls – Sugar City

Between 2000 and 2016, a 45-mile stretch of US 20 in eastern Idaho was converted from a 2×2 divided highway to a freeway with grade-separated connections. A total of 18 intersections have been replaced with 7 connections between the north side of Idaho Falls and Sugar City. The actual bypass of Idaho Falls is older, having been built as a freeway before 1992. On November 18, 2016, the last grade-separated connection opened at Thornton.

connection opening up
Idaho Falls (Exit 310) pre 1992
St Leon Road (Exit 311) opened in 2004-2005
Hitt Place / 25th East (Exit 313) opened in 2004-2005
Ucon (Exit 315) pre 1992
County Line / 145 North (Exit 318) opened in 2001
Rigby (Exit 320 pre 1992
Rigby (Exit 322) pre 1992
Menan Lorenzo (Exit 325) opened in 2010-2011
Thornton (Exit 328) opened in 2016
Rexburg / University Blvd (Exit 332) pre 1992
Rexburg (Exit 333) pre 1992
Rexburg / Salem Road (Exit 337) pre 1992
Sugar City / Salem (Exit 338) opened in 2001
Driggs Jackson (Exit 339) opened in 2001

Traffic intensities

Every day, 4,300 vehicles cross the Oregon border, rising to 6,600 vehicles in Parma and 6,200 vehicles in Caldwell. Between Mountain Home and Carey, there are usually 1,500 to 1,800 vehicles and 1,100 to 1,300 vehicles between Carey and Arco. 2,000 vehicles continue to Idaho Falls. From I-15 at Idaho Falls to Rexburg there are 17,000 to 20,000 vehicles, the busiest portion of US 20. This drops sharply north to 2,700 vehicles at Targhee Pass on the border with Montana.

US 20 in Idaho