U.S. Route 20 or U.S. Highway 20 (US 20) is an east-west United States highway that stretches from the Pacific Northwest all the way to New England. The "0" in its route number indicates that US 20 is a coast-to-coast route and major route. Spanning 3,365 miles (5,415 km), it is the longest road in the United States, and the route is roughly parallel to that of the newer Interstate 90 (I-90), which is in turn the longest Interstate Highway in the U.S. There is a discontinuity in the official designation of US 20 through Yellowstone National Park, with unnumbered roads used to traverse the park.
US 20 and US 30 break the general U.S. Route numbering rules in Oregon, since US 30 actually starts north of US 20 in Astoria, and runs parallel to the north throughout the state (the Columbia River and Interstate 84). The two run concurrently and continue in the correct positioning near Caldwell, Idaho. This is because US 20 was not a planned coast-to-coast route while US 30 was. US 20 originally ended at the eastern entrance of Yellowstone Park; it was extended in 1940.
The highway's eastern terminus is in Boston, Massachusetts, at Kenmore Square, where it meets Route 2. Its western terminus is in Newport, Oregon, at an intersection with US 101, within a mile of the Pacific Ocean.
Oregon
US 20 begins at an intersection with US 101 in Newport, Oregon, and runs generally eastward towards Idaho. On the way it goes over the Central Oregon Coast Range, through several Willamette Valley cities including Corvallis and Albany, climbs the Cascade Mountains over Santiam Pass, goes through Bend, and traverses the Oregon High Desert passing through Burns. It eventually overlaps with US 26 in Vale, and the two roads continue concurrently to the Idaho border.
Idaho
Main article: U.S. Route 20 in Idaho
US 20 crosses into Idaho from Oregon northwest of Parma. It runs 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.
Montana
Main article: U.S. Route 20 in Montana
In the state of Montana, US 20 runs for less than 10 miles (16 km). It runs from the Idaho state line to West Yellowstone, Montana, the western entrance to Yellowstone National Park. US 20 is known as the Targhee Pass Highway in Montana.
Wyoming
In the state of Wyoming, the eastern segment of US 20 starts at the eastern entrance to Yellowstone National Park along with the western termini of US 14 and US 16.
The Oregon high desert is a region of the U.S. state of Oregon located east of the Cascade Range and south of the Blue Mountains, in the central and eastern parts of the state. Divided into a southern region and a northern region, the desert covers most of five Oregon counties and averages 4,000 feet (1,200 m) above sea level. The southwest region is part of the Great Basin and the southeast is the lower Owyhee River watershed. The northern region is part of the Columbia Plateau, where higher levels of rainfall allow the largest industry on private land to be the cultivation of alfalfa and hay. Public land within the region is owned primarily by the Bureau of Land Management, which manages more than 30,000 square miles (78,000 km2) including five rivers designated as Wild and Scenic.
At 9,733 feet (2,967 m) above sea level, the summit of Steens Mountain is the highest point in the high desert. The broad fault-block mountain is characteristic of the basin and range plate tectonics of the high desert. About 16 million years ago, during the early Miocene epoch, lava flows from volcanic eruptions covered about half the surface area of Oregon. The Earth's crust then began stretching, giving way to further volcanic activity from 15 million to 2 million years ago.