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 a major east-west route. Spanning 3,365 miles (5,415 km), it is the longest U.S. Route in the United States Numbered Highway System, and particularly from Newport, Oregon to Boston, Massachusetts, the route is roughly parallel to that of the newer Interstate 90 (I-90), which is in turn the longest Interstate Highway in the Interstate Highway System. 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, Oregon 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 National Park; it was extended in 1940.
The highway's eastern terminus is in Boston, Massachusetts, at Kenmore Square, where it meets Massachusetts Route 2 (MA 2). Its western terminus is in Newport, Oregon, at an intersection with US 101, within a mile of the Pacific Ocean.