<Li> United States Numbered Highway System </Li> <Ul> <Li> List </Li> <Li> Special </Li> <Li> Divided </Li> <Li> Replaced </Li> </Ul> <P> 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 . Spanning 3,365 miles (5,415 km), it is the longest road in the United States, 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 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 . </P> <P> It and US 30 break the general U.S. Route numbering rules in Oregon, since US 30 actually starts north of US 20 and runs parallel to the north throughout the state . 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 . </P>

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