<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 Idaho to Massachusetts, the route roughly parallels that of 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>

What highway goes from the east coast to the west coast