The longest U.S. highway is Route 20, which is over 3,365 miles

The U.S. Route 20 (US 20) is an east–west United States highway. 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 the route roughly parallels that of Interstate 90 (I-90). However, since numbered US routes are not designated as such within the boundaries of national parks, Yellowstone National Park technically splits the route into two sections.

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 overlap 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.