High Quality Content by WIKIPEDIA articles! U.S. Route 20 (US 20) is an east–west United States highway. As the "0" in its route number implies, 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 sparsely parallels Interstate 90 (I-90). However, a gap in signage exists through Yellowstone National Park which splits the route into two sections.