U.S. Route 20
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 , 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.
U.S. Route 77 in Iowa
US 77 has never been a long route in Iowa. When it was extended into the state in 1929, it only traveled in Sioux City before crossing the Big Sioux River into South Dakota. After I-29 was built in Sioux City during the late 1950s, US 77 was moved onto the new Interstate Highway. At both historic ends of the highway in Iowa, bridge failures have caused US 77 to be detoured around Sioux City. In the mid-1980s, the national end of the highway was moved to the interchange with I-29.
This is not the shortest highway in the USA. Pretty certain the Old Edgerton Highway in Alaska is the shortest at less than 12 miles long.