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 16 in South Dakota
US 16 is also known as Mount Rushmore Road in western South Dakota. The highway enters South Dakota east of Newcastle, Wyoming. It travels near Jewel Cave, the third-longest cave in the world. The highway goes through the city of Custer and shares alignment with US 385. East of Hill City, US 16 splits off US 385. It then becomes a four-lane divided highway, with the two roadways separated by up to in some places, including the old gold-mining town of Rockerville, South Dakota, which is contained entirely in the median of US 16. In Rapid City, US 16 follows Mount Rushmore Road to a concurrency with SD 44 (Omaha Street) to the southern terminus of I-190. US 16 stays concurrent with I-190 until both highways end at I-90.