State Highway 75 is a two-lane highway in the western United States that travels through the Sawtooth Valley of central Idaho. The highway's southern terminus is in Shoshone, and its northern is near Challis. It is designated as one of Idaho's scenic byways and provides access to Sawtooth National Recreation Area and primarily follows the Big Wood River in the south and the main Salmon River in the north, divided by Galena Summit.

U.S. Route 20 or U.S. Highway 20 (US 20) is an east–west United States Numbered Highway that stretches from the Pacific Northwest east to New England. The "0" in its route number indicates that US 20 is a major coast-to-coast route. Spanning 3,365 miles (5,415 km), it is the longest road in the United States, and, in the east, the route is roughly parallel to Interstate 90 (I-90), which is 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.