From Casper, US 20/US 26 parallels Interstate 25 (I-25) and US 87 for 26 miles (42 km), until all four link up together just southeast of Glenrock.

US 18/US 20 run concurrently from Orin to Lusk, where US 18 turns north and US 20 continues east into Nebraska.

[3] Unofficially, Google Maps marks the start of the western part of US 20, along with US 191 and US 287, at the state line near West Yellowstone, Montana.

[4] The three routes then turn south along US 89 (Grand Loop Road), paralleling the Firehole River.

Past Old Faithful, the four routes curve east before reaching the West Thumb of Yellowstone Lake.

[5] The eastern section of US 20 begins at the east entrance of Yellowstone National Park.

US 20, along with US 14 and US 16, then meanders east along the Shoshone River, heading toward Wapiti, Buffalo Bill Reservoir, and Cody.