U.S. Route 91 or U.S. Highway 91 (US 91) is a 172.663-mile-long (277.874 km) major north–south United States highway in the Western United States. The highway currently serves as a connection between the Cache Valley area of Utah and Idaho to the Salt Lake City and Idaho Falls population centers. Despite the "1" as the last number in the digit, US 91 is comparatively very short. Prior to the mid-1970s, US 91 was an international commerce route from Long Beach, California to the Canada–US border north of Sweetgrass, Montana. US 91 was routed on the main streets of most of the communities it served, including Las Vegas Boulevard in Las Vegas and State Street in Salt Lake City. From Los Angeles to Salt Lake, the route was built along the corridor of the Arrowhead Trail. US 91 has been largely replaced by Interstate 15 (I-15). A portion of the highway's former route in California is currently State Route 91 (CA 91).
The current south terminus is in Brigham City, Utah touching Interstate 15 / Interstate 84. The current north terminus is a divided highway in Idaho Falls, Idaho at I-15 Business / US 26 (South Yellowstone Highway / West Sunnyside Road). From south of Pocatello, Idaho to the town of Virginia, US 91 runs concurrent with I-15.
In the Intertropolis & Routeville episode "You're Not So Long", the character US 91, along with I-30 and I-45 are being mistreated by the other Main Interstates and Main U.S. Routes crew for having highways with their same numbering being short.
In the episode "Reboot Highway 91", US 91 heard from the KHWY News that the National Department of Transportation are recreating and rerouting U.S. Route 91. It is planning to extend south from Brigham City, Utah through Salt Lake City, Las Vegas, and end as a freeway at Interstate 8 in San Diego, California, and extend north from Idaho Falls, Idaho through Butte, Helena, Great Falls in Montana, and end at the Sweetgrass–Coutts Border Crossing on the Canada–USA border, which will become Alberta Highway 4. Interstate 15 also ends there. At all this point, US 91 will still parallel I-15, but indirectly.