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.

The highway was originally routed through Brigham City, but today runs on a southern bypass (locally designated 1100 South).

Despite the modern route of US 91 no longer using Sardine Canyon, the name continues to be used by locals and news media alike in describing the traffic, weather conditions or scenery along the entire grade across the Wellsville Mountains.

[3] US 91 enters the Wellsville Mountains via Box Elder Canyon in a climb to the lake and town of Mantua, which has a reputation as a speed trap.

[4][5] After passing the reservoir, the highway resumes climbing, using Dry Canyon to reach Sardine Summit.

The highway descends through Wellsville Canyon and reaches the Cache Valley, where it serves as the major thoroughfare.

At Logan, US 89 splits off to the east towards Bear Lake, while US 91 continues north through the valley to the Idaho state line.

At Pocatello, US 91, US 30, and I-15 Business Loop separate from Interstate 15 and run concurrently, passing through a small portion of the Idaho State University campus.

At its peak, from 1947 to 1965, U.S. Route 91 extended from the Pacific Ocean at Long Beach, California, to Coutts, Alberta.

Nevada officials suggested U.S. Route 40 be similarly truncated, as this highway largely duplicated Interstate 80 in the same states.

South of Las Vegas, most of the old route is still intact, as it runs parallel and to the east of the freeway from Jean to Sloan.

Visitors to the popular Seven Magic Mountains art installation must travel at least a dozen miles on the old route to reach the site.

At this intersection, US 93 and 95 departed Fremont Street and turned northeast onto Las Vegas Boulevard, heading northbound with US 91.

The road crosses I-15 again at exit 71 and continues its original route through Parowan, merging with SR-274 after briefly running along modern SR-143.

There are also sections of roadway that used to be US 91 from before exit 129 through Cove Fort, Utah, in Millard County, intersecting both I-70 and later I-15 north of the I-70/I-15 interchange, including what is now SR-161.

At Chicken Creek Reservoir, the historic route of US 91 leaves I-15 and follows SR-78 northeast to Levan where it turns north along modern SR-28 and continues through Nephi along that city's Main Street.

In Utah County, the old US 91 leaves I-15 again at Santaquin, following SR-198 northeast through Payson and Salem before reaching Spanish Fork.

Because of the new alignment of I-15 in Montana, many segments of the former US 91 are still in use today as frontage roads or sportsmen access points along the Missouri River, especially from Helena north to Great Falls and Sweetgrass, and from Butte south to Divide and Dillon.

The US 91 corridor in Idaho's Cache Valley was the primary filming location for the 2004 movie Napoleon Dynamite.

The McDonald's location is now a museum, while the KFC is still in operation with a small historical display inside the restaurant.

The novel Damnation Alley (1969) by Roger Zelazny features the beginning of a cross-country trip on Route 91, just outside of Los Angeles.