|Length||172.663 mi  (277.874 km)|
|South end||I-15 / I-84 in Brigham City, UT|
US 30 in McCammon, ID
I-86 in Pocatello, ID
|North end||I-15 Business Loop / US 26 in Idaho Falls, ID|
U.S. Route 91 (US 91) is a north–south United States highway. 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. 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.  A portion of the highway's former route in California is currently State Route 91.
U.S. 91 begins at Brigham City and winds its way through the Wellsville Mountains using Box Elder and Wellsville Canyons for its path to the Cache Valley, concurrent with U.S. 89. Along the way it passes Mantua and Mantua Reservoir and crosses over Sardine Summit. Once entering Cache Valley it serves as the major thoroughfare of the valley. Logan is the largest city in the valley. Both the Utah and Idaho portions of Cache Valley are part of the Logan Utah Metropolitan Statistical Area.
The Utah section of U.S. 91 is defined at Utah Code Annotated § 72-4-115(1). 
U.S. 91 continues serving the Idaho portion of the Cache Valley with Preston as the largest city. The highway passes by the Bear River Massacre Site, a National Historic Landmark, near Preston, and over the geologically significant Red Rock Pass, near Downey. The U.S. 91 corridor in Idaho's Cache Valley was the primary filming location for the 2004 movie Napoleon Dynamite.
After leaving Cache Valley, the highway then crosses back to rejoin Interstate 15. At Pocatello, U.S. 91, U.S. 30, and I-15 Business Loop separate from Interstate 15 and run concurrently, passing through a small portion of the Idaho State University campus. U.S. 30 separates from U.S. 91 near Pocatello's city hall at what was once the starting point of one of Idaho's first designated highways, the Yellowstone Park Highway. From this point the highway generally follows the original route of the Yellowstone Park Highway northeastward, until it reaches its terminus at its intersection with U.S. 26, south of Idaho Falls.
At its peak, from 1947 to 1965, U.S. 91 extended from the Pacific Ocean at Long Beach, California to Coutts, Alberta. North of Riverside, California, U.S. 91 followed a route that would be largely paralleled by modern I-15 and I-215 where these routes did not completely supplant it. There are some parts where U.S. 91/I-15 had divergent paths. The more southerly is from Littlefield, Arizona to St George, Utah where the two routes took different paths through the Arizona Strip. The more northerly and longer is the surviving portion of U.S. 91 from Brigham City to Downey, where Interstate 15 follows an old route of U.S. Route 191. 
Utah first submitted a petition to AASHTO to have the U.S. 91 designation truncated in 1971. This petition was approved on June 21, 1971. However, the state did not take action until 1974. Utah officials co-ordinated a second petition with the transportation departments of California, Nevada and Arizona. In this unified petition officials in Utah proposed the number changeover take place in 1974. Nevada requested postponing the deletion of U.S. 91 as Nevada expected to complete the last piece of I-15 in 1975. Nevada officials suggested U.S. Route 40 be similarly truncated, as this highway largely duplicated Interstate 80 in the same states. Despite Nevada's request, the petition was submitted and approved in 1974. 
Retracing the path of historic U.S. Route 91 is possible but difficult. In most cities the route of U.S. 91 is still in use as a local street or highway. In many rural areas, I-15 was built on top of the former U.S. 91.
The highway passed through the following states:
- California (until 1974)
- Nevada (until 1974)
- Arizona (10 miles (16 km) in the Arizona Strip, until 1974)
- Montana (until 1980)
U.S. 91's original southern terminus was at the intersection of Atlantic Avenue (historically SR 15) and East Pacific Coast Highway ( SR 1) in Long Beach. This intersection also marked the historic western terminus of US 6, which proceeded west from that intersection. East of the intersection, U.S. 91 proceeded east along East Pacific Coast Highway to Los Alamitos Circle.
The portions in California were replaced with:
- State Route 1 (Pacific Coast Highway from Atlantic Avenue to the Traffic Circle)
- State Route 19 (Lakewood Boulevard from the Traffic Circle to Lincoln Avenue)
- State Route 214 (route no longer a state highway — Carson Street and Lincoln Avenue)
- State Route 91
- Interstate 215
- Interstate 15
The route of U.S. 91 in Nevada was replaced with I-15 in 1974.
In Las Vegas, U.S. 91 was better known as Las Vegas Boulevard ( State Route 604 after the 1976 renumbering of most Nevada state highways). The intersection of Las Vegas Boulevard and Fremont Street is the historic intersection with U.S. 93 and U.S. 95. Here, U.S. 466 parted ways with U.S. 91 and headed southeast on Fremont Street with southbound U.S. 93 and 95. At this intersection, U.S. 93 and 95 departed Fremont Street and turned northeast onto Las Vegas Boulevard, heading northbound with U.S. 91. A few blocks further up, at Bonanza Road, U.S. 95 turned west until it hit Rancho Drive, then continued north. U.S. 91/93 stayed aligned with Las Vegas Boulevard, which turned into Salt Lake Highway after departing the city of North Las Vegas.
Until its decertification in 1974, U.S. 91 followed I-15 to exit 112, where it took present-day NV 170 through Riverside to Mesquite, turned right onto NV 144 (Mesquite Boulevard and Sandhill Boulevard), and then followed Hillside Drive into Arizona (where the road became Fairview Avenue).
U.S. 91 entered Arizona from Nevada on Fairview Avenue and followed it northeast to Littlefield. Once Fairview Avenue crosses I-15, it became Cane Beds Road through downtown Beaver Dam and into Utah, where the road continued as Old Highway 91.
From the State Line to Saint George US-91 followed a semi-circular route that is now simply called Old Highway 91. Old Highway 91 continues to Santa Clara and what is now SR-18 to St. George. Where SR-18 intersects St. George Blvd, U.S. 91 then continued east on St. George Blvd. until it intersected Red Cliffs Rd. From there, U.S. 91 went in a north-east direction, with Red Cliffs becoming Telegraph St. in Washington. U.S. 91 followed on Telegraph until becoming Main St. in Leeds. After leaving Leeds, Old 91 is still intact until about a half mile north of the junction with SR-228 where its routing runs right into I-15. 
U.S. 91 picks up again in Black Rock in northern Washington County at exit 36 on I-15. It continues parallel to I-15 until just past exit 40, then picks up again on the other side of the freeway. This section of the road is not maintained and some of the original paving is still visible between the end of the road at Lower Taylor Creek Trailhead and when normal paving resumes near SR 144 by exit 42. The road, marked as Old US-91, then runs north through Kanarraville and into Cedar City near exit 57, merging into Main St (SR 130) until near the city border with Enoch just past exit 62. Old U.S. 91 continues to parallel I-15 to Summit. The road crosses I-15 again at exit 71 and continues its original route though Parowan merging with SR 274 at the intersection with SR 143. There is no additional signage in Parowan to indicate the original route, though historical maps show that it follows SR 271 through Paragonah.  There are some frontage roads along I-15 through Beaver that may be sections of the original highway in Iron County.
In Beaver County, sections of SR 160 through Beaver and the decommissioned (but still maintained) highway 357 through Manderfield use original sections of U.S. 91. There are also sections of highway 357 that used to be U.S. 91 from before exit 129 through Cove Fort in Millard County, intersecting both I-70 and later I-15 north of the I-70/I-15 interchange. Some frontage roads along I-15 may be parts of U.S. 91. The route picks up again at exit 146 on SR 133 through Kanosh and Meadow before being supplanted by frontage roads and I-15. SR-64 through Holden is also a piece of U.S. 91 before running concurrently with U.S. 50 until it merges again with I-15.
From Anderson Junction to Springville, U.S. 91 is in the shadows of I-15. At places the old pavement is visible off to the side of the freeway; at others I-15 was paved over the old U.S. 91.
U.S. 91 first joined U.S. 89 in Springville at one of the oldest grade-separated interchanges in Utah (this portion is now SR 51).  From this junction to Logan the two highways ran mostly concurrent. This portion is mostly intact as U.S. 89.
U.S. 91 did have a unique route between Farmington and Ogden and where U.S. 89 and 91 split and then rejoined on what is now State Route 126 and State Route 26. These highways form the Main Street through Kaysville and Layton; State Street from the Layton- Clearfield border to Main Street in Clearfield though the city of Sunset turning into 1900 West at the Sunset- Roy border, then following 1900 W to a junction with Riverdale Road in Roy, then following Riverdale Road through Riverdale, and then meeting back up with U.S. 89 at Washington Blvd in Ogden.
U.S. 91 formerly entered Idaho Falls along Yellowstone Highway, and returned to the course of modern I-15 north of town. The highway then proceeded north toward Monida Pass where the highway entered Montana. In some small towns streets used by the route are called "Old Highway 91". 
In Montana, the highway's alignment slightly changed with the construction of the Clark Canyon Dam.
Starting just south of Dillon, I-15 was built on a new alignment that mostly parallels US 91 and crosses over it several times. In Dillon, the former route of the highway is now known as Atlantic Street and Montana Street.  The highway formerly joined with U.S. Route 10 between Rocker and downtown Butte and took a northeasterly path to Elk Park Pass before it was rerouted to its current alignment east of Butte in 1980 (today's southbound lanes of I-15). It formed the main street of Boulder before I-15 was completed, and was routed through Helena along what is now Montana Secondary Highway 518 and Montana Secondary Highway 229.
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.
  
|Utah||Box Elder||Brigham City||0.000||0.000||I-15 / I-84 – Ogden, Tremonton|
|1.965||3.162||US-89 south / SR-13 north – Brigham City, Willard||South end of US 89 overlap|
|3.920||6.309||SR-90 west – Brigham City||Grade separated interchange; westbound exit and eastbound entrance|
|Mantua||5.601||9.014||Mantua Road||Grade separated interchange; eastbound exit and westbound entrance|
|Cache||Wellsville||16.939||27.261||SR-23 north – Mendon|
|19.177||30.862||SR-101 – Hyrum|
|Logan||23.769||38.252||SR-252 north (1000 West)|
|26.651||42.891||SR-165 south – Hyrum, Nibley|
|26.886||43.269||SR-30 west – Tremonton|
|27.148||43.690||US-89 north – Garden City, Utah State University, Bear Lake||North end of US 89 overlap|
|North Logan||29.819||47.989||SR-252 south (2500 North)|
|Smithfield||34.021||54.751||SR-218 west – Newton|
|Richmond||40.004||64.380||SR-142 west – Trenton|
|Lewiston||43.694||70.319||SR-61 west – Lewiston|
|Utah/Idaho state line|
|Idaho||Franklin||Preston||8.389||13.501||SH 36 west – Weston||South end of SH-36 overlap|
|9.265||14.911||SH 34 north / SH 36 east – Montpelier, Soda Springs||North end of SH-36 overlap|
|I-15 – Pocatello, Malad||South end of I-15 overlap (mileposts begin reflecting distance along I-15), exit 36.|
|||US 91 overlaps with I-15 (exits 36–67)|
|I-15 / I-15 Business Loop / US 30||North end of I-15 overlap (mileposts end reflecting distance along I-15), exit 67;|
South end of I-15 Bus. overlap (mileposts begin reflecting distance along I-15 Bus.)
|3.442||5.539||US 30 west||North end of US 30 overlap|
|I-15 Business Loop||North end of I-15 Bus. overlap (mileposts end reflecting distance along I-15 Bus.)|
|Chubbuck||80.020||128.780||I-86 – Twin Falls, Boise||I-86 exit 61.|
|I-15 – Pocatello, Idaho Falls||I-15 exit 89; south end of I-15 Bus. overlap (mileposts begin reflecting distance along I-15 Bus.)|
|I-15 Business Loop – Pocatello, Idaho Falls||North end of I-15 Bus. overlap (mileposts end reflecting distance along I-15 Bus.)|
|York Road/Old I-15 Bus.||Mileposts begin reflecting distance along old I-15 Bus. alignment|
|Idaho Falls||4.526||7.284||I-15 Business Loop / US 26 – Idaho Falls||Mileposts end reflecting distance along old I-15 Bus. alignment|
|1.000 mi = 1.609 km; 1.000 km = 0.621 mi|
- United States System of Highways Adopted for Uniform Marking by the American Association of State Highway Officials (Map). 1:7,000,000. Cartography by U.S. Geological Survey. Bureau of Public Roads. November 11, 1926. OCLC 32889555. Retrieved November 7, 2013.
- Sum of Utah and Idaho mileage logs in the Major intersections section of this article
- "Highway Resolution route 91". Utah Department of Transportation. Retrieved 2008-03-08.
- "Utah Code". State of Utah. Archived from the original on 2008-07-11. Retrieved 2007-12-11.
- Road Atlas (hosted by Broer Map Library) (Map). H.M. Gousha Company. 1967. p. 42. Retrieved 2008-01-27.
- Utah Road and Recreation Atlas (Map) (2002 ed.). 1:250000. Benchmark Maps. 2002. p. 80. § G3. ISBN 0-929591-74-7.
- "Google Maps, northern Idaho". Google Maps. Retrieved 2008-03-09.
- "Google Maps, Dillon Montana". Google Maps. Retrieved 2008-03-09.
- "Highway Reference Information". Utah Department of Transportation. Retrieved 2011-09-19.
- "Idaho Transportation Department Milepost Log State Highway System: US 91 Main Route" (PDF). Idaho Transportation Department. 2011-08-26. Retrieved 2012-06-10.
- "Idaho Transportation Department Milepost Log State Highway System: Interstate 15 Main Route" (PDF). Idaho Transportation Department. 2011-08-26. Retrieved 2012-06-10.
|Wikimedia Commons has media related to U.S. Route 91.|
|Browse numbered routes|
|← SR 90||AZ||SR 92 →|
|← SR-90||UT||SR-92 →|
|← US 89||ID||US 93 →|
|← I-90||MT||US 93 →|