U.S. Route 91 partially coincident with U.S. Route 89 in Utah
Route description
Utah
US 91 begins at Brigham City. The highway was originally routed through Brigham City, but today runs on a southern bypass (locally designated 1100 South). The original routing is now State Route 90. In Brigham City, US 89 becomes concurrent with US 91. 
After leaving Brigham City, the highway begins to climb the Wellsville Mountains. The entire grade across these mountains is colloquially called "Sardine Canyon". However, Sardine Canyon is the canyon originally used by the first settlers to mark a trail to Cache Valley, so named as they used the litter from their lunch of canned sardines to mark the trail. A new route was constructed in the 1950s that uses the same mountain pass, Sardine Summit, as the old road, however follows different canyons down from the summit. 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.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. 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. Logan is the largest city in the valley. 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. Both the Utah and Idaho portions of Cache Valley are part of the Logan metropolitan area.
The Utah section of US 91 is defined at Utah Code Annotated § 72-4-115(1). In addition, the segment south of the SR-142 intersection in Richmond is included in the National Highway System.Utah
From the State Line to Saint George US 91 followed a semi-circular route that given the name Old Dixie Highway 91 in 2023. Washington County officials gave the former segment of US 91 the name as part of an effort to preserve the nickname Utah's Dixie for this region of Utah, co-inciding with the removal of the name Dixie from Utah Tech University. 
Dixie Highway 91 continues to Santa Clara and followed what is now SR-8 and SR-18 to St. George. Where SR-18 intersects St. George Blvd, US 91 then continued east on St. George Blvd. (SR-34) until it intersected Red Cliffs Rd. From there, US 91 went in a north-east direction, with Red Cliffs becoming Telegraph St. in Washington. US 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.US 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 the road to New Harmony (old SR-144), at I-15 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 US 91 continues to parallel I-15 to Summit. 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 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 road running north through Manderfield use original sections of US 91. 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. Some frontage roads along I-15 may be parts of US 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-99 through Fillmore and SR-64 through Holden are also pieces of US 91 before running concurrently with US 50 until it merges again with I-15.
The original alignment of US 91 also diverges from I-15 through most of Juab County. 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. North of Nephi, it follows what is now Mona Road, passing through Mona, before rejoining the modern I-15 alignment near Rocky Ridge. 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. In Spanish Fork, it ran north on Main Street (now SR-156, east on 800 North, and northeast on what is now SR-51 to Springville.
In Springville US 91 first joined US 89 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 US 89.
However, US 91 diverged from US 89 between Farmington and Ogden. US 91 split away on what is now State Route 273, 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 West to a junction with Riverdale Road in Roy, then following Riverdale Road through Riverdale, and then meeting back up with US 89 at Washington Blvd in Ogden.
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