Interstate 15 (I-15) is a freeway that begins near the United States-Mexico border in San Diego County and runs north to Alberta, Canada. The freeway runs through California, Nevada, Arizona, Utah, Idaho, and Montana. The southern terminus of I-15 is at an intersection with I-8 and State Route 15 in San Diego County; its northern terminus is in Sweet Grass, Montana, on the US border with Canada (where it becomes Alberta Highway 4). From one end to the other, I-15 is 1,433 miles long.
Interstate 15 runs through a variety of terrain along its entire length. It connects the southwest of the desert with the Intermountain West and at the same time provides an important link between Canada’s inland areas, a number of transcontinental corridors, southern California and Mexico. Those traveling west on I-40, I-70, and I-80 can easily switch to I-15 south for connections to Las Vegas, Los Angeles, San Diego, and Mexico.
High priority corridor
Since its construction, the I-15 has served as a long-haul route for North American trade. Now she’s officially chartered for it – from its intersection with I-515 in Las Vegas to the Canadian border, I-15 is part of High Priority Corridor 26, also known as the CANAMEX Corridor.
In addition, I-15 from San Diego to Mesquite, Nevada via the Inland Empire and Las Vegas is part of High Priority Corridor 16/70 known as the Economic Lifeline Corridor. Between Great Falls and Sweetgrass, I-15 is part of High Priority Corridor 27, also known as Camino Real.
An early interstate highway photo in California showing an exit to I-15. (Photo: interstate-guide.com)
Story from I-15-15
Interstate 15 was built along the former US Route 91 (US 91). After the I-15 was essentially completed, the US 91 was decommissioned (with the exception of a section in northern Utah / southern Idaho, where the I-15 followed the route of the former US Route 191).
Originally the I-15 had two routes (I-15W and I-15E). In California, I-15 had an eastern branch that bypassed San Bernardino and was designated Interstate 15E. However, I-15E has been renumbered and is now called Interstate 215 or I-215. In Idaho, I-15 had a western branch near Pocatello that connected I-15 and then I-80N. This freeway was known as I-15W. However, it has also been renumbered and is now I-86.
An overhead view of the traffic on I-15 in San Diego County.
(Photo: rebuild SoCal partnership)
In Southern California it was originally proposed that I-15 only be built to an intersection with I-10 near San Bernardino to the south. However, an extension of I-15 south to San Diego was included in the 1968 legislation that expanded the interstate highway system by 1,500 miles.
For much of its route, the I-15 replaced US 91 (and US 466), which used to be the most direct route from Los Angeles to Las Vegas. I-15 meets old US 91 (and US 66-395) in Devore, California; the three routes have been replaced by the I-15 over the Cajon Pass. US 395 splits at Hesperia and old US 66 splits east at Barstow, but I-15 runs parallel to US 91 for the remainder of its northern route. When the I-15 through California, Nevada, Arizona and Utah was completed, the US 91 was demoted to the status of a front road and then decommissioned.
A top view of a number of I-15 flyovers. (Photo: rebuild SoCal partnership)
I-15 in California
Most of the I-15 that runs through California was built in the 1970s. However, sections were built across the Mojave Desert as early as 1961.
Interstate 15 is 287 miles long in California. On many sections of the highway, high-voltage lines – almost all of them starting from Hoover Dam – run parallel to the highway.
Trucks drive past Las Vegas along I-15 (Photo: Nevada Department of Transportation)
I-15 in Nevada
Within Nevada, I-15 is entirely in Clark County and is 123.8 miles long. It starts in Primm and runs through Las Vegas along the Las Vegas Strip Corridor. I-15 crosses the Arizona border in Mesquite.
The I-15 bridge over the Virgin River in Arizona. (Photo by Arizona Department of Transportation)
I-15 in Arizona
I-15 in Arizona is only 29.4 miles long and runs through the northwest corner of the state. There is only one big exit. However, its short length in Arizona includes a section where it runs between the narrow walls of the Virgin River Gorge.
A well-known scene across America. This is road construction on I-15 in northern Utah.
(Photo: standard.net)
I-15 in Utah
I-15 runs 401 miles through Utah and serves as the main north-south connection for the state. The interstate generally follows the old US 91 line from St. George to Brigham City. On its route, I-15 passes St. George, Cedar City, and eventually most of the state’s major cities and suburbs, including Provo, Orem, Sandy, Salt Lake City, Layton, and Ogden. It intersects I-70, merges with I-80 for about 3 miles from South Salt Lake to west of downtown Salt Lake City, and also joins I-84 from Ogden to Tremonton.
Entrances to I-15 in Provo, Utah. (Photo: universum.byu.edu)
For almost its entire length through Utah, I-15 runs parallel to the western edge of the almost continuous Wasatch Mountain Range. The only exceptions are north of Cove Fort and when it runs between Cedar City and St. George, a transition zone with great elevation and climate changes. This is where the eastern Great Basin, the Colorado Plateau and the Mojave Desert meet.
Along I-15 in Idaho. (Photo: Idaho Transportation Department)
I-15 in Idaho
Interstate 15 crosses Idaho’s Oneida County. It runs through Pocatello, Blackfoot and Idaho Falls where it crosses I-86. It runs 196 miles through Idaho and reaches Montana at the Monida Pass.
Interstate 15 in Montana. (Photo: interstate-guide.com)
I-15 in Montana
The length of Interstate 15 in Montana is 396 miles and passes through the major cities of Butte, Helena, and Great Falls. Along its length it intersects with I-90, I-115 and I-315. At Sweet Grass, I-15 ends at the Canadian border (province of Alberta).
Growth along I-15
For the past 50 years, California, Nevada, and Utah have consistently been among the fastest growing areas in the country. Population and commuter traffic have grown significantly along the I-15 route. Current estimates suggest that more than 75% of the population of Utah, 19% of the population of California, and more than 70% of the population of Nevada live in counties where the I-15 is the main freeway.
Traffic on I-15 in Temeculaca, California. The caption says it all. (Photo: local.temeculaca.gov)
The sections of I-15 in Arizona, Idaho and Montana have largely retained their rural long-haul character.
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