Interstate 15 (I-15) is an interstate highway that begins near the border of the United States and Mexico in San Diego County and runs north to Alberta, Canada. The highway passes through California, Nevada, Arizona, Utah, Idaho and Montana. I-15’s southern terminus is at a junction with I-8 and State Route 15 in San Diego County; its northern terminus is in Sweet Grass, Montana, at the U.S. border with Canada (where it becomes Alberta Highway 4). From end to end, I-15 is 1,433 miles long.
Along its length, Interstate 15 passes a wide variety of terrain. It joins the desert Southwest with the Intermountain West, while providing a key link between Canada’s interior, a number of transcontinental corridors, southern California and Mexico. Those traveling westbound on I-40, I-70 and I-80 are able to easily transition onto southbound I-15 to connect to Las Vegas, Los Angeles, San Diego and Mexico.
High Priority Corridor
Since it was built, I-15 has served as a long-haul route for North American commerce. Now it is officially chartered for this purpose – from its junction with I-515 in Las Vegas to the Canadian border, I-15 forms part of High Priority Corridor 26, known also 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, which is known as the Economic Lifeline Corridor. Between Great Falls and Sweetgrass, I-15 is part of High Priority Corridor 27, known also as Camino Real.
Interstate 15 was built along the route of former U.S. Route 91 (US 91). Once I-15 was substantially done, US 91 was decommissioned (except for one section in northern Utah/southern Idaho where I-15 followed the route of former U.S. Route 191).
Originally, I-15 had two routes (I-15W and I-15E). In California, I-15 had an eastern branch that bypassed San Bernardino, which was designated Interstate 15E. However, I-15E was renumbered and is now Interstate 215, or I-215. In Idaho, I-15 had a western branch near Pocatello that connected I-15 and then I-80N). That highway was designated I-15W. However, it too was renumbered and is now I-86.
In southern California, I-15 was originally proposed to be built southward only to a junction with I-10 near San Bernardino. 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, I-15 replaced US 91 (and US 466), which formerly provided the most direct route from Los Angeles to Las Vegas. I-15 meets old US 91 (and US 66-395) at Devore, California; the three routes were replaced by I-15 over Cajon Pass. US 395 splits off at Hesperia and old US 66 splits east at Barstow, but I-15 parallels US 91 for the rest of its northward route. When I-15 was completed through California, Nevada, Arizona and Utah, US 91 was relegated to frontage road status and then decommissioned.
I-15 in California
The majority of I-15 that runs through California was built during the 1970s. However, sections across the Mojave Desert were built as early as 1961.
Interstate 15 is 287 miles long in California. Along many sections of the highway, high-voltage power lines – almost all originating at Hoover Dam – parallel the highway.
I-15 in Nevada
Within Nevada, I-15 is entirely in Clark County and its length is 123.8 miles. It begins in Primm and continues through Las Vegas along the Las Vegas Strip corridor. I-15 crosses the border with Arizona in Mesquite.
I-15 in Arizona
I-15 is only 29.4 miles long in Arizona, running through the northwestern corner of the state. There is only one major exit. However, its short length in Arizona includes a section where it runs between the narrow walls of the Virgin River Gorge.
I-15 in Utah
I-15 continues through Utah for 401 miles and serves as the main north-south connection for the state. The interstate generally follows the old alignment of US 91 from St. George to Brigham City. Along its route I-15 passes St. George, Cedar City, and eventually most of the major cities and suburbs of the state, 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 just west of downtown Salt Lake City, and also merges with I-84 from Ogden to Tremonton.
For nearly its entire length through Utah I-15 parallels the western edge of the nearly continuous Wasatch Range of mountains. The only exceptions are north of Cove Fort and when it passes between Cedar City and St. George, which is a transition zone of major changes in elevation and climate. That area is where the eastern Great Basin, Colorado Plateau and Mojave Desert come together.
I-15 in Idaho
Interstate 15 crosses into Idaho’s Oneida County. It runs through Pocatello, Blackfoot and Idaho Falls, where it intersects I-86. It runs through Idaho for 196 miles and reaches Montana at Monida Pass.
I-15 in Montana
Interstate 15’s length in Montana is 396 miles and runs through the state’s 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 terminates at the Canadian border (the province of Alberta).
Growth along I-15
Over the last 50 years, California, Nevada and Utah have consistently ranked among the fastest-growing areas of the nation. Population and commuter traffic have grown substantially along I-15’s route. Current estimates are that more than 75% of Utah’s population, 19% of California’s population and more than 70% of Nevada’s population live in counties in which I-15 is the primary interstate highway.
However, the sections of I-15 in Arizona, Idaho and Montana have largely maintained their rural, long-haul nature.