Interstate 15

Interstate 15

Route information
Length:	1,433.52 mi^[1] (2,307.03 km)
Existed:	1957 – present
Major junctions
South end:	I-8 / SR 15 in San Diego, CA
	I-10 in Ontario, CA I-40 in Barstow, CA US 95 in Las Vegas, NV I-70 near Cove Fort, UT I-80 in Salt Lake City, UT I-90 near Butte, MT
North end:	Hwy 4 at Canadian border near Sweetgrass, MT
Highway system
Main route of the Interstate Highway System Main • Auxiliary • Business

Interstate 15 (I-15) is the fourth-longest north–south transcontinental Interstate Highway in the United States, traveling through the states of California, Nevada, Arizona, Utah, Idaho, and Montana. The Idaho, Utah and Arizona portions have been officially designated the 'Veterans Memorial Highway'.^[2]

Since I-15's inception, it has served as a long-haul route for North American commerce. It is now officially chartered for this purpose. From the junction of Interstate 515 in Las Vegas to the Canadian border I-15 forms part of the CANAMEX Corridor, a High Priority Corridor, as a result of the North American Free Trade Agreement.^[3]

Local portions were built to connect the Inland Empire with San Diego in California, facilitate tourism access to Las Vegas, interconnect all of the Metropolitan Statistical Areas in Utah except Logan, and provide freeway bypasses for Pocatello, Idaho Falls, and Great Falls.

Since I-15's initial construction, California, Nevada, and Utah have consistently ranked in the fastest growing areas of the United States. As a result, the I-15 corridor has substantially increased in population and commuter traffic has increased the traffic burden on the freeway. Current population estimates are that more than 75 percent of Utah's population^[4], 19 percent of California's population, and more than 70 percent of Nevada's population^[5] live in counties where Interstate 15 is the primary transportation corridor. Similarly, in California, I-15 is seeing more commuter traffic due to the growth of the Mojave Desert communities of Hesperia and Barstow, California. In all of these states, I-15 has recently been or is currently in the process of being upgraded to increase capacity.^[6]^[7]^[8] The Arizona, Idaho and Montana portions have retained their rural, long-haul character. Although Arizona has also grown substantially since I-15's inception, I-15 only serves an isolated corner of the state.

Due to this rapid area growth, the I-15 corridor is the focus of several mass transit projects. The Las Vegas Monorail, FrontRunner commuter rail system and TRAX light rail in Salt Lake City are mass transit lines loosely parallel to I-15 that are now in operation. Between Los Angeles and Las Vegas has long been proposed as a maglev train route; in 2004 the California-Nevada Interstate Maglev project held public meetings on the plan.^[9]

Route description

Lengths
	mi	km
CA	287	462
NV	124	200
AZ	29	47
UT	401	645
ID	196	315
MT	396	637
Total	1,433	2,307

This highway's southern terminus is in San Diego, California at Interstate 5, 13 miles (21 km) north of the international border with Mexico. The northern terminus is in Sweetgrass, Montana at the international border between the United States and Canada, where it becomes Alberta Highway 4. It is 1,433 miles (2,306 km) long from San Diego to Sweetgrass.

California

North of its junction with the Riverside Freeway, State Route 91, in the Inland Empire near Corona, the route roughly follows the former routes of U.S. Highway 91 and U.S. Highway 395. North of Devore, the highway follows the approximate alignment of historic U.S. Highway 66 along with U.S. 91 and 395. U.S. 395 breaks away at Hesperia and the route continues the approximate route of co-signed 66 and 91 until about the Mojave River, 35 miles (56 km) to the north. At that point, I-15 follows the old route of U.S. 91 exclusively. For many parts of the highway, high-voltage power lines, like Path 46 and Path 27, almost all from the Hoover Dam, follow the freeway. Many of these link distant power stations to the Los Angeles metropolitan area.

Northbound I-15 makes a steep descent from the Mountain Pass, in California into the Ivanpah Valley and the site of Ivanpah Dry Lake. In the middle distance, the casinos of Primm straddle the freeway right at the Nevada border, while those of Jean are further off, to the left; Las Vegas is immediately on the far side of the hills on the horizon.

The southern starting point of Interstate 15 was in 1947 planned to be in San Bernardino, at the interchange with the San Bernardino Freeway (then US 70/99), now I-10). This was logical as I-15 was following the old alignment of the historic Route 66 which passed through San Bernardino. The segment was completed accordingly.

But in 1964, legislation was later passed to extend the interstate to San Diego. But instead of extending the existing freeway from the I-10 interchange south, the California Department of Transportation drew a new segment in Devore that 'branched' off of the original alignment and bypassed San Bernardino altogether. This segment's alignment is generally northeast to southwest for about 15 miles (24 km). Then, in Fontana/Rancho Cucamonga, its directional alignment shifts to north–south where it eventually junctions with Interstate 10 (about 15 miles (24 km) west of the original interchange in San Bernardino). The segment that had been built from Devore to San Bernardino was retained as an interstate, but was re-numbered as Interstate 215. Note that during the construction of I-15's present alignment, and for some time afterwards, I-215 was numbered as I-15E, and its actual mileage would begin at Interstate 10. I-15 runs for a total of 287 miles (462 km) in California.

Nevada

I-15 passes through the Virgin River Gorge, Arizona revealing scenic reddish brown cliffs.

Interstate 15 begins in Primm, continues through Las Vegas along the Las Vegas Strip corridor. Then the interstate crosses the border with Arizona in Mesquite. The whole interstate in Nevada runs entirely in Clark County, for a distance of 123.8 miles (199 km).

Arizona

I-15 just clips the northwestern corner of Arizona with a total length of 29.4 miles (47 km).^[10] The stretch is separated from the rest of the state and has one major exit, at Beaver Dam/Littlefield, Arizona. It includes a spectacular section where the road twists between the narrow walls of the Virgin River Gorge.

Utah

I-15 in Salt Lake City

I-15 continues through Utah for 401 miles (645 km). It is the main north–south connection for the state. The highway approximately follows the old alignment of U.S. Highway 91 from St. George to Brigham City. The highway passes through the fast-growing Dixie region, which includes St. George, Cedar City, and eventually most of the major cities and suburbs along the Wasatch Front, including Provo, Orem, Sandy, West Jordan, Salt Lake City, Layton, and Ogden. Near Cove Fort, Interstate 70 begins its journey eastward across the country. The interstate merges with I-80 for about 3 miles (5 km) from South Salt Lake to just west of Downtown Salt Lake City and also merges with Interstate 84 from Ogden to Tremonton. Along nearly its entire length through the state, I-15 winds its way along the western edge of a nearly continuous range of mountains (the Wasatch Range in the northern half of the state). The only exceptions are when it passes through the mountains south of Cedar City and again north of Cove Fort.

Idaho

Main article: Interstate 15 in Idaho

I-15 passes through Idaho for 196 miles (315 km). Interstate 15 crosses the Utah state line in Oneida County. The highway runs through Pocatello, Blackfoot, and Idaho Falls, intersecting with Interstate 86. The last county in Idaho that Interstate 15 passes through is Clark County. Finally, the interstate reaches the Montana state line at Monida Pass.

Montana

Main article: Interstate 15 in Montana

Interstate 15 continues onward through 396 miles (637 km) of Montana through the cities of Butte, Helena and Great Falls, intersecting with Interstate 90, Interstate 115 and Interstate 315. At Sweetgrass, I-15 terminates upon crossing the international border into Alberta, Canada; however, I-15 signage is present on Alberta Highway 4 southbound from Lethbridge to the U.S.-Canada Border.

History

Interstate 15 was constructed along the route of U.S. Route 91. Once Interstate 15 was relatively intact U.S. 91 was decommissioned, except for one part in Northern Utah/Southern Idaho where Interstate 15 instead followed the route of former U.S. Route 191.

Interstate 15 originally had two suffixed routes. In California, Interstate 15 had an eastern branch bypassing San Bernardino, which was designated Interstate 15E. I-15E was renumbered and is now Interstate 215.^[11] In Idaho, I-15 had a western branch near Pocatello that connected I-15 and I-84 (then I-80N). This highway was designated Interstate 15W. It is now the western I-86.^[12]

Future

Redesignation of SR 15 in San Diego as I-15 might eventually occur if the freeway's interchange with SR 94 is updated to Interstate standards. The interchange currently has left-exits and blind merges, and is due to be updated with a long-awaited widening of SR 94 in 2016. At that time SR 15 could be eliminated and re-signed as Interstate 15.