Joining the desert southwest with the intermountain west, Interstate 15 provides a major link between the interior of Canada, several transcontinental corridors, Southern California, and Mexico. Travelers westbound on I-40, I-70 and I-80 may easily transition onto southbound I-15 to connect to Las Vegas, Los Angeles, San Diego and International Destinations in Mexico. Between these destinations, I-15 is an extremely busy highway, frequently backing up on holiday weekends in the Mojave Desert.
Originating within Mission Valley in San Diego, Interstate 15 leads north through Murphy Canyon to Miramar Naval Air Station and Kearny Mesa as a busy commuter route to Poway, Rancho Bernardo and Escondido. This stretch includes HOT lanes (Interstate 15 Express Lanes) running between the north and southbound main lines.
Continuing through the North County area of San Diego, I-15 reaches Riverside County and Temecula Valley. There the route splits with Interstate 215 (former I-15E) at Murrieta. I-215 ventures north through Menifee Valley to Perris and San Bernardino while I-15 stays west through Temecula Valley to Lake Elsinore, El Cerrito and Corona. Interstate 15 turns northeastward at Rancho Cucamonga to reconvene with I-215 at Cajon Canyon and Devore.
The freeway meanders north through Cajon Canyon between the San Gabriel and San Bernardino Mountains. Historic U.S. 66 parallels this stretch, with portions still drivable to Cajon. A wide roadway split ensues north of Alray to Cajon Summit, with I-15 straightening out northeast to Apple Valley and Victorville.
Long straight aways become the norm as Interstate 15 advances northeast to Barstow and across the Mojave Desert. Some elevation changes remain along the route, such as where the freeway drops into Cronise Valley or passes between the Soda Mountains. Travelers traverse the dry bed of Ivanpah Lake before crossing the Clark County line into Nevada.
Heading more northerly, Interstate 15 progresses through Ivanpah Valley by the dry Roach Lake between Primm on the state line and Jean. Heavy traffic is common along this stretch during weekends with traffic running between Los Angeles and Las Vegas. I-15 curves northward again into Paradise Valley and the south suburbs of Las Vegas. The freeway increases in both traffic and capacity, eventually reaching the Las Vegas Strip while en route to Downtown.
Beyond North Las Vegas, I-15 turns more easterly again by Nellis Air Force Base and Las Vegas Motor Speedway to vacate the Las Vegas metropolitan area toward Dry Lake Valley, Mormon Mesa and Mesquite near the Arizona state line. A short but scenic stretch through Arizona ensues, with I-15 winding through Virgin River Gorge between the Beaver Dam and Virgin Mountains.
Interstate 15 enters Utah at Big Valley and quickly enters the growing city of St. George south of Downtown. St. George includes a freeway loop (Utah State Route 7) encircling newer suburban areas and St. George Regional Airport (SGU) northeast to SR 9 in Hurricane. Northeast from there, I-15 parallels the Hurricane Cliffs to Cedar City and Parowan. The route leaves Parowan Valley to kink northward through Nevershine Hollow east of the South Hills.
Smaller cities in Utah along I-15 include Beaver, Fillmore, Holden, Scipio and Nephi as the freeway varies in terrain through valley, canyon and hillsides. North of Juab Valley, Interstate 15 lowers into an agricultural area through a series of cities starting with Santaquin and culminating with Provo and Orem. The freeway stays in urban or suburban settings for the majority of the drive northward into Salt Lake City and Ogden. Overlaps along the route include ones with I-80 and I-84.
I-15 and I-84 separate at Tremonton through Bear River Valley, with I-15 staying east of the West Hills through Malad Valley into Idaho. The freeway angles northeast across the Bannock Range into Marsh Valley ahead of Pocatello. Advancing from there, I-15 parallels the Snake River through mostly agricultural areas to Idaho Falls. Sparsely populated lands lie north from there as the freeway extends to Targhee National Forest and the ascent to the Continental Divide and Montana.
The bulk of the route through Montana directly overlaid U.S. 91. Interstate 15 varies between mountainous terrain and plateaus with farmland to Silver Bow. An eight-mile overlap takes I-15 along side Interstate 90 east to Butte. The freeway resumes a northern heading on the east side of the city through Deerlodge National Forest.
A winding stretch takes I-15 east to Boulder and Boulder Valley, where the route straightens out and again turns north to the capital city of Helena. Interstate 15 follows the course of the Missouri River from east of Wolf Creek to Chestnut Valley and Great Falls. There hidden I-315 spurs into the city. A northwestern turn then takes I-15 from Great Falls to Vaughn. The remainder of the route is rural, as the freeway traverses Teton Ridge en route to Shelby, Sweetgrass and the Canadian Border.
Interstate 15 from San Diego to Mesquite, Nevada via the Inland Empire and Las Vegas is part of High Priority Corridor 16 and 70: Economic Lifeline Corridor. The section of I-15 from Las Vegas to Sweetgrass, Montana, is part of High Priority Corridor 26: CANAMEX Corridor. Between Great Falls and Sweetgrass, the freeway is also part of High Priority Corridor 27: Camino Real.
Between San Diego and Temecula, Interstate 15 replaced U.S. 395. U.S. 395 largely still exists today as a busy expressway route from Hesperia north to Mammoth Lakes, Reno and Spokane, Washington. Remnant original sections of U.S. 395 in San Diego County are readily apparent along the Cabrillo Freeway (now California State Route 163), Kearny Villa Road, Pomerado Road, Old Highway 395 and Rainbow-Temecula Road. At Temecula, Interstate 15 follows old SR 71 to Corona and SR 31 northward to Devore (I-215).
For much of its course, Interstate 15 replaced U.S. 91 (and U.S. 466), which used to provide the most direct route from Los Angeles to Las Vegas. I-15 meets old U.S. 91 (and U.S. 66-395) at Devore; the three routes were replaced by Interstate 15 over Cajon Pass. While U.S. 395 splits off at Hesperia and old U.S. 66 splits east at Barstow, I-15 parallels U.S. 91 for the remainder of its route northward. With the completion of the freeway through California, Nevada, Arizona, and Utah, U.S. 91 was relegated to frontage road status. Since western states generally do not maintain frontage or parallel service roads as state highways, U.S. 91 was decommissioned.
Sections of old U.S. 91 are appear periodically along Interstate 15, both along the business loops through small and mid-sized cities and as frontage roads. One large extant segment of Old U.S. 91 bypasses Virgin River Gorge to the west in the vicinity of Arizona and Utah. For the brief portion of U.S. 91 between Ogden and Twin Falls, Interstate 15 actually replaced an earlier version of U.S. 191. As a result, this short section is all that remains of U.S. 91; U.S. 91 has also been completely decommissioned in Montana.
Prior to 1969, Interstate 15 ended at I-10 in San Bernardino, following what is now the northern extent of I-215. Photo scanned from the California Department of Public Works Biennial Report by Joel Windmiller.
Through Southern California, I-15 was originally proposed southward only to Interstate 10 in the San Bernardino vicinity. Extension of the route south to San Diego was included in the 1,500 mile Interstate system expansion legislation of 1968. This was confirmed by the American Association of State Highway Officials (AASHO) on June 24, 1969. The extension superseded the southernmost extent of U.S. 395.
The bulk of Interstate 15 through California was constructed in the 1970s, with exceptions being across the Mojave Desert, where the freeway was constructed as early as 1961 from East Baker (Exit 248) to Cima Road (Exit 272). Through San Bernardino and Rancho Cucamonga, the route of I-215 defaulted as the mainline of I-15 until the section from SR 91 to SR 60 opened to traffic on February 28, 1989. A surface route, SR 31, was labeled as TEMP I-15 until that time.
Interstate 15 and I-15E
The initial planned alignment for I-15 took the route directly south from the junction with I-10 at Colton on the ridge between Reche Canyon and Pigeon Pass and then descend from the Box Springs Mountains near the present eastbound split of Interstate 215 and SR 60 to overtake U.S. 395 south from March AFB. This proposal was short lived due to the presence of unstable compressed sandstone. Instead the freeway was rerouted to the west through less rocky terrain. Diverging from U.S. 395 between Grand Terrace and Highgrove, the proposed route shifted southeast of Iowa Avenue to follow a Santa Fe Railroad line along the base of the Box Springs Mountains to the east of the University of California, Riverside (UCR) campus. The alignment converged with U.S. 395 and SR 60 juts north of the present route split of I-215 and SR 60 in Moreno Valley. Potential impacts to the UCR community led to the cancellation of this proposal by 1969 as well.1
Having been bypassed by the new alignment of Interstate 15, the cities of Riverside and San Bernardino voiced concerns citing negative impacts to development plans. Officials countered with a plan to renumber the western route as Interstate 15W while recommissioning the U.S. 395 alignment (which was redesignated as SR 194) as I-15E, despite it not being an Interstate highway. The American Association of State Highway and Transportation Officials (AASHTO) provided the option to retain Interstate status on U.S. 395 through San Bernardino until the Ontario-Devore segment of new I-15 could be completed. They approved I-15E as a temporary designation for the route southeast from Devore to I-10 and it was posted by late 1972.1
These actions benefited San Bernardino but still omitted Riverside. Objections from the city for being left off the Interstate system coincided with 1973 legislation passed that introduced chargeable/not chargeable mileage for Interstate construction. This opened the door extending I-15E south along U.S. 395 to the new I-15 route at Murrieta as a nonchargeable route. With backing from the local Congressional delegation, I-15E was signed in place of U.S. 395 south to the SR 60 split at Moreno Valley by Spring 1974. The remainder of the route south to Murrieta was posted as Temporary I-15E by mid 1975, and SR 71 south from there as Temporary I-15.1
District 8 planners devised an alternate route in 1970 along the proposed SR 31 freeway. With significantly less impacts, the alignment extended north from Corona through eastern Ontario and northeast around the Lower Lytle Creek Range to meet Interstate 15 at Devore in Cajon Canyon. The reroute of I-15 to the west shaved about 23 minutes off the Los Angeles to Las Vegas routing by eliminating the loop east into San Bernandino. The south end of SR 31 also tied into the proposed SR 71 freeway south of Corona, ten miles of which was already constructed to Lake Elsinore. Overall the western alignment of I-15 was ten miles longer than the planned course south of San Bernardino, but was otherwise well received. The Division of Highways pursued the SR 31 option by mid 1970, leading to Federal Highway Administration (FHWA) concurrence by Fall of that year.1
FHWA initially raised an object to the new SR 31 alignment for I-15 related to the preexisting Interstate 15 southeast from Devore to I-10 at Sen Bernandino. It was thought that the section previously received chargeable Interstate funds, but a perusal of records indicated that the segment between I-10 and 5th Street (U.S. 66) commenced construction in early 1956, a year prior to the first distribution of Interstate funds. Only the segment north of there was part of chargeable construction. Negotiations between the state and FHWA led to an agreement where California would wave any rights to Interstate maintenance or construction funds for the old I-15 segment between Devore and I-10 in exchange for full funding of the new alignment using the SR 31 and SR 71 corridor. Construction followed in Spring 1971 on the systems interchange at Devore and initial alignment leading southwest.1