Cutting a diagonal swath across the intermountain west, Interstate 15 provides a major link between the interior of Canada, several transcontinental east-west corridors, Southern California, and Mexico. Travelers westbound on Interstates 40, 70, and 80 may easily transition to southbound Interstate 15 to connect to Las Vegas, Los Angeles, San Diego, and International Destinations in Mexico. Between these destinations, Interstate 15 is an extremely busy highway, frequently backing up on holiday weekends in the Mojave Desert.
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. Remnant original sections of U.S. 395 in San Diego County are readily apparent along the Cabrillo Freeway (now California 163), Kearny Villa Road, Pomerado Road, Old Highway 395, and Rainbow-Temecula Road. At Temecula, Interstate 15 follows old California 71 to Corona and California 31 northward to Devore (Junction Interstate 215).
For much of its journey, Interstate 15 replaced U.S. 91 (and U.S. 466), which used to provide the most direct route from Los Angeles to Las Vegas. Interstate 15 meets old U.S. 91 (and U.S. 66-395) at Devore; the three routes were replaced by Interstate 15 over the Cajon Pass. While U.S. 395 splits off at Hesperia and old U.S. 66 splits east at Barstow, Interstate 15 roughly parallels U.S. 91 for the remainder of its journey 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 readily visible along Interstate 15, both along the business loops that all small and mid-sized cities and frontage roads. One large extant segment of Old U.S. 91 bypasses the 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 tiny section is all that remains of U.S. 91; U.S. 91 has also been completely decommissioned in Montana.
Interstate 15 through the Virgin River Gorge in Arizona opened to traffic on December 14, 1973. The final section of Interstate 15 to open was the bypass of Plymouth, Utah, which opened to traffic on November 20, 1990.1
Interstate 15 Split Routes
Interstate 15 had two split routes identified in the 1970s; none of these split routes remain today. These routes included (from south to north):
Interstate 15E from Murrieta, California, to Devore, California (now Interstate 215)
Currently, the official southern terminus of Interstate 15 is at its junction with Interstate 8 in San Diego (Mission Valley) near Qualcomm Stadium. For many years, the freeway south of Interstate 8 was incomplete, with California 15 following 40th Street, a city street, through the Mid-City of San Diego. With the completion of the California 15 Freeway in January 2001, Interstate 15 is planned for extension to meet Interstate 5 in Barrio Logan . However, this redesignation has not occurred due to the substandard nature of the California 15/California 94 interchange. This interchange is due to be reconstructed as part of a large widening project along California 94. Plans call for upgrading this interchange, along with the removal of left exits and blind merges, by 2008, but this timeline may be delayed due to the California budget crisis of 2003.
Designated as a high priority corridor in 1995, Interstate 15 has seen significant improvements in major metropolitan areas due to exploding population growth throughout the west. Various construction projects along the corridor in Salt Lake City, St. George, Las Vegas, Barstow, the Inland Empire (Riverside/San Bernardino Counties), and San Diego have already expanded the capacity of Interstate 15. Future plans include a minimum of six lanes from San Diego to Las Vegas, a high-speed mass transit system between Los Angeles and Las Vegas, increased high occupancy vehicle lanes in North San Diego County, light rail in Salt Lake City, and the Legacy Highway relief expressway in greater Salt Lake. These improvements are discussed in detail on the High Priority Corridor 16 and Corridor 26 pages.
In 2001, Interstate 15 reconstruction in the Salt Lake City area was completed. The five year overhaul began in 1997 and entailed $1.59 billion in funds. The project was sought to upgrade the Salt Lake City downtown Interstate system in preparation for the 2002 Olympic Winter Games.
Current Southern Terminus - Interstate 8 - San Diego (Mission Valley), California
Perspective from Interstate 8/Mission Valley (Alvarado) Freeway east
At the base of the towers that support the bridge carrying Interstate 805 high above Interstate 8 and Mission Valley is the first advance sign for Interstate 15 and California 15 along Eastbound Interstate 8. The signage is very clear in delineating that Interstate 15 does not extend south of Interstate 8. This is Exit 7, Junction Interstate 15/California 15 and Fairmount Avenue/Mission Gorge Road/Montezuma Road. This segment of Interstate 8, between Interstate 805 and Interstate 15, is routinely one of the most traveled sections of freeway in the entire city, even surpassing the Interstate 5/805 merge in terms of average daily traffic counts in some years. Photo taken 08/20/04.
As Interstate 8 begins its gradual ascent toward Grossmont Summit, it passes through its busiest segment in the heart of Mission Valley between Interstate 805 and Interstate 15. It is here that transcontinental Interstate 15 leaves the shadows of Qualcomm Stadium (home of the San Diego Chargers) and begins its journey toward the Inland Empire, Mojave Desert, Utah, and Continental Divide. As it journeys north, Interstate 15 slowly narrows from a major, ten to twelve lane freeway into a quieter, four-lane freeway. In fact, some sections of Interstate 15 in Idaho and Montana were not widened to Interstate standards until the 1980s. Meanwhile, California 15 also begins its short southerly trek from this interchange. It will pass through Mid-City San Diego before reaching its terminus at the foot of the 32nd Street Naval Station, adjacent to massive shipyards and the Coronado-San Diego Bay Bridge. This stretch of Interstate 8 can be very hectic, as traffic coming from Interstate 805 must weave out of the auxiliary lanes and into the main lanes, while traffic trying to get to Interstate 15 northbound, California 15 southbound, or Fairmount Avenue/Mission Gorge Road must merge over to the right. The result is a lot of weaving traffic on a very busy highway. Photo taken 08/20/04.
When these overhead signs were installed, California 15 was designated as 40th Street. The green overlay covers up the words 40th St because it is now a freeway south through City Heights toward Barrio Logan. In 1999, the freeway was completed. Photo taken 08/20/04.
Eastbound Interstate 8 reaches Exit 7, Southbound California 15. This used to be 40th Street, but with the completion of the freeway section between Adams Avenue and Interstate 805 in 1999, California 15 does not carry the 40th Street designation. For travelers on Interstate 8, this interchange is much less impressive than the Interstate 8/Interstate 805 interchange and is anticlimatic compared to the bridges that serve as the southern terminus of Interstate 15. The difference in interchange complexity is largely Interstate 805 spans Mission Valley on a very high bridge that necessitates tall ramps and flyovers. Unlike Interstate 805, Interstate 15 enters the valley and crosses the San Diego River on a relatively low level before ascending into Mid-City to the south. Photo taken 08/20/04.
After the ramp to southbound California 15, the next exit along eastbound Interstate 8 is the ramp to Interstate 15 northbound and three city streets (see below). This overhead sign is hanging from a ramp connecting southbound Interstate 15 with Camino del Rio South, the Interstate 8 frontage road. As for the offramp to Interstate 15, it also connects to north-south Fairmount Avenue, a major city street that leads south into the Mid-City community (including City Heights). Mission Gorge Road is a major southwest to northeast route that cuts a path roughly parallel to the San Diego River from the Mission Gorge neighborhood near Interstate 8 all the way to California 52 at Santee. Although not signed, Fairmount Avenue southbound is an excellent route to take to San Diego State University, which may be reached by taking southbound Fairmount Avenue to eastbound Montezuma Road. Photo taken 08/20/04.
Eastbound Interstate 8 reaches Interstate 15/California 15 (Exit 7). Finally the interchange structure for Interstate 15 becomes clear after passing through the Interstate 805 interchange. The first ramp from Eastbound Interstate 8 is for California 15 south (not pictured); the second ramp not only carries traffic for northbound Interstate 15 but also traffic destined for Fairmount Avenue, Mission Gorge Road, and Montezuma Road. In fact, the two-lane exit primarily leads to these city streets; the ramp to northbound Interstate 15 is a quick exit with a loop ramp! Fairmount Avenue leads south into Mid-City, meeting El Cajon Boulevard (unsigned Business Loop I-8) after a brief expressway segment. Mission Gorge Road leads northeast toward Mission Trails Regional Park, one of the focal points of the many parks located in America's Finest City. Montezuma Road leads southeast for an alternate route toward San Diego State University. This is the fastest route to Cox Arena, home to the San Diego State Aztecs basketball teams. For now, this interchange marks the current southern terminus of Interstate 15, which is expected to be extended south to the Interstate 5 interchange once the remainder of California 15 is upgraded to Interstate standards. The next exit is Exit 9, Waring Road. Photo taken 08/20/04.
This mileage sign is hoisted above eastbound after the Interstate 15 interchange. The sign bridge spans across the auxiliary lanes too, and on that portion of the signbridge, signs direct traffic to Mission Gorge Road, Fairmount Avenue, and Eastbound Interstate 8 for traffic from both Interstate 8 and Interstate 15. Two new lanes of traffic merge onto the mainline; one of them exits onto Waring Drive (Exit 9), while the other lane remains with the freeway until 70th Street/Lake Murray Boulevard. Photo taken 08/20/04.
Perspective from Interstate 8/Mission Valley (Alvarado) Freeway west
Now traveling west on Interstate 8, the first appearance of Interstate 15 north is on this mileage sign. Distance is provided to Mission Gorge Road, Fairmount Avenue, Interstate 15, and California 15. This sign was put in place in 1998, right after the transition from westbound Interstate 8 to southbound California 15 was reconstructed to eliminate a blind merge. It was one of the last non-reflective, button-copy signs to be placed on a California freeway. The ramp behind the Waring Road overpass carries Alvarado Canyon Road between Mission Gorge Road and Waring Road. This ramp was built as a mitigation measure as part of the trolley construction process, thus reducing local traffic that used to have to use the freeway to make that connection (prior to 2001). Photo taken 10/12/03.
This stretch of Interstate 8 is among the busiest stretches of freeway in all of San Diego County. At this point (approaches two major arterials and Interstate 15), it is six lanes wide. Photo taken 10/12/03.
Westbound Interstate 8 reaches Exit 8, Mission Gorge Road northeast to Mission Trails Regional Park and Santee and Fairmount Avenue south to City Heights (with access to eastbound Montezuma Road which leads back to San Diego State University, in case you missed the College Avenue Exit 10). Photo taken 10/12/03.
Immediately after the Mission Gorge Road exit, westbound Interstate 8 reaches Exit 7B, Junction Interstate 15 north to the northern reaches of San Diego, Escondido, Temecula, and Riverside/San Bernardino Counties. Interstate 15 does not pass through its control city of Riverside. The following Exit is Exit 7A, Junction California 15 South. Photo taken 10/12/03.
The second ramp in the Interstate 15 interchange connects westbound Interstate 8 with Exit 7A, Junction California 15 south. Although California 15 is slated for upgrading to Interstate 15, that designation must wait until after the interchange between California 15 and California 94 is reconstructed. Photo taken 10/12/03.
Perspective from Mission Gorge Road/Fairmount Avenue north
Traveling north on Fairmount Avenue after the Montezuma Street interchange, the ramp to Interstate 8 and Interstate 15 is signed solely for Interstate 8. However, this ramp also connects to Interstate 15 north and California 15 south. Photo taken 03/27/05.
The transition ramp splits, with the right lane connecting to Interstate 8 east, while the two lanes continue toward Interstate 8 west. Use Interstate 8 west to Interstate 15 north and California 15 south. Photo taken 03/27/05.
This photo shows the signage along the transition ramp from Mission Gorge Road/Fairmount Avenue southbound to Interstate 8 westbound/Interstate 15 northbound/California 15 southbound. This set of signs is not accessible from the mainline. Photo taken 03/27/05.
A roadside sign for Interstate 15 is posted under the Green Line trolley bridge. Note the use of the state name in the Interstate 15 shield. Photo taken 03/27/05.
The transition ramp from Mission Gorge Road/Fairmount Avenue splits: the right two lanes connect to Interstate 15 north, while the left two lanes aim toward Interstate 8 west and California 15 south. As part of the construction of the California 15 freeway through Mid-City along 40th Street, a new ramp was placed to facilitate traffic flow from westbound Interstate 8 onto southbound California 15. Prior to the improvement, traffic had to enter this transition ramp from the left, then cut across three lanes of traffic to get to this right clover exit. This was corrected with the use of a flyover ramp that moved traffic over the three lanes of traffic and directly into the exit only lane for California 15. Photo taken 03/27/05.
Perspective from California 15 north
Now traveling north on California 15, we begin with Exit 6A, Adams Avenue. This is the end of the section of California 15 freeway that opened in 2001; the route between here and Interstate 8 was improved in 2001, but it was a freeway prior to that. The next exit is Junction Interstate 8 and the beginning of Interstate 15. Photo taken 04/09/06.
This mileage sign along northbound California 15 provides the distance for the next four exits: Exit 6B, Junction Interstate 8/Camino del Rio South (three-quarters of a mile); Exits 7A-B, Friars Road (East and West); and Exit 8, Aero Drive. This sign is located on the Adams Avenue overpass. Photo taken 04/09/06.
Interstate 8 leads east toward the eastern end of the city of San Diego, as well as the suburbs of La Mesa and El Cajon. Heading west, Interstate 8 enters Mission Valley, then follows the San Diego River until it terminates in Mission Bay Park at Sunset Cliffs Boulevard/Nimitz Boulevard. As noted in this picture, the overhead sign points to California 15 rather than Interstate 15 continuing north of Interstate 8. Since this particular spot is still California 15, this sign is technically correct. There is no END California 15 shield; the next set of overhead signs display Interstate 15. Photo taken 04/09/06.
This is the third and final northbound California 15 reassurance shield. The freeway descends from City Heights into the Mission Valley community, which follows the San Diego River on an east-west course through the center of San Diego. Photo taken 04/09/06.
Interstate 8 leads east toward San Diego University, La Mesa, Grossmont, and El Cajon before leaving the urban areas via the Cuyamaca and Laguna Mountains of Cleveland National Forest. Heading west, Interstate 8 connects with Interstate 805, California 163, and Interstate 5 before terminating in Mission Bay Park just northeast of Ocean Beach. Photo taken 04/09/06.
Northbound California 15 reaches Exit 6B, Junction Interstate 8 and Camino del Rio South. This is the last northbound reassurance guide signage for California 15. Photo taken 04/09/06.
Now on the transition ramp for Exit 6B, the first exit connects to Camino del Rio South, which is the frontage road for Interstate 8. Continue straight ahead for the connection to Interstate 8 east and west. Photo taken 04/09/06.
After the exit ramp for Camino del Rio South, the ramp further splits into eastbound and westbound Interstate 8. Photo taken 04/09/06.
Now back on mainline California 15, this mileage sign provides the distance to the next four exits: Exits 7A-B, Friars Road (East and West); Exit 8, Aero Drive; and Exit 9, Balboa Avenue (former California 274) and Tierrasanta Boulevard. Photo taken 04/09/06.
The interchange between California 15 and Interstate 8 comes into view. While Interstate 8 takes the bottom level, California 15/Interstate 15 takes the middle deck. Flyover ramps connect southbound Interstate 15 to eastbound Interstate 8 and northbound California 15 to westbound Interstate 8. Photo taken 04/09/06.
As California 15 crosses over Interstate 8 and the ramp from eastbound Interstate 8 merges onto California 15, the signed state route ends and Interstate 15 begins. Photo taken 04/09/06.
Perspective from Interstate 15 north
The first exit along northbound Interstate 15 is Exit 7A, Friars Road. This roadside sign attempts to explain that the first right is for Friars Road east (Exit 7A) followed by the second right for Friars Road west (Exit 7B). Photo taken 04/09/06.
This is the first reassurance signage for northbound Interstate 15. The right two lanes exit only to Friars Road (Exits 7A-B). Photo taken 04/09/06.
Northbound Interstate 15 reaches Exit 7A, Friars Road east. Use Friars Road east to the San Diego Mission, Mission Gorge, and Mission Trails Regional Park via Mission Gorge Road. From here, Interstate 15 travels north through Murphy Canyon, then ascends to Mira Mesa, Scripps Ranch, and Rancho Bernardo before leaving the City of San Diego. Thereafter, the freeway enters Riverside County, then aims northeast toward Las Vegas and St. George, Utah. Photo taken 04/09/06.
Perspective from Interstate 15 south
Southbound Interstate 15 signage for trucks, warning that at that time, California 15 was not suitable for trucks due to the narrowness of 40th Street and the ongoing construction. This signage was removed when the freeway was completed through Mid-City. Photo taken 08/08/04.
Southbound Interstate 15 approaches Junction Interstate 8, one and one-half miles. Photo taken 08/08/04.
Southbound Interstate 15 at Junction Interstate 8. The overhead sign here shows South Interstate 15, but the Interstate designation ceases after the freeway crosses Interstate 8. There is no END Interstate 15 shield present here, and the next overhead reassurance signs in Mid-City indicate California 15. Photo taken 08/08/04.
Southbound Interstate 15 on its bridge over Interstate 8. At this point, Interstate 15 begins to transition into California 15. There are a handful of Interstate 15 shields at the onramps at Camino del Rio South, which parallels Interstate 8 just to the south of the east-west freeway. However, all signage along Adams Avenue and points south only have California 15 shields. Photo taken 08/08/04.
As southbound Interstate 15 crosses over Interstate 8, the freeway changes into California 15 (even though there are no end shields or any state California 15 shields). Photo taken 08/08/04.
California 15 splits into three branches at Junction Interstate 5: South Interstate 5 (National City/Chula Vista control cities), Main Street/Harbor Drive (via 32nd Street), and North Interstate 5 (to Coronado Bridge and Downtown San Diego). The freeway does not end until meeting 32nd Street about one-half mile south of this point along the Main Street/Harbor Drive ramp. It is a matter of semantics as to whether Interstate 15, if extended south, would end at this interchange or at the stoplight in the following photo. Photo taken 08/08/04.
The freeway ends at 32nd Street, a block or so northeast of Harbor Drive. This stoplight marks the end of the continuous freeway from Coutts, Montana south to the Barrio Logan community of San Diego. Someday this light may mark the end of Interstate 15. Photo taken 08/08/04.
Perspective from Interstate 5 south
As Interstate 5 leaves downtown San Diego, it parallels San Diego Bay to the west. As a result, the freeway makes several turns toward the southwest as it approaches Mexico. Just prior to entering National City, Interstate 5 meets California 15 coming from the northeast. The interchange provides access from southbound Interstate 5 to northbound California 15, as Pictured in this picture. Photo taken 05/22/04.
California 15 is the Escondido Freeway, which heads north from along the old Wabash Boulevard Expressway and 40th Street Corridor to join Interstate 15 at its interchange with Interstate 8. California 15 is not designated as Interstate 15 because of the substandard interchange with California 94/Dr. Martin Luther King, Jr. Freeway. Once that interchange is improved to interstate standards, California 15 will become designated as Interstate 15. Photo taken 05/22/04.
Ice plant, commonly used on San Diego's freeways as a landscaping feature to keep rights of way looking green, appears on both sides of Interstate 5 as it approaches Exit 13A, Junction California 15 north. Photo taken 05/22/04.
A new exit number sign features an Interstate 15 shield rather than California 15 shield on it. This sign was posted in Summer 2006. Photo taken 12/03/06.
Southbound Interstate 5 reaches Exit 13A, Junction California 15. The next exit is for Main Street and National City Boulevard. Photo taken 05/22/04.
Perspective from Interstate 5 north
As Interstate 5 follows the historic alignment of U.S. 101 (Montgomery Freeway), it approaches another historic freeway: Wabash Boulevard. Wabash Boulevard was constructed to older freeway standards by the City of San Diego as a route between the Naval Center at 32nd Street and Mid-City. For a time, the freeway was not part of the state highway system. Ultimately, Wabash Boulevard was considered as the southernmost segment of Interstate 15 once it was extended into San Diego County. Wabash Boulevard (known by most locals as "the 15") was upgraded to Interstate standards between Interstate 5 and California 94 through the removal of some on-ramps and addition of longer acceleration ramps. The route is still signed as California 15 but will become part of Interstate 15 once the interchange with California 94 is upgraded to Interstate standards. Photo taken by Andy Field and Casey Cooper (09/01/2003).
Northbound Interstate 5 reaches Exit 12, Main Street/Division Street. This interchange marks the city boundary between San Diego to the north and National City to the south. Officially, California 15 begins in San Diego in the community of Barrio Logan, just shy of National City. The next exit is Junction California 15. Photo taken 05/22/04.
The control city for northbound California 15 and Interstate 15 in San Diego County is Riverside. Riverside is a curious choice for a control city because Interstate 15 does not enter Riverside; Interstate 215 provides access to that city near the California 60/91 interchange. Two lanes exit from northbound Interstate 5 to California 15; many travelers departing from Mexico may break off from Interstate 5 toward inland and desert communities at this juncture. Photo taken 05/22/04.
Northbound Interstate 5 at Junction Northbound California 15. With sweeping connecting ramps and a smooth transition, Casey Cooper opines that "this is the way an Interstate should end." This is actually an incomplete interchange, as canceled California 252 was originally planned to connect to both Interstate 5 and California 15 at this interchange. Known as the Southcrest Freeway, California 252 was killed by local opposition in 1980, but a monument to its passing still remains at the Interstate 805/43rd Street interchange, which features high freeway-to-freeway flyover ramps. The California 252 corridor now features a new park, an Albertsons grocery store, and a swath of weed-infested open space. Redevelopment of this area continues, and a much-needed missing link to San Diego's freeway system will soon be too expensive to build. Photo taken 05/22/04.
Perspective from 32nd Street/Wabash Boulevard north
Traveling north on 32nd Street a block north of Harbor Boulevard (former U.S. 101), the right lanes prepare to transition onto northbound California 15. The left lanes continue north on 32nd Street. Note the erroneous Interstate 15 shield posted on the series of signs on the overhead sign bridge. The shield is a recent arrival, with the California triangular Interstate shield design specs. Photo taken 12/03/06.
Meeting Norman Scott Road, 32nd Street splits: the left lanes continue north on 32nd Street, while the right lanes angle to the northeast to become the beginning of northbound California 15 (designated as Wabash Boulevard). Note the small Interstate 15 shield placed on the traffic signal mast arm. Photo taken 12/03/06.
The City of San Diego maintains Wabash Boulevard, which connects 32nd Street with the Interstate 5/California 15 interchange. The city placed an Interstate shield along northbound Wabash Boulevard just after the traffic signal with 32nd Street, but it was removed by 2003. (For a brief time, a newer replacement shield was located here, but that shield was also removed.) From that traffic signal northward, Wabash Boulevard is a limited access roadway that feeds onto northbound California 15 just south of Interstate 5. Even though this section has an Interstate 15 shield on it, it is not part of Interstate 15 since it is not maintained by the State of California. State maintenance begins just south of the Interstate 5/California 15 interchange. Photos taken 03/98 and by Andy Field/Jeff Royston (12/03/06).
Wasbash Boulevard travels north for a short distance as a city-maintained street. At the top of this rise, it changes into a state freeway. Photo taken 12/03/06.
Perspective from California 15 north
Upon crossing over Main Street, Wabash Boulevard transitions directly onto California 15 north as identified by this BEGIN FREEWAY sign. Photo taken 12/03/06.
There is no access from northbound California 15 to either direction of Interstate 5, which becomes visible after passing the freeway begins sign. Photo taken 12/03/06.
Traffic merges from southbound Interstate 5 onto northbound California 15. The overhead ramp is the connection from southbound California 15 to southbound Interstate 5. Photo taken 12/03/06.
This is the first California 15 north shield assembly. The bridges ahead carry Interstate 5 over California 15. The mileage sign provides the distance to the first four exits: Ocean View Boulevard, Market Street, and Junction California 94/Dr. Martin Luther King, Jr. Freeway (East and West) and Home Avenue. Photo taken 12/03/06.
On the connection from northbound Interstate 5 to California 15, the freeway immediately encounters its first exit, Ocean View Boulevard (Exit 1). Notably, at the time these photos were taken, exit numbers were not present. There is not another reassurance shield for California 15 until after the Interstate 805 exit. Photo taken 12/03/06.
Again note the first overhead sign that features a California 15 control city of Riverside. The next exit is Exit 2, Junction California 94 (Dr. Martin Luther King, Jr. Freeway), a major east-west corridor between downtown San Diego and El Cajon. The California 15/California 94 interchange is the last remaining obstacle to California's goal of adding California 15 to the Interstate Highway System as the missing link of Interstate 15, since the interchange is not up to Interstate standards. Photo taken 04/09/06.
Prior to 1969, Interstate 15 ended at its junction with Interstate 10 in San Bernardino. At that time, Interstate 15 followed the path of current Interstate 215; the section of Interstate 15 through Colton and Corona was unconstructed at that time. However, in 1969, with the planned upgrade of U.S. 395 to freeway standards between San Bernardino to San Diego, Interstate 15 was extended along with it. For a time, Interstate 15 was planned to follow Interstate 215; this plan was later changed to allow for a more direct route between the Inland Empire and San Diego. At that time, the section of Interstate 15 through San Bernardino became Interstate 15E; Interstate 15E was renumbered as Interstate 215 by the late 1970s. Photo scanned by Joel Windmiller, from the California Department of Public Works Biennial Report.
Northern Terminus - International Border - Sweetgrass, Montana
Perspective from Alberta 4/Interstate 15 south
Alberta 4 and Interstate 15 combine to form the main highway between Lethbridge, Alberta, and Great Falls, Montana. The two highways converge at the international border crossing at Coutts, Alberta, and Sweetgrass, Montana. Southbound Alberta 4 is a four-lane, divided expressway as it approaches the international border; Interstate 15 continues the southerly journey as a four-lane, divided freeway. Photo taken by Rich Piehl (07/16/02).
View of the border crossing area at Coutts along southbound Alberta 4. This border crossing is significantly less crowded than the San Ysidro border crossing into Mexico, which is located only a few miles south of the southern terminus of California 15. Photo taken by Rich Piehl (07/16/02).
Passing by the last of the port of entry facilities and border crossing checkpoint, the northern terminus of the Interstate 15 freeway is visible. Southbound Alberta 4 ends here. Photo taken by Rich Piehl (07/16/02).
Begin Southbound Interstate 15 after the port of entry and international border crossing. The first exit leads to Sweetgrass, a small border community along the 49th parallel. This exit also connects to historic U.S. 91, which was replaced by Interstate 15 in stages through the 1960s and 1970s throughout the Intermountain West and California. Photo taken by Rich Piehl (07/16/02).
Welcome to Montana sign along Southbound Interstate 15 after the Sweetgrass exit (Exit 397). Photo taken by Rich Piehl (07/16/02).
Southbound Interstate 15 approaching the first mileage sign: seven miles to Sunburst (Exit 389), 34 miles to Shelby (Junction U.S. 2), and 116 miles to Great Falls (Junction Montana 200 and U.S. 87). Photo taken by Rich Piehl (07/16/02).
Scenes Pertaining to Interstate 15
Temporary Interstate 15E
A remnant of the former Interstate 15E designation remained in place until mid-2008. This mileage sign was found on southbound Perris Boulevard just before the junction with California 74 in Perris. This mileage sign provided the distance to Temporary Interstate 15E, which is now Interstate 215. Photo taken by Mike Ballard (01/07/07).
Source: December 31, 2014 Interstate Route Log and Finders List
Interstate 15 Annual Average Daily Traffic
Salt Lake City
Source: Caltrans, Traffic Operations Program - Traffic and Vehicle Data Systems 
The Annual Traffic Report  (Nevada Department of Transportation)
2001 Arizona Interstate Annual Average Daily Traffic (AZDOT)
Utah Department of Transportation - Traffic on Utah's Highways 2001
2002 Rural Traffic Flow Map (ITD)
Complete Interstate 15 AADT data.