Interstate 15 replaced US 395 between San Diego and Temecula, and US 66 between San Bernardino and Barstow. North of Barstow, the highway replaced US 91. The highway was initially planned from I-10 in San Bernardino to Nevada, but the state of California wanted the highway to start in San Diego, which proved successful. The old I-15 between San Bernardino and Devore became I-215.
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San Diego
Already in 1951 the first part was opened in San Diego. For a long time, however, this was the only part in the San Diego area, partly because the suburbs north of San Diego only developed later, as most of the growth took place in nearby Los Angeles. In the late 1970s, construction was mainly done north of San Diego, in the Escondido region. Most of the remaining parts were constructed in the 1980s, and most of them were completed in 1985. Except for one short piece in San Diego, which is the stretch of road between I-805 and I-8. This section was missing for a long time, and traffic had to detour via I-805 and I-8. Not a big detour, but it did cause unnecessary traffic on these highways. This missing link was not opened to traffic until 2000.
Express lanes
Until 2012, the I-15 between SR-163 in San Diego and SR-78 in Escondido has been drastically widened, a separate toll road with 2×2 lanes has been built in the central reservation. Since then, the highway has 14 lanes here.
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Opening history
This is the opening history in the San Diego area.
| From | Unpleasant | Length | Opening |
| Ocean View Blvd | SR-94 Martin Luther King Fwy | 2 km | 00-00-1951 |
| SR-94 Martin Luther King Fwy | I-805 | 2 km | 16-07-1970 |
| SR-78 Packard Fwy | Center City Pkwy | 4 km | 28-02-1977 |
| Via Rancho Parkway | SR-78 Packard Fwy | 8 km | 18-03-1977 |
| Miramar Road | Poway Road | 6 km | 19-04-1978 |
| I-5 | Ocean View Blvd | 1 km | 05-05-1978 |
| Center City Drive | Deer Springs Road | 4 km | 04-10-1979 |
| Gopher Canyon Road | Lilac Road | 5 km | 13-05-1980 |
| Deer Springs Road | Gopher Canyon Road | 7 km | 07-08-1980 |
| Camino del Norte | Via Rancho Parkway | 8 km | 28-01-1981 |
| Lilac Road | Palo Mesa Drive | 4 km | 14-04-1981 |
| San Diego Mission Road | Aero Drive | 3 km | 22-05-1981 |
| Aero Drive | Tierra Santa Blvd | 1 km | 06-05-1982 |
| Poway Road | Camino del Norte | 7 km | 28-07-1982 |
| SR-163 Cabrillo Fwy | Miramar Road | 4 km | 28-10-1982 |
| Palo Mesa Drive | Mission Road | 6 km | 06-02-1983 |
| I-8 | San Diego Mission Road | 1 km | 28-10-1984 |
| Mission Road | Riverside County Line | 7 km | 24-02-1985 |
| Tierra Santa Blvd | SR-163 Cabrillo Fwy | 4 km | 07-05-1985 |
| I-805 | I-8 | 5 km | 18-01-2000 |
Los Angeles / Inland Empire
From the San Diego state line, I-15 was gradually extended north to Lake Elsinore between 1973 and 1982. At the same time, sections northwards between Lake Elsinore and Corona were also opened in 1975. Thereafter, there was a fairly long hiatus between Corona and Ontario, which opened only in 1989, one of the last completions of a mainline Interstate in the state of California. In the mid to late 1970s, I-15 opened up around Ontario, Rancho Cucamonga, and west of San Bernardino. The oldest part dates from 1969 and includes the part from San Bernardino to the Cajon Pass.
Devore Interchange
Between 2013 and 2016, the interchange between I-15 and I-215, the ‘Devore Interchange’ north of San Bernardino, was reconstructed. Originally this was a simple interchange with direct connecting roads, where freight traffic on I-15 in particular had to shift a number of lanes to the left to follow the highway. To tackle this problem, truck bypass routes have been constructed with separate lanes for freight traffic around the interchange, so that there are fewer lane changes and weaving movements. The project cost $324 million and was completed on May 20, 2016, 18 months ahead of schedule.
Express lanes
Between 2018 and 2021, express lanes were built on I-15 between Cajalco Road in southern Corona and SR-60 in Jurupa Valley. This project involved 25 kilometers of toll lanes built in the median strip of the existing motorway. Unlike some projects in which HOV lanes have been converted into toll lanes, this was a capacity expansion. The $472 million project began in April 2018 and the express lanes opened on April 10, 2021. The project is also known as Riverside Express.
Opening history
This is the opening history in the Corona region.
| From | Unpleasant | Length | Opening |
| San Diego County Line | Temecula Pkwy | 5 km | 23-03-1973 |
| El Cerrito Road | Magnolia Avenue | 4 km | 01-12-1975 |
| Temescal Canyon Road | El Cerrito Road | 7 km | 30-12-1975 |
| Temecula Pkwy | Winchester Road | 5 km | 30-09-1976 |
| Winchester Road | Los Alamos Road | 5 km | 23-12-1977 |
| Los Alamos Road | Clinton Keith Road | 6 km | 26-09-1978 |
| Indian Truck Trail | Temescal Canyon Road | 6 km | 26-09-1979 |
| Lake Street | Indian Truck Trail | 6 km | 28-05-1980 |
| Central Avenue | Lake Street | 6 km | 07-11-1980 |
| Clinton Keith Road | Olive Street | 6 km | 04-09-1981 |
| Olive Street | Central Avenue | 8 km | 08-07-1982 |
| Magnolia Avenue | SR-91 Riverside Fwy | 2 km | 21-04-1987 |
| SR-91 Riverside Fwy | SR-60 Pomona Fwy | 16 km | 28-02-1989 |
Opening history
This is the opening history in the Ontario & San Bernardino region.
| From | Unpleasant | Length | Opening |
| Glen Helen Pkwy | Cajon Pass | 14 km | 07-11-1969 |
| I-10 | 4th St | 1 km | 14-04-1975 |
| 4th St | Foothill Blvd | 4 km | 30-10-1975 |
| Sierra Avenue | Glen Helen Pkwy | 3 km | 24-12-1975 |
| Foothill Blvd | Sierra Avenue | 12 km | 22-09-1976 |
| Jurupa St | I-10 | 2 km | 02-02-1978 |
| SR-60 Pomona Fwy | Jurupa St | 2 km | 23-03-1979 |
Mojave Desert
The section through the Mojave Desert is the oldest of I-15 in California. Between 1958 and 1965, the highway was opened between Barstow and the Nevada state line. In 1972 the last section south of Barstow opened to traffic.
Interstate 15 is undergoing a major upgrade project between Devore and the Nevada border. This highway is very busy as it connects the cities of Los Angeles, San Diego and Las Vegas. Most of this route consisted of 2×2 lanes. The project began in 2002 at a cost of $349 million and involved repaving the entire route and adding 63 kilometers of climbing lanes for trucks. In many places freight traffic drives over the widened hard shoulder. Service areas have also been improved and exit numbers have been added. California did not have exit numbers until recently, but only adds them when maintenance needs to be carried out to reduce costs.
In 2022, an 8-kilometer-long rush- hour lane on I-15 near the Nevada border will be completed.
Opening history
I-15 at Temecula.
| From | Unpleasant | Length | Opening |
| National Trails Highway | Pacific Rail underpass | 9 km | 00-00-1958 |
| Hodge Road | L Street | 18 km | 00-00-1958 |
| L Street | Ghost Town Road | 16 km | 00-00-1961 |
| East Baker Blvd | Cima Road | 39 km | 00-00-1961 |
| Cima Road | Nevada State Line | 38 km | 00-00-1963 |
| Ghost Town Road | Field Road | 36 km | 09-06-1964 |
| Field Road | Basin Road | 26 km | 19-10-1964 |
| Oak Hill Road | US 395 | 5 km | 12-01-1965 |
| Basin Road | East Baker Blvd | 29 km | 07-10-1965 |
| US 395 | Palmdale Road | 14 km | 03-01-1966 |
| roadway split | Oak Hill Road | 7 km | 10-08-1966 |
| Cajon Pass | roadway split | 3 km | 03-08-1970 |
| Palmdale Road | National Trails Highway | 5 km | 04-08-1971 |
| Pacific Rail underpass | Hodge Road | 17 km | 22-12-1972 |
Future
Cajon Pass
Construction of express lanes over Cajon Pass north of San Bernardino is being considered. Due to the rapid growth of the High Desert region, traffic to the rest of the Inland Empire and Los Angeles is increasing. Two toll lanes in each direction are assumed.
Traffic intensities
The express lanes north of San Diego.
| Exit | Place | 2007 | 2012 | 2016 |
| Exit 1 | San Diego ( I-5 ) | 106,000 | 107,000 | 108,000 |
| Exit 3 | San Diego ( I-805 ) | 159,000 | 159,000 | 173,000 |
| exit 6 | San Diego ( I-8 ) | 213,000 | 197,000 | 217,000 |
| exit 12 | San Diego ( SR-163 ) | 302,000 | 293,000 | 305,000 |
| Exit 19 | San Diego ( SR-56 ) | 236,000 | 223,000 | 234,000 |
| exit 32 | Escondido ( SR-78 ) | 132,000 | 128,000 | 150,000 |
| exit 63 | Temecula ( I-215 ) | 195,000 | 180,000 | 198,000 |
| Exit 64 | Murrieta | 116,000 | 106,000 | 114,000 |
| Exit 96 | Corona ( SR-91 ) | 184,000 | 182,000 | 187,000 |
| Exit 106 | Ontario ( SR-60 ) | 225,000 | 215,000 | 219,000 |
| Exit 109 | Ontario ( I-10 ) | 207,000 | 198,000 | 206,000 |
| Exit 115 | Rancho Cucamonga ( I-210 ) | 151,000 | 134,000 | 140,000 |
| Exit 123 | San Bernardino ( I-215 ) | 162,000 | 152,000 | 152,000 |
| Exit 141 | Hesperia ( US 395 ) | 106,000 | 101,000 | 101,000 |
| Exit 184 | Barstow ( I-40 ) | 67,000 | 65,000 | 66,000 |
| Exit 246 | Baker | 40,000 | 40,000 | 42,000 |
| Exit 296 | border with Nevada | 40,000 | 40,000 | 43,000 |
Lane Configuration
| From | Unpleasant | Lanes | Comments |
| Exit 1 | Exit 3 | 2×3 | San Diego |
| Exit 3 | exit 12 | 2×5 | San Diego |
| exit 12 | exit 13 | 6+2+2+6 | |
| exit 13 | Exit 19 | 5+2+2+5 | |
| Exit 19 | exit 32 | 4+2+2+4 | |
| exit 32 | exit 63 | 2×4 | |
| exit 63 | Exit 91 | 2×3 | |
| Exit 91 | Exit 96 | 4+1+1+4 | express lanes through Corona |
| Exit 96 | Exit 106 | 3+1+1+3 | express lanes |
| Exit 106 | Exit 122 | 2×4 | Ontario |
| Exit 122 | Exit 123 | 2×3 | |
| Exit 123 | Exit 141 | 2×4 | |
| Exit 141 | Exit 184 | 2×3 | barstow |
| Exit 184 | Exit 296 | 2×2 | Mojave Desert |
