The lowest freeway in the country at one point, as it drops below sea level in the Imperial Valley near El Centro, Interstate 8 also traverses several mountain passes en route between San Diego, California, and Casa Grande, Arizona. Beginning in the Ocean Beach neighborhood, I-8 proceeds east along an urban course through the San Diego metro area. Beyond El Cajon and Alpine, the freeway winds through Cleveland National Forest and Laguna Mountain range before angling southeast. I-8 passes through the In-ko-pah Gorge ahead of Ocotillo and the rolling desert hills and farmland in the Imperial Valley. The freeway enters Arizona at the Colorado River and Yuma, and advances east across more desert terrain, including the famed Arizona saguaro. I-8 concludes at I-10 midway between Phoenix and Tuscon.
Parallel U.S. Routes
Interstate 8 only intersects one U.S. route for its entire length, U.S. 95, thus lending credibility to the paucity of U.S. highways in the Southwestern United States. However, it parallels a well-preserved historic U.S. route, Old U.S. 80.
U.S. 80 follows Interstate 8 from San Diego to Gila Bend, Arizona, where U.S. 80 turned north along what is now Arizona Route 85 to Interstate 10 near Buckeye. Well-preserved stretches of Highway 80 are otherwise accessible as frontage roads and county roads along the freeway corridor.
The easternmost stretch of I-8 between Gila Bend and Casa Grande was not a former U.S. route; the Interstate instead replaced former SR 84. (A shorter section remains close to Casa Grande.)
Origins of Interstate 8 in San Diego date back to 1957 with construction for a new U.S. 80 freeway between U.S. 395 and Ward Road. The majority of the U.S. 80 freeway was completed east from U.S. 101 to El Cajon by the early 1960s, with I-8 formally posted in 1964. Originally, I-8 was planned to end at Interstate 5. However, with the construction of the Ocean Beach Freeway (former California State Route 109), the route was extended west to the intersection of Nimitz Drive and Sunset Cliffs Boulevard.
The Pine Valley Creek Bridge opened to traffic in 1975. This segmental balanced cantilever bridge carries the freeway over 200 feet above the valley floor.1 A more detailed history for I-8 posted at the Interstate 8 California guide.
Within Arizona, the earliest sections of Interstate 8 completed ran between Sentinel (Exit 87) and Piedra (Exit 102) and from Gila Bend (Exit 119) to SR 84 (Exit 151) in 1959. The Colorado River bridge west of Yuma opened in 1979 as the last portion of I-8 opened overall. See the Interstate 8 Arizona guide for a more detailed time line.
California – 169.92
Arizona – 178.33
Source: December 31, 2021 Interstate Route Log and Finders List
I-8 Annual Average Daily Traffic (AADT)
Source: 2017 Caltrans Traffic Volumes
ADOT Traffic Monitoring 2018 AADT
Interstate 8 ended at I-5 until 1971, when the Ocean Beach Freeway was renumbered from SR 109 to I-8.
U.S. 80 was decommissioned formally by the state of California in 1964, but signs remained along I-8 until 1972.
Long pieces of Interstate 8 were open by 1963 along U.S. 80 through San Cristobal Valley and across Sentinel Plain and over SR 84 between the Sand tank and Maricopa Mountains and across Vekol Valley.
Within Arizona, U.S. 80 was truncated from Yuma east to Benson in 1977. The remainder was dropped in 1989.
East End – Casa Grande, Arizona
East End Throwback
Interstate 10 west at I-8 prior to the 2011/12 six lane expansion. 01/17/05
West End – Sunset Cliffs Blvd / Nimitz Blvd – San Diego (Ocean Beach), California
Two lanes depart from I-8 west for Mission Bay Drive north and Sports Arena Drive south. Spanning the San Diego River, Mission Bay Drive heads to Mission Beach and Pacific Beach. Sports Arena Drive continues the arterial southeast through the Midway-Sports Arena District, which houses Pechanga Arena San Diego (San Diego Sports Arena). 07/19/21
Sunset Cliffs Blvd – North at
Nimitz Blvd – North at
- California Celebrates 50 Years of the Interstate Highway System.
http://www.dot.ca.gov/interstate/Caltrans (California Department of Transportation).
- “Both directions of I-10 now three lanes all the way between Tucson and Casa Grande.” ADOT (Arizona Department of Transportation) News. September 27, 2019.
Page updated May 17, 2022.