The lowest Interstate in the country at one point (as the freeway drops below sea level elevation in the Imperial Valley near El Centro), Interstate 8 also traverses several mountain passes on its journey between San Diego, California, and Casa Grande, Arizona. I-8 provides a dynamic look at everything that’s good about an Interstate: It has the multi-lane, urban feel in San Diego, the mountainous terrain with peaks and valleys in Cleveland National Forest/Laguna Mountains, the In-ko-pah Gorge, rolling desert hills, farmland in the Imperial Valley, the vast Imperial dunes, a Colorado River Bridge and more desert scenery (including the famed Arizona saguaro).
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.
- East End – Casa Grande, AZ
- West End – San Diego, CA
Branch Routes – 0
- Total Mileage – 348.25
California – 169.92
- Cities – San Diego, El Centro
- Junctions –
Arizona – 178.33
- Cities – Yuma, Casa Grande
- Junctions –
Source: December 31, 2018 Interstate Route Log and Finders List
I-8 Annual Average Daily Traffic (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
Completed in September 2019, a $43 million project expanded I-10 to six overall lanes from the Earley Road overpass in Casa Grande four miles to the trumpet interchange (Exit 199) with Interstate 8.2 Interstate 8 ventures 341 miles west from I-10 to meet Interstate 15 in Mission Valley, San Diego. 04/26/17
East End Throwback
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). 07/13/18
Sunset Cliffs Blvd north at
Sunset Cliffs Boulevard crosses the San Diego River to enter the Mission Bay Interchange with Sea World Drive, Ingraham Street and West Mission Bay Drive. During hot summer afternoons, Sea World Drive can be extremely congested with beach traffic trying to make its way back to Interstate 5. 08/11/06
Nimitz Blvd north at
- California Celebrates 50 Years of the Interstate Highway System by 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 January 23, 2020.