Interstate 8 (I-8) is an Interstate Highway in the southwestern United States in the U.S. states of California and Arizona. It runs from the southern edge of Mission Bay at Sunset Cliffs Boulevard in San Diego, California, almost at the Pacific Ocean, to the junction with I-10, just southeast of Casa Grande, Arizona. In California, the freeway travels through the San Diego metropolitan area as the Ocean Beach Freeway and the Mission Valley Freeway before traversing the Cuyamaca Mountains and providing access through the Imperial Valley, including the city of El Centro, where it junctions with I-7. Crossing the Colorado River into Arizona, I-8 continues through the city of Yuma across the Sonoran Desert to Casa Grande, in between the cities of Phoenix and Tucson at Interstate 10.
The first route over the Cuyamaca Mountains was dedicated in 1912, and a plank road served as the first road across the Imperial Valley to Yuma; east of there, the Gila Trail continued east to Gila Bend. These were later replaced by U.S. Route 80 (US 80) across California and part of Arizona, and Arizona State Route 84 (AZ 84) between Gila Bend and Casa Grande. The US 80 freeway through San Diego was largely complete by the time it was renumbered as I-8 in the 1964 state highway renumbering; east of San Diego, the US 80 roadway was slowly replaced by I-8 as construction progressed in the Imperial Valley. The Arizona portion of the road was built starting in the 1960s. Several controversies erupted during the construction process; questionable labor practices in Imperial County led to the federal conviction of a mobster and a U.S. House of Representatives subcommittee found that the Arizona government had mismanaged financial resources.
The route was completed in 1975 through California, and by 1977 through Arizona, though the bridge over the Colorado River was not completed until 1978. Since then, the freeway through San Diego has been widened due to increasing congestion, and another portion in Imperial County had to be rebuilt following damage by the remnants of Hurricane Kathleen.
I-8 is part of the National Highway System, a network of highways that are considered essential to the country's economy, defense, and mobility by the Federal Highway Administration. The freeway from the eastern junction with California State Route 98 (CA 98) to the eastern end is designated as part of the Juan Bautista de Anza National Historic Trail auto tour route, promoted by the National Park Service.
