Interstate 8 (I-8) is an Interstate Highway in the southwestern United States. 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 before traversing the Cuyamaca Mountains and providing access through the Imperial Valley, including the city of El Centro. 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. 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. These were later replaced by U.S. Route 80 (US 80) across California and part of Arizona. 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 route was completed in 1975 through California, though the bridge over the Colorado River was not completed until 1978.