Interstate 8 or I-8 is an Interstate Highway that runs from Interstate 10 in the vicinity of Casa Grande , Arizona to San Diego , California . 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 Blvd , in San Diego , California , almost at the Pacific Ocean , to the junction with Interstate 10 , just southeast of Casa Grande , Arizona .