High Quality Content by WIKIPEDIA articles! State Route 78 (SR 78) is a state highway in the U.S. state of California in the United States (U.S.) that runs from Oceanside east to Blythe, traversing nearly the entire width of the state. Its western terminus is at Interstate 5 (I-5) in San Diego County and its eastern terminus is at I-10 in Riverside County. The route is a freeway through the heavily populated cities of northern San Diego County and a two-lane highway running through the Santa Rosa Mountains to Julian. In Imperial County SR 78 travels through the desert near the Salton Sea and passes through the city of Brawley before turning north and passing through an area of sand dunes on the way to its terminus in Blythe.