State Route 78 (SR 78) is a state highway in the U.S. state of California 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 Cuyamaca Mountains to Julian. In Imperial County, SR 78 travels through the desert near the Salton Sea and passes through the city of Brawley and the Algodones Dunes before turning north on the way to its terminus in Blythe.

Oceanside is a city on the South Coast of California, located in San Diego County. The city had a population of 174,068 at the 2020 United States census, up from 167,086 at the 2010 census, making it the most populous city in the North County region of San Diego. The city is a popular tourist destination, owing to its historic landmarks, beaches, and architecture.