 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 before turning north and passing through an area of sand dunes on the way to its terminus in Blythe . 
