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.
SR 78 was one of the original state highways designated in 1934, although portions of the route existed as early as 1900. However, it was not designated east of Brawley until 1959. The freeway section in the North County of San Diego that connects Oceanside and Escondido was built in the middle of the twentieth century in several stages, including a transitory stage known as the Vista Way Freeway, and has been improved several times. An expressway bypass of the city of Brawley was completed in 2012. There are many projects slated to improve the freeway due to increasing congestion in the region.
SR 78 begins in Oceanside as a continuation of Vista Way. As it encounters a traffic signal and crosses over I-5, the route becomes a suburban freeway traveling east through Oceanside. The freeway loosely parallels Buena Vista Creek before entering the city of Vista. Turning southeast, SR 78 continues into the city of San Marcos near California State University San Marcos and enters Escondido, where it has an interchange with I-15. A 2011 Caltrans study estimated that the average commuter encountered a delay of 10 minutes on the portion from I-5 to I-15. After passing the Center City Parkway (I-15 Business) interchange, the freeway abruptly ends at the intersection with Broadway. SR 78 then makes a turn south onto Broadway and continues through downtown Escondido by turning east onto Washington Avenue and south onto Ash Street, which becomes San Pasqual Valley Road.
Turning east once again, SR 78 leaves the Escondido city limits and enters the San Pasqual Valley as it provides access to the San Diego Zoo Safari Park and San Pasqual Battlefield State Park. After leaving the San Pasqual Valley, the road follows a serpentine alignment, heading south to enter the community of Ramona as Pine Street. In Ramona, SR 78 intersects SR 67 and makes a turn east onto Main Street, going through downtown Ramona. The highway leaves Ramona as Julian Road, which continues on a winding mountain alignment through Witch Creek to Santa Ysabel where it meets SR 79.
SR 78 runs concurrently with SR 79 across the headwaters of the San Diego River and through the hamlet of Wynola, briefly entering Cleveland National Forest before reaching Julian and entering the town as Washington Street. The route, still concurrent with SR 79, turns east onto Main Street and travels through downtown Julian before SR 79 diverges south towards Cuyamaca and SR 78 heads northeast as Banner Road. The road intersects with County Route S2 (CR S2) at a junction called Scissors Crossing; CR S2 runs concurrently in a wrong-way concurrency. Shortly afterwards, SR 78 enters Anza-Borrego Desert State Park and is designated as a scenic highway for its length in the state park. Although this route travels many miles south of the town of Borrego Springs, it provides access to the town via CR S3. SR 78 travels through the town of Ocotillo Wells before exiting the state park and entering Imperial County.