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 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.

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. 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) in what is known as 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.

In Imperial County, SR 78 intersects with SR 86, running concurrently with it southwest of the Salton Sea and northwest of San Felipe Creek. SR 78 passes through the desert community of Elmore Desert Ranch before entering the city of Westmorland. The route, still concurrent with SR 86, enters into the city of Brawley as Main Street, where SR 86 splits to the south towards El Centro. SR 78 continues north onto the Brawley Bypass, a freeway that passes to the north of downtown Brawley. SR 111 runs concurrently with SR 78 for a short duration before the latter exits from the freeway and continues east.

Then, SR 78 intersects with SR 115 east of Brawley, running concurrently with it for a brief distance. Shortly after passing through the small community of Glamis, the road turns northeast and eventually north towards Blythe, passing near the Chocolate Mountain Naval Reserve. As it nears the Colorado River and the Arizona border, SR 78 briefly passes through Cibola National Wildlife Refuge before entering the community of Palo Verde, where the river turns away from the highway and SR 78 enters Riverside County.

As it nears Blythe, the highway makes a sharp turn east onto 32nd Avenue before turning north on Rannels Boulevard. It makes a right on 28th Avenue before turning north on Washington Boulevard and passing through Ripley. SR 78 continues north for a few more miles to its terminus at I-10 a few miles west of Blythe.

SR 78 is designated as the Ronald Packard Parkway (after a former Congressman named Ronald Packard from the area) from I-5 in the city of Oceanside to I-15 in the city of Escondido, and Ben Hulse Highway (after a former state senator named Ben Hulse) from SR 86 near Brawley to I-10 near the city of Blythe. The portion of SR 78 from SR 86 in Brawley to CR S3 near Anza-Borrego Desert State Park is designated as part of the Juan Bautista de Anza National Historic Trail auto tour route, promoted by the National Park Service. SR 78 is part of the California Freeway and Expressway System, although only the metropolitan section of SR 78 is a freeway. The section of SR 78 from the western junction of SR 79 to the western junction with SR 86 is designated by the California State Legislature as eligible by law for the State Scenic Highway System; however, only the section in Anza Borrego Desert State Park has officially been designated by Caltrans as being part of the system, meaning that it is a substantial section of highway passing through a "memorable landscape" with no "visual intrusions", where the potential designation has gained popular favor with the community. SR 78 from I-5 to I-15 is part of the National Highway System (NHS), a network of roadways important to the country's economy, defense, and mobility.

Before the designation of SR 78, a road known as the Brawley-Westmorland-Julian-Oceanside Highway (connecting Oceanside, Escondido, Ramona, Julian, Westmorland, and Brawley) existed during the early twentieth century. This road roughly followed the current routing of SR 78 from Escondido to the east of Brawley, although it traveled along a different routing from Westmorland into Brawley.

No road connected Brawley with Glamis at this time; it was necessary to travel north through Calipatria to reach Blythe. East of the Sand Hills, there was a road from Glamis passing by Smith Well into Palo Verde, which roughly follows the routing of SR 78. Around the 1930s, the road from Escondido to Ramona was a gravel road, and the portion from Julian to U.S. Route 99 (US 99), which is currently designated as SR 86, was a dirt road.

SR 78 was originally formed along with the originally signed state highways in California (Sign Routes) in 1934; however, it only extended to what was then US 99 near Kane Springs. In the North County, SR 78 was legislatively designated as Legislative Route 196 from then-US 101 (present-day I-5) to Vista, and as Legislative Route 77 from Vista to US 395 in Escondido. SR 78 was legally known as Legislative Route 197 from Escondido to Ramona, and Legislative Route 198 from Ramona to US 99, which is now SR 86. From US 99 in Brawley to SR 115, what would become SR 78 was defined as Legislative Route 187.

By 1947, US 395 ran concurrently along the portion of SR 78 from Vista to Escondido before continuing along Santa Fe Avenue to Bonsall and Fallbrook and rejoining its alignment during the 1970s. At this time, all of SR 78 that existed had been paved. Before the present-day freeway was built, SR 78 was routed on the Vista Way Freeway (which was an expressway) from Oceanside east to downtown Vista. After this, it followed Santa Fe Avenue and Mission Road east (now signed as CR S14), continuing onto Grand Avenue in Escondido. Following the intersection with US 395, SR 78 turned south on Ash Street and rejoined the current alignment of the highway.

The Vista Way Freeway opened in April 1962 between I-5 and Melrose Drive, but was not entirely access controlled. The first section of the SR 78 freeway from San Marcos to west of Vista was scheduled to be completed in early 1963, at a cost of $3.9 million (about $56 million today). The rest of the freeway between San Marcos and Escondido was constructed between 1963 and 1968.

The San Diego Chamber of Commerce and the San Diego Highway Development Association urged for the conversion of SR 78 west of Vista, a length of 5.6 miles (9.0 km), from an expressway to a freeway in April 1967. The College Boulevard diamond interchange on this western segment opened to traffic on October 24, 1967, and connected the recently opened MiraCosta College to the freeway. The interchange, previously an at-grade intersection, improved traffic flow to the college. The construction of the interchange cost $800,941 at the time (about $8.96 million today). May 1968 saw the state designating the Jefferson Street and Emerald Drive interchanges as a priority. In August 1968, the state allocated $750,000 (about $7.75 million today) for building the Jefferson Street interchange. Both interchanges had funding by August 1970. The section from I-5 east to Melrose Drive (along the routing of the Vista Way Freeway) had been upgraded to full freeway standards as of 1973.

The missing portions of the current SR 78 routing in Imperial County were constructed after 1963, when the state legislature allowed for a road to be built from SR 115 to the Riverside County line. This portion of the road was specifically designed to address the challenges of building it through sand dunes. The engineers routed the highway according to the terrain and made cuts in the sand up to 80 feet (24 m) deep. In 1957, the United States Navy obtained ownership from Imperial County of the road running through the Chocolate Mountain Aerial Gunnery Range for $660,000 (about $11.5 million today). This money was used to fund the construction of what would become SR 78, which Ben Hulse (who later had the highway named after him) predicted would become a state highway. Following this, in 1965, the newly constructed section was signed as CR S78. Legislatively, it had designated as Legislative Route 146 from the Riverside county line into Blythe, and eventually was signed as SR 195 from Palo Verde into Blythe; the SR 195 and Legislative Route 146 designations continued along the path of current US 95 to the Nevada state line. In 1959, the rest of the current routing of SR 78 between Brawley and Palo Verde was added to the state highway system as SR 195 and Legislative Route 146. The section from Palo Verde to Blythe shows up as part of SR 78 on maps as early as 1965, and the section from southwest of Midway Well to Palo Verde is shown as part of SR 78 as early as 1966.

In 1969, plans to extend the freeway portion of SR 78 east from the Broadway interchange through Escondido were delayed by Caltrans director Jacob Dekema due to a lack of funding; however, the bypass does not exist today.

During the last decade, several significant upgrades have been made to SR 78 in the North County. A new interchange with Vista Village Drive was opened in 1998, and the College Boulevard interchange in Oceanside was revised along westbound SR 78. In addition to this, a new interchange was constructed at Las Posas Road in San Marcos, which opened in 2006.

During the late 2000s, planning began for a bypass around the downtown portion of the city of Brawley. An expressway would carry the routing of SR 78 north and east of the city, with an interchange at SR 111, before intersecting with the current alignment of SR 78. A Swedish company began construction on this bypass in April 2008; the first phase consisted of the portion of the bypass that is solely SR 111. The second phase of the bypass, from the western junction with SR 111 to the eastern junction with SR 78, lasted from February 2008 to June 2011. On the third phase of the project, from the junction with SR 86 west of Brawley to the western end of the completed bypass, construction began in late 2010. This project was identified in August 2010 as a project that could be affected by California state budget cuts. The Brawley Bypass, as it was known, opened on October 30, 2012.

The western portion of SR 78 in North County is currently slated for several improvements. There were plans to construct an additional interchange at Rancho Del Oro Road in Oceanside; however, the Oceanside City Council decided to cancel these plans in 2005, despite studies suggesting that this would be detrimental to the traffic in the region. There are also plans to improve the interchange with I-5, which currently involves a traffic signal connecting Vista Way and SR 78 with the ramps to I-5 southbound. Plans call for adding more lanes to I-5 and SR 78 as well as for the construction of a new ramp from SR 78 westbound to I-5 southbound and from I-5 southbound to SR 78 eastbound. In the past, there was a direct ramp from SR 78 westbound to I-5 southbound which avoided this traffic signal; however, it was removed to construct a park and ride lot. The interchange at Nordahl Road will also be improved, and extra lanes will be added between Nordahl Road and I-15; construction commenced in early 2012.

In the Imperial and Riverside County portion from Brawley to Blythe, the road goes through several washes. During the monsoon season, these washes can be left with several inches of water, sand, and rock debris following rainstorms. There is a proposal to improve drainage by raising the roadway and installing culverts.

Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, and do not necessarily reflect current mileage. The numbers reset at county lines; the start and end postmiles in each county are given in the county column.