State Route 178 (SR 178) is a route that exists in two constructed segments. The gap in between segments is connected by various local roads and State Route 190 through Death Valley National Park.

SR 178 serves many different purposes. It connects SR 99 and Downtown Bakersfield with East Bakersfield and Lake Isabella. Lake Isabella is a weekend vacation spot for, among other things, fishing, boating, and white water rafting in the lake and Kern River. It is the only seasonal crossing and one of two crossings over the Sierra Nevada Mountains south of Yosemite (SR 120, Tioga Pass Rd.), connecting the Southern San Joaquin Valley with the upper Mojave Desert and the Owens Valley. This also provides access to Death Valley National Park. If the unconstructed portion were built, it would also provide an easy route between Ridgecrest, California and Las Vegas, via Pahrump, Nevada.

This route is part of the California Freeway and Expressway System and is eligible for the State Scenic Highway System.

The first segment starts at State Route 99 just west of Downtown Bakersfield. The road continues as 24th street, but then splits at B St. utilizing 24th street as its westbound usage, and 23rd St. as its eastbound through the eastern part of Downtown Bakersfield. SR 178 becomes a freeway as it leaves Downtown, and winds through East Bakersfield. The freeway then travels east as it enters Northeast Bakersfield. Just east of the Fairfax Interchange, the road narrows to a 2-lane conventional highway. The highway continues through the rural, but growing Rio Bravo section of Bakersfield. It then crosses SR 184. Turning northeast, the route continues to the mouth of Kern Canyon (which is also the city limits of Bakersfield). For the next approximately 8 miles, the route is a narrow 2-lane road (average width of 18 ft. to 24 ft.), as it ascends the lower Sierra Nevada Mountains. Average speed is 35 mph, with sharp turns and steep dropoffs. After approximately 8 miles, the road becomes a much gentler 4-lane, 60 ft. expressway. The route continues east, and reaches the town of Lake Isabella, which is just south of the Lake Isabella Reservoir. The road briefly expands to a divided freeway through Lake Isabella, before narrowing to a 2-lane conventional highway at the intersection with Lake Isabella Blvd. The road winds a little bit until it ascends to Walker Pass, an elevation of over 5,000 feet (1,500 m). The highway then descends from the mountains to its junction with State Route 14. It then proceeds eastward across US 395 into the town of Ridgecrest, CA. The constructed highway ends at the turnoff for the Trona Pinnacles National Natural Landmark.

The second segment resumes four miles (6 km) west of Salisberry Pass in the southeasterly part of Death Valley National Park in Inyo County at what had been the former boundary of Death Valley National Monument until 1994. It then meets up with State Route 127. 178 then branches northward from 127 to the California-Nevada State Line. In Nevada, the roadway continues as State Route 372 ending at State Route 160 near the center of Pahrump in Nye County. Plans to connect the two constructed segments in California are unlikely due to the would-be link's passage through Death Valley National Park.

The segment of State Route 178 from State Route 127 to the California-Nevada State Line as well as all of Nevada State Route 372 are both known as the Charles Brown Highway. Charles Brown, a former California State Legislator, was a major proponent for the incorporation of the segment of State Route 178 between State Route 14 and the California-Nevada state line into the California Highway System.

SR 178 was one of the routes created with the third bond act of 1919. It defined a route 202 miles long between Santa Maria and Freeman Junction through Bakersfield. Freeman does not exist today; it was originally located near (and later at) the junction of SR 178 and SR 14. The route was defined as Legislative Route 57. The 1919 bond act also created the first segment of LRN 58. The route was extended several times since 1919. In 1933, the final segment was added to LRN 58, which created a route from US 101 near Santa Margarita to the Nevada state line via Bakersfield. Construction on the route between Bakersfield and Isabella through Kern Canyon started in 1922. Progress moved slowly, as sheer rock walls had to be blasted with dynamite. In 1931, 9 years after construction started, the 26 mile highway segment was completed.

In 1933, with the creation of signed routes, portions of LRN 57 and LRN 58 would be signed as Route 178. LRN 58 would be signed between Route 33 and US 99, and LRN 57 would be signed between US 99 and US 395. Later, in 1947, LRN 212 was created, and defined to run from US 395 at the junction with Route 178 east to the Nevada state line. It was an unsigned route.

In the period 1950-53 a portion of SR178 in Lake Isabella was rerouted around the Isabella Auxiliary Dam. The dam was built over the old route and parts of it are inundated by Isabella Lake. The new route goes over the dam's southern abutment and along the shoreline of Isabella Lake toward Onyx.

In 1964 all of the California routes were renumbered. LRN 58 was dropped from Route 178 and combined with the eastern portion of the decommissioned US 466 (also defined as part of LRN 58) to created SR 58. The remaining Route 178 was combined with LRN 212 to create SR 178. It was originally defined to start at SR 99 in Bakersfield, but later that year it was changed to simply start in Bakersfield. This change was probably done to avoid a cosign with SR 58 along 23rd/24th St. After SR 58 was moved to the freeway south of Brundage Ln. in 1976, SR 178 was extended west to SR 99.

Construction on the initial freeway in Bakersfield was completed in 1968. It ran from M St, on the eastern edge of Downtown, through East Bakersfield to Haley St. Eastern extensions have been constructed since 1968, one interchange at a time as the need arises. In the Kern Canyon section, which is between Bakersfield and the Kern River Valley, a bypass route was identified in 1964. By 1968, a deed from the US forestry service issued an easement to the State for the construction of the highway through National Forest lands. The first phase of construction was completed in 1974, which created a short freeway near Lake Isabella and a 60’ wide expressway extending west to China Garden. Subsequent phases were not funded.

There has been a plan to extend SR 178 freeway west, past SR 99 to the proposed Westside Parkway. Although the extension has been studied several times since the existing freeway was completed in 1968, a formal route adoption has never been done. When that study is conducted, most likely the following two alignments will be considered, since both have been recommended in previous reports. The central alignment, sometimes referred to as the Westchester alignment, was Caltrans preferred route in 1973. The southern alignment was recommended as part of the Bakersfield System Study in 2001. It was also recommended in a study conducted by the Kern Council of Governors (KernCOG) in 1986.

The central alignment would extend the freeway west from its current terminus at M St, through Downtown Bakersfield between 23rd St. and 24th St. It would continue west through Westchester, to Oak St. At Oak St, the freeway would turn southwest, cross under SR 99 and terminate at the proposed Westside Parkway.

The southern alignment would start at SR 178 at Baker St, about 0.7 miles east of its current terminus. From there, it would turn southwest, and run parallel to Baker St, through East Bakersfield, to the BNSF railroad yard. From there, it would turn west and run south of the railroad tracks through the southern end of Downtown Bakersfield. At Bakersfield High School, it would run north of the tracks (avoiding the high school). It would continue, under SR 99 and terminate at the proposed Westside Parkway.

Although there is currently no date for the construction of the freeway extension, there are plans to increase capacity on the existing streets. 23rd and 24th St, through Downtown, will be widened to four lanes each. 24th St. through Westchester will be widened to six lanes, and close several of the local streets access, or possibly be grade separate. An interchange will also be constructed at Oak St. Construction is scheduled to begin in 2012.

There are plans to extend SR 178 as a divided highway east of Fairfax Road. When all of the projects are completed, the highway will be a four-lane divided freeway to future Vineland Road (near the old Mesa Marin Raceway) and a four-lane arterial to Miramonte Drive. These improvements are expected to be completed by 2014.

There is a plan to realign SR 178 from its current route through the Kern Canyon, to a route parallel to Rancheria Road, north of the canyon. The new alignment would run from future Vineland Road, 15 miles to China Garden, where it will meet with the existing four-lane expressway (which was complete in 1974). The goal will be to have a minimum of a four lane expressway from Bakersfield, to Lake Isabella. This would result in increasing the speed limit from 35 to 55 mph (56 to 89 km/h) and eliminate the sharp curves and steep drop-offs that exist with the current route. Currently there is no expected construction date.

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.

CalTrans guide sign along State Route 178 in Onyx.

CalTrans G9-2 guide sign looking westbound on SR178 at the eastern extent of Mountain Mesa.

SR178 is the road left of center in this view of Bodfish and Lake Isabella.

Guide sign at the east extent of South Lake in eastern Kern County.

SR178 upgrades to freeway eastbound from M Street in Bakersfield.