California State Route 178

State Route 178 (SR 178) is a state highway in the U.S. state of California that exists in separate constructed segments. The western segment runs from State Route 99 in Bakersfield and over the Walker Pass in the Sierra Nevada to the turnoff for the Trona Pinnacles National Natural Landmark. Though some maps and signs mark this segment of SR 178 as continuous through Downtown Bakersfield, control of the portion between the Kern River Bridge and M Street was relinquished to the city and is thus no longer officially part of the state highway system. The eastern segment of SR 178 runs from the southeasterly part of Death Valley National Park to Nevada State Route 372 at the Nevada state line west of Pahrump. The gap between Trona Pinnacles and the southeasterly part of Death Valley is connected by various local roads and State Route 190 through the park.

SR 178 serves many different purposes. It connects Downtown Bakersfield with East Bakersfield and Lake Isabella. It is one of three crossings over the Sierra Nevada south of Yosemite (SR 120, Tioga Pass Road), 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 through Death Valley was built, it would provide a continuous route between Bakersfield and the Las Vegas Valley via Pahrump.

SR 178 is part of the California Freeway and Expressway System, and through Bakersfield and Ridgecrest is part of the National Highway System, a network of highways that are considered essential to the country's economy, defense, and mobility by the Federal Highway Administration. It is eligible for the State Scenic Highway System, but it is not officially designated as a scenic highway by the California Department of Transportation.

The western segment starts in Bakersfield at State Route 99 as 24th Street, but splits at B Street, utilizing 24th Street for westbound traffic and 23rd Street for eastbound traffic, through the central business district. Officially, the segment between the Kern River Bridge and M Street was relinquished to the city and is thus no longer legally part of the state highway system. SR 178 then becomes a freeway as it leaves Downtown and winds through East Bakersfield. The freeway then travels east as it enters Northeast Bakersfield. A mile east of the Morning Drive interchange, the freeway segment ends with the first at-grade intersection at Canteria Drive, where the road widens to a six-lane undivided expressway. The highway continues through the rural, but growing Rio Bravo neighborhood of Bakersfield, and intersects State Route 184. Turning northeast, the road narrows — first to four lanes at its intersection with Masterson Street, then to two lanes at Miramonte Drive — and continues to the mouth of the Kern River Canyon, where the highway exits the northeastern city limits of Bakersfield.

Over approximately 13 miles through the Kern Canyon, SR 178 closely follows the Kern River as a narrow two-lane road, with an average width of 18 to 24 feet. Though the speed limit on this length is signed for 55 miles per hour (89 km/h), the average travel speed is often much lower, owing to sharp turns in the road around sheer rock faces, and steep drop-offs into the river below. From the canyon mouth at 650 feet (200 m), SR 178 ascends through the lower Sierra Nevada, and climbs to its exit of the canyon at 2,250 feet (690 m). Here, the highway becomes gentler, and widens to a four-lane undivided expressway.

The route continues east and reaches the town of Lake Isabella in the Kern River Valley. The road briefly expands to a divided freeway, and meets a junction with State Route 155 at an overpass, before narrowing back to a two-lane undivided highway at the intersection with Lake Isabella Boulevard. The road then follows the south shore of the Lake Isabella Reservoir, until it reaches the town of Mountain Mesa, then continues to community of Onyx. Here, the route begins its ascent out of the Kern River Valley, and climbs from 2,795 feet (852 m) at Onyx over approximately 18 miles to the summit of Walker Pass at 5,250 feet (1,600 m). Then, the highway descends eight miles to an intersection with State Route 14 at Freeman, an elevation of 3,195 feet (974 m).

SR 178 runs concurrently with SR 14 for three miles, then proceeds eastward across the Indian Wells Valley into the city of Ridgecrest. The constructed highway then ends east of Ridgecrest at the turnoff for the Trona Pinnacles National Natural Landmark in Searles Valley. The right-of-way then continues north as the county-maintained routes of Trona Road, Trona-Wildrose Road, and then Panamint Valley Road, running through the community of Trona to SR 190 east of Panamint Springs.

The Eastern 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. SR 178 then branches northward from SR 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.