Western segment

SR 178 follows the Kern River through the Kern Canyon, just northeast of Bakersfield

The first segment starts at State Route 99 just west of Downtown Bakersfield. The road continues as 24th Street but then splits at B Street, utilizing 24th Street for westbound traffic and 23rd Street for eastbound traffic through 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. A mile east of the Morning Drive interchange, the freeway segment ends with the first at-grade intersection at Canteria Drive. The highway continues through the rural, but growing Rio Bravo section of Bakersfield, crossing SR 184. Turning northeast, the road narrows to a 4-lane highway and continues to the mouth of Kern Canyon (which is also the northeastern 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. 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 Boulevard. The road continues to wind until it ascends to Walker Pass, an elevation of 5,250 feet (1,600 m). The highway then descends from the mountains to its junction with State Route 14. It then proceeds eastward across US 395 into the city of Ridgecrest. The constructed highway then ends east of Ridgecrest at the turnoff for the Trona Pinnacles National Natural Landmark. 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.

Eastern segment

Looking west from the state line

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. 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.

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.

Abandoned western freeway extension

The freeway portion of Route 178, completed in the late 1950s, ends as it approaches downtown Bakersfield from the east. Weak public support and subsequent lack of funds hindered efforts to complete the freeway through downtown and Westchester to its proposed terminus at the beginning of the modern-day Westside Parkway. Although freeway alternatives through various neighborhoods have been studied several times following the existing portion's completion in 1968, the city of Bakersfield never formally endorsed a route. Caltrans formally announced preference for an alignment through downtown and Westchester in 1973, but fierce public opposition coupled with the historic integrity of the neighborhoods slated to be demolished greatly hindered momentum of the project. In 1977, then-Governor Jerry Brown issued a moratorium on all new freeway construction. Formal studies on possible routes continued and resulted in recommendations of a more southern alignment near Truxtun Avenue executed by the Kern Council of Governors (KernCOG) in 1986 and the 2001 Bakersfield System Study.

A Westchester alignment would have extended the freeway west from its current terminus at M Street and through Downtown Bakersfield in the vicinity of 23rd street. As it continued through Westchester the freeway would turn southwest, cross under SR 99 and terminate at the newly completed Westside Parkway.^[13]

A southern alignment would start at SR 178 at Baker Street, about 0.7 miles east of its current terminus. From there, it would turn southwest and run parallel to Baker Street, through East Bakersfield, to the BNSF railroad yard. It would then 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.^[14]

City of Bakersfield 24th Street Improvements

Still needing to address the increasing crosstown congestion and with a freeway through downtown effectively out of the question, the City of Bakersfield approved construction in the late 2000s to increase capacity on 24th Street through Westchester and 23rd and 24th streets through Downtown using TRIP funds. The project widened, realigned and re-striped 24th Street from west of State Route 99 to east of M Street, as well as realigned and re-striped 23rd Street from west of C Street to east of M Street, in order to add two travel lanes (one in each direction) to the roadway. The project also encompassed improvements to the 24th Street/State Route 99 interchange and widened the Oak Street/24th Street intersection.^[15] This solution required the acquisition of several properties north of 24th Street, which was completed in early 2015, as well as the closure of access of B Street through Elm Street from the north. The project was completed in December 2020.^[16]

Death Valley Segment

This segment is unconstructed from 15 miles east of Ridgecrest to 15 miles west of SR 127 (10 miles from the eastern boundary of the Death Valley National Park at what had been its former boundary prior to 1994). The state is considering three options: select an alignment to connect with the current eastern segment at its present location; select an alignment to avoid traversing the Death Valley Wilderness; or delete this unconstructed segment from the State Highway System.^[17]

The County of Inyo has offered three alternatives:

• The State of California rescinds the unconstructed portion of SR 178.
• The State of California rescinds the unconstructed portion of SR 178 and adopts Trona Road, Trona-Wildrose Road, and Panamint Valley Road.
• The State of California rescinds the unconstructed portion of SR 178 and adopts Trona Road, Trona-Wildrose Road, and Panamint Valley Road; Inyo County adopts portions of SR 178 and SR 190, and Death Valley National Park adopts the portion of SR 178 that connects to Badwater Road in the park.^[17]