The scenic highway begins in the west at its junction with Interstate 5 located south of Gorman in the Sierra Pelona Mountains, continues eastward through the Antelope Valley and Cajon Pass, to its junction with State Route 18 in the east, located in the San Bernardino Mountains south of Crestline.

The entire segment from Interstate 15 to the eastern terminus of State Route 138 at Mount Anderson Junction is known as the El Cajon-Skyline Forest Highway.

Because of its twisting, mountainous segments and overloaded traffic conditions on its eastern leg, State Route 138 east of Palmdale and west of Interstate 15 is the site of numerous serious auto accidents as of 2004, according to CHP data.

One of the chief contributors to accidents on Route 138 of late is drivers passing on the two-lane highway in unsafe conditions.

[11] Improvements to the six-mile (9.7 km) stretch between Highway 18 and the Los Angeles/San Bernardino County Line, including widening the shoulders and installing rumble strips, were proposed in February 2013, with no timeline for completion being given.

The $8 billion project would have included an eight-lane freeway between Lancaster and Victorville, along with a bikeway, rapid transit, and part of Brightline West, a proposed high-speed rail line linking Los Angeles to Las Vegas.

[14] Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, based on the alignment that existed at the time, and do not necessarily reflect current mileage.

David Hockney composed the picturesque photographic collage Pearblossom Highway in 1986 off the segment of Route 138 bearing that moniker.