California State Route 39

Although defined to be a continuous route in the California Streets and Highway Code, the segment from Whittier Boulevard in La Habra to West Covina is not officially adopted or signed by Caltrans. This is indicated at the intersection of Whittier Boulevard and Harbor Boulevard, where an "END 39" sign appears.

Prior to the present before reaching Harbor Boulevard, SR 39 continued north from Whittier Boulevard along Hacienda Road to the Los Angeles/Orange County line then north on Hacienda Boulevard and Glendora Avenue to US 60, 70, and 99 (Garvey Avenue, now Interstate 10) in West Covina.[12] It then continued east with US 60, 70, and 99 to Azusa Avenue where it turned north to follow the present alignment as described beginning in the fourth paragraph of the preceding section. The Hacienda Glendora segment can still be seen as Route 39 on some maps.

Prior to 1991, Harbor Boulevard became Fullerton Road northbound from the Los Angeles/Orange County Line, through the Puente Hills as a one-lane winding road into Rowland Heights. However, due to complaints of nearby residents due to the increased volume of traffic, a straighter, wider stub was built slightly to the east, and was named Harbor Boulevard. This route is under consideration to become part of SR 39 to complete the gap.

The new Harbor Boulevard was opened to the public early in 1992, and is now the primary corridor between Orange County and Rowland Heights, although the original winding Fullerton Road segment still exists as a strictly residential street.[13]

The definition of Route 39 in the California Streets and Highway Code was thus changed accordingly, and the traversable route to fill the Route 39 gap would be via Harbor Boulevard north, Fullerton Road north, and then Colima Road west to Azusa Avenue in Hacienda Heights. Route 39 would then continue north on Azusa Avenue through the City of Industry to Interstate 10 in West Covina.[4][14] This segment however has yet to be officially designated by Caltrans.[15]

SR 39 closure in the San Gabriel Mountains

Locked gate at northern terminus of Highway 39

Map of the closed and incomplete section; the ca. 1967 reconstruction is shown in blue

The northernmost 4.5-mile (7.2 km) stretch of SR 39, from Snow Springs north of Azusa to SR 2 at Islip Saddle, has been closed to public vehicle traffic since 1978 due to a massive mud and rockslide. While the road is passable to emergency vehicles for the entire closed length, sections near recurring rock slides have become narrow dirt roads which are not suitable for passenger vehicles. According to Caltrans officials, it will take three years to complete the repairs.

People heading to nearby Mount Waterman must instead travel west to Pasadena and join SR 2 in La Cañada Flintridge - a nearly two-hour trip. Reopening SR 39 would cut the drive-time to the Waterman ski area in half and shorten the trip east to Wrightwood.

The segment was opened to emergency crews in February 2003 after a Caltrans study showed reopening it would not harm wetlands, air and water quality, natural vegetation or threatened plants and animals.

On July 10, 2006, the Pasadena Star-News reported that the state provided funding for an Environmental Impact Report (EIR) in June 2006 to assess the feasibility of reopening the highway in the Angeles National Forest.

According to the Caltrans District Seven "Inside Seven" newsletter, work was to continue through 2009 and 2010 on reconstruction and installation of features to prevent future slides from damaging the roadway:[16]

Two projects that will address those issues and get the highway reopened are scheduled for construction soon. The first, building two retaining walls near the city of Azusa from Old San Gabriel Canyon Road to approximately four miles south of SR-2, could begin in mid-2009. The second, a $45 million project to reconstruct the roadway, construct soldier pile retaining walls, repair drainage systems, install rock fall protection, and provide asphalt concrete overlay and traffic striping, should begin in fall 2010.

However, due to budgetary issues, by 2012 Caltrans was instead trying to transfer responsibility for the entirety of SR 39 north of postmile LA 17.81 (including the closed section) to either the U.S. Forest Service or Los Angeles County, both of which declined, also for financial reasons, even though both the USFS and the county deem the route "essential". Abandoning the route would also not be cost-effective for Caltrans due to, among other reasons, federal environmental restoration requirements related to the presence of bighorn sheep.[17] The proposal was abandoned after opposition by local residents and the county, as well as substantial legal pressure by the Forest Service, who threatened to enforce an old contract that requires Caltrans to completely deconstruct the highway should it refuse to maintain it.[17] There are proposals to move the closed gate north two miles to a popular trailhead.

On December 15, 2022, Caltrans held a meeting to consider several proposed plans to reopen SR 39. Proposals include a full restoration, minimal restoration for emergency services, a single-lane road, or recreational use. Under the recreational use proposal, the restored route would be closed to general traffic, but would allow for hiking, biking, and other recreational activities.[18]