East of Santo Road and west of SR 125, the highway goes through Mission Trails Regional Park, a large open preserve.

Plans for a route between La Jolla and Santee date from 1959, and SR 52 was officially designated in the 1964 state highway renumbering.

Funding issues delayed the completion of the entire route until 2011, more than fifty years after construction began; until then, the city of Santee faced traffic snarls.

A widening project was completed in 2007 between Santo Road and Mast Boulevard; further expansion has been put on hold due to state budget concerns.

Before entering San Clemente Canyon, part of Marian Bear Natural Park,[3] the road becomes a freeway as it intersects I-5.

The freeway runs north of the Rose Canyon Fault, composed of Late Cretaceous rock estimated to be 90 million years old, and Mount Soledad.

[3] After this interchange, the freeway leaves the edge of the military base and enters the San Diego neighborhood of Tierrasanta, where there is a junction with Santo Road, before traversing Mission Trails Regional Park, an open space preserve, for a few miles.

[5] The mountain is part of the Peninsular Range; the highway cuts through Eocene rocks estimated to be 50 million years old and marine fossils.

The road on the eastern side of the mountain is carved out of "igneous granitic rocks" that are thought to be 150 million years old, an unusual formation compared to the Eocene layer.

[4] A dedicated two-way bicycle path exists on the northern side of the roadway between Santo Road and Mast Boulevard, with access possible from both termini.

The freeway continues east through Santee, with interchanges at Fanita Drive, Cuyamaca Street, and Magnolia Avenue, before it comes to an end at SR 67.

[9] In 2016, Caltrans officially designated the segment adjacent to Mission Trails Regional Park between Santo Road and Mast Boulevard as a scenic highway.

[14] In 1959, Legislative Route 279 was designated as a highway from La Jolla to Santee and incorporated into the California Freeway and Expressway System.

City and state officials indicated that the Soledad Freeway would be constructed in the northern part of the canyon to minimize environmental damage.

[21] On November 18, San Clemente Canyon Road was connected to I-5 when the aluminum guard rails arrived and were subsequently installed.

The U.S. Navy was consulted in the planning process due to the road's proposed routing through what was then NAS Miramar that would provide a delineation against further urban development.

[34] In 1977, the county supervisor, a San Diego City councilman, and the mayors of La Mesa and National City wrote a letter to Brown to ask for the construction of this portion of SR 125 and other freeways, due to concerns about the types of congestion seen in Los Angeles coming to San Diego due to the incomplete freeway system.

[43] The trial was considered successful as more than 99 percent of trips arrived on time, customer feedback was favorable and no safety concerns were encountered.

The dip had developed in a section of the highway constructed on top of the Miramar Landfill, and had been caused by trash settling; it was repaired by the next day.

The City of Santee opposed the original plans, hoping to route the freeway on Prospect Avenue or north of the San Diego River.

[54] In January 1987, the Santee City Council voted to commence a study of a more northern route, even though local residents and workers objected that this would postpone construction.

[55] In March, the study, done by BSI Inc., supported the Caltrans decision to abandon plans for the northern path due to the increased cost from the "mountainous, undeveloped" terrain.

[56] The council voted to support a southern alignment through the town, with both the Prospect Avenue and San Diego River alternatives still viable.

[57] In the same month, the mayor of La Mesa, Fred Nagel, started a petition drive supporting the extension of the freeway due to the recurring traffic on I-8.

Fish and Wildlife Service expressed concerns that construction near the San Diego River crossing might result in the destruction of the endangered least Bell's vireo habitat.

[72] Environmental concerns raised in June 1988 related to the least Bell's vireo included decrease of insects, a darker environment after overpasses are constructed, and the fragmenting of habitat.

[73] Upset because of the delays, the Santee City Council wrote to U.S. senators Pete Wilson and Alan Cranston, asking for their assistance.

[75] In March 1989, the Fish and Wildlife Service agreed to the project, on the conditions of altering the route to cross the San Diego River and pass east of the center of the town, and constructing 44 acres of additional habitat as mitigation.

[29] In April 1990, the Santee City Council agreed to begin purchasing land, over the environmental concerns of Councilman Jim Bartell.

There were reports of faster transportation through the East County area, yet commuters noted a rush hour backup at SR 125 headed westbound because there were only two lanes traveling west through the interchange.