[4] Passing to the west of Mount Helix, SR 125 interchanges with I-8 before continuing north through the city of El Cajon near Grossmont College.

[11] Added to the state highway system in 1933,[12] and defined in 1935,[13] Route 198 extended from US 80 onto La Mesa Boulevard and Palm Avenue to SR 94 by 1938.

[26] A few months later, state engineer Jacob Dekema indicated that there were four routes under consideration for the portion from Brown Field to Sweetwater Lake.

[29] The following year, the California Highway Commission approved a routing north of Mission Gorge Road into Poway and ending at SR 56.

[33] In 1973, then-governor Ronald Reagan vetoed a bill to delete part of SR 125 from state plans, over concerns that not enough transportation studies had been done on the matter.

[51] By early 1980, SR 125 was denoted with signs saying "To 94" at Grossmont Summit; plans were to provide access north to Fletcher Parkway in the revamp of the interchange.

[57] Jacob Dekema was asked to advise the city of Poway in the dispute following his departure from the Caltrans district director position.

[58] The proposal to extend SR 125 north into Poway was discussed as late as 2003 by a local advisory group working with the San Diego Association of Governments.

[67] La Mesa residents also expressed concerns about the destruction of homes under some of the proposals for the Amaya Drive interchange design,[68] which the city council also endorsed.

[70] In August 1993, Caltrans agreed to design the new freeway to keep it west of Sweetwater Reservoir, which satisfied Spring Valley while raising concerns from those living in Bonita, who would be closer to this new routing.

The segment north to Navajo Road was due to open later that year, and the part from there to Grossmont College Drive was to begin construction in early 2001.

[78] The H. Lee House, a historic site built in 1928, was relocated by Caltrans for $1.5 million to the City Center Park in Lemon Grove.

[81] In 1990, Caltrans proposed building the southern portion as a toll road, operated by private firms; California Transportation Ventures (CTV) and PrivaCAL both applied for the role.

[89] CTV spent $14 million on improving parks, and building Little League fields and trails to offset effects of the construction of the road.

[90] However, the construction of both SR 125 and Olympic Parkway were considered by Chula Vista officials to be essential for the further development for the eastern part of that city.

[93] Early the next year, an environmental impact report suggested that Bonita might face negative effects from the freeway, such as noise and having the roadway divide the community.

[97] However, the San Diego Regional Water Quality Control Board withheld its permit approval in December 2000 over concerns that Sweetwater Reservoir could be polluted in the process.

[99] The suit by Preserve South Bay was dismissed in March 2001, on the grounds that the legislature did not need to review the annexation of state parkland for the highway because it was done under eminent domain.

Fish and Wildlife Service to block the construction of the toll road over alleged breaches of the Endangered Species Act in October 2001; however, a San Diego judge sided with the contractor and the federal government in March 2003.

The developer resolved the environmental concerns for $3.07 million and an agreement to work outside breeding times for the endangered species involved in May of that year.

[106] Over the next few years, 1.5 million cubic meters of rock were blasted through in 160 separate explosions to allow for the interchange with SR 54 to be built.

A partnership known as the South Bay Expressway operated the road, while an Australian company, Maquarie Group Ltd., handled financial and construction matters.

[116] On March 22, 2010, the toll road's operator filed for Chapter 11 bankruptcy protection, citing traffic counts running at less than 40 percent of initial estimates due to the economic downturn.

[117] An editorial in the San Diego Union-Tribune also blamed the delay in opening the toll road and the associated penalties, interest, and legal fees for the company's financial status.

[120] On July 29, 2011, SANDAG agreed to purchase the lease of the freeway from South Bay Expressway LLC;[121] even though a study suggested that the agency might have to spend millions of dollars on maintaining and repairing parts of the highway.

SANDAG claimed at the time that they would reduce the tolls to attract increased use and decrease traffic on I-805, which would mean that additional lanes would not need to be constructed on that route.

[126] It was estimated that the road would be fully paid off in 2042, and there were reports of decreased traffic on Chula Vista streets and an increase in retail business.

[127] In early 2022, Chula Vista mayor Mary Salas said that she hoped to end the toll by 2027 instead of 2042, wanting SANDAG to find a way to pay off the loans by then, stating that it is affecting the economically disadvantaged and driving away companies.

[128][129] In April, SANDAG officials reported that the regional body lost up to $1.8 million in revenue when the toll sensors were disabled between June and August 2021.

[4] For example, any cash- or credit-paying driver who passes through the Otay Mesa Toll Plaza ends up paying the maximum rate of $3.50 regardless of the distance they traveled.