The winding two-lane road through the Cuyamaca Mountains was one of the factors that led to a four-hour journey from San Diego to El Centro.

[5] By 1933, the routing had shifted to use Park Boulevard south of University Avenue into downtown, to end at Market Street; the city council voted that July to use University Avenue west of Park Boulevard to connect to US 101, to obtain federal funding for improving that road.

[22] At the time, the Ellis Wayside Rest/Ellis Spring provided a rest area and water for travelers; later, the state recognized it as a historic site.

[23] Exiting the national forest, US 80 continued in a southeasterly direction towards Live Oak Springs and Boulevard, intersecting with the eastern terminus of SR 94.

[1] US 80 then descended rapidly into Imperial County along the In-Ko-Pah Gorge just west of Coyote Wells and Ocotillo before entering the city limits of El Centro.

US 80 cut a southeasterly trajectory, running parallel and very close to the Mexican border, and traversing the Algodones Dunes and the Colorado Desert.

[26] Before a highway was constructed through the Imperial San Dunes, travelers had to pass to the north in order to reach Yuma.

[27][29][30] The delay in constructing a road to San Diego caused increased development in Los Angeles and resulted in that city becoming the trade and population center of Southern California.

[39] Meanwhile, Imperial County businessmen hoped to promote the use of this road over the route from Phoenix to Los Angeles, to increase tourism.

Although the city raised concerns about the proposal, due to the shift in transportation design towards "free-access highways" where vehicles could travel up to 60 miles per hour (97 km/h), and the success of the Arroyo Seco Parkway in Los Angeles, the La Mesa Scout newspaper withdrew their objections to the construction.

[45] Funding was obtained by 1949 for the 5.4 miles (8.7 km) portion between 70th Street and Lake Murray Boulevard, and Fairmout Avenue.

Reports indicated that this decreased the time to travel from La Mesa to San Diego by a factor of two, with 7,000 trips on the route each day.

[45] That same year, the San Diego city manager expressed concerns at the Highway Commission meeting about shifting US 80 away from El Cajon Boulevard to the Alvarado Canyon and Mission Valley Road route, due to the potential loss of commerce for businesses located near the current route.

[50] In September, the county supervisors approved the Mission Valley road as a limited access highway, but held off on processing the results from surveying.

[57] More funds were allocated by the State Highway Commission a few months later, and the work was to be put out for bid to complete the project.

[60] However, many El Cajon Boulevard motel owners raised objections over the reassigning of the US 80 designation to the new freeway, as it would potentially harm business.

In response, the Highway Development Association proposed designating El Cajon Boulevard as a business route.

[63] Following the city of San Diego council and county approval, the California State Assembly transportation committee heard complaints from citizens claiming that they had not been adequately notified about the plans for US 80 or SR 94, or given enough time to provide input.

[65] The next year, the state director of public works announced that the route would be rebuilt to the south for 7 miles (11 km) as a freeway.

[71] In April 1955, a Caltrans official announced that US 80 would be built as a freeway for its entire length in California, as it was to be included in the Interstate Highway System.

[72] In the meantime, while plans moved further along on US 101, San Diego Mayor John D. Butler asked the California Highway Commission to keep the US 80 project on track.

[77] Finally, in May 1957, bids opened for the part of US 80 between Grossmont Boulevard in La Mesa and Chase Avenue in El Cajon, that would connect to the new SR 67 freeway.

[87] In January 1960, the Taylor Street to Cabrillo Freeway portions were under construction, including the interchange at the latter, and were scheduled to be complete by October, while the Cabrillo Freeway to Fairmout Avenue portion was up for bid;[88] the next month, the contract was awarded for $4.6 million, with interchanges to be constructed at Texas Street and Ward Road to replace traffic signals.

[91] But in 1960, Jacob Dekema, the district highway engineer, stated that due to lower traffic on US 80, the completion of the freeway through the mountains would be delayed until at least 1971.

[93] That year, work continued on the freeway, and in August, only one traffic signal was left on the highway west of El Cajon, at Ward Road.

[101] That year, US 80 west of the Cabrillo Freeway interchange was declared to be the busiest road in the City of San Diego, at 71,000 daily vehicles.

This supports the California sections by extending the Historic U.S. Route 80 designation through Arizona to the New Mexico state line.