 By 1947, US 395 ran concurrently along the portion of SR 78 from Vista to Escondido before continuing along Santa Fe Avenue to Bonsall and Fallbrook and rejoining its alignment during the 1970s. At this time, all of SR 78 that existed had been paved. Before the present-day freeway was built, SR 78 was routed on the Vista Way Freeway (which was an expressway) from Oceanside east to downtown Vista. After this, it followed Santa Fe Avenue and Mission Road east (now signed as CR S14), continuing onto Grand Avenue in Escondido. Following the intersection with US 395, SR 78 turned south on Ash Street and rejoined the current alignment of the highway. In 1949, the rerouting of SR 78 from US 395 to US 99 was listed as a priority by local officials. The road was known for its curves, even though it went over relatively flat terrain; this reduced its efficiency.
