California State Route 42 was a east/west State Highway located in the greater Los Angeles area. California State Route 42 was aligned from California State Route 1 near Los Angeles International Airport eastward to via Manchester Boulevard and Firestone Boulevard to Interstate 5 in Norwalk. The designation of California State Route 42 was an early 1960s numbering swap from California State Route 10 to avoid duplication with Interstate 10. California State Route 42 was slated to become part of the Century Freeway before the corridor was added to the Interstate System in 1968 as Interstate 105. Upon the completion of Interstate 105 the surface segments of California State Route 42 were relinquished. Despite being relinquished California State Route 42 remains signed in places such as above via southbound La Cienega Boulevard (courtesy M3100 of the AAroads forum). Below California State Route 42 can be observed on the 1990 Caltrans Map with a legislative designation of Route 105 prior to the completion of Interstate 105.
The history of California State Route 10 and California State Route 42
The genesis point for California State Route 10 ("CA 10") in the Los Angeles Area dates to 1933. 1933 was significant due to the State Legislature allowing the Division of Highways to assume maintenance of highways in Cities for the first time. This action by the Legislature led to a large addition of highway mileage throughout the State of California during 1933. One such addition was Legislative Route Number 174 ("LRN 174") which was added to the State Highway System was the following definition:
a. [LRN 60] via Manchester Avenue (note; Manchester Avenue and Boulevard are often shown interchange in official documents) to [LRN 2/US Route 101] near Miraflores
b. [LRN 2] near Orange County Hospital to Main Street, Santa Ana via Santa Ana Boulevard
In the August 1934 California Highways and Public Works Guide the Sign State Routes were announced. CA 10 was applied to the entirety of LRN 174 between CA 3/LRN 60 and US Route 101/LRN 2 in Santa Ana.