1952, Four-level interchange. Where the Hollywood/Santa Ana Freeway meets the Pasadena/Harbor Freeway. This interchange is the first 4-level interchange ever built in the world. Note the route signing; US Route 99 is now Interstate 10; the Hollywood Freeway is still US Route 101. Of course US Route 66 doesn't exist, the Pasadena/Harbor Freeway now being California State Route 110/Interstate 110, respectively. California already had an extensive highway/freeway system which predated the Federal Interstate system; this was the reason why California was exempt from having exit numbers until fairly recently.
From the USC archive.
The Four-level, 1953
From the USC archive.
Aerial view of the four-level, 1970
From LAPL
The four-level under construction, late 1940s
From LAPL
Wilshire and Bonnie Brae, 1937. From the USC archive.
By the 1920s, LA already had the most cars per capita than any other city on earth at the time. I'm sure it was scenes like this that led to the encouragement of the building of freeways. Reading through old LA traffic plan books from the 1950s at the LA Central Library, it's funny to know that back then, freeways were really seen as a salvation for many, as they would "forever free up traffic from surface streets and cut down travel times across the city."