From this circa 1931 photograph, we can see that oil was king in Los Angeles. If there was an oil well in the way of building a road—even a major one like La Cienega Boulevard—you build around it and everybody wins. This well was just south of Beverly Blvd so the Beverly Center shopping mall is now where that empty lot is on the left.

I tend to think that the “SLOW” sign is somewhat superfluous:

That same view in January 2018: