Cool find! I looked at this for a number of seconds and then I knew where this is. The drawing isn't to scale, but this is the southern edge of the modern Chinatown, where Broadway intersects Cesar Chavez Ave. (of which that stretch used to be Sunset Blvd.), the view is looking south. The tunnel is the Broadway tunnel, which no longer exists, because most of the hill has been removed. The viaduct was never built.
I don't remember if I posted these photos a while back and hehe I'm too lazy to look through the pages of this thread, but here are some comparison photos,
courtesy of the LAPL:
Sunset and Broadway looking south, 1929:
Same view, 1951:
Same view, 1971:
As you can see the hill and tunnel are gone. I remember when I first learned that there used to be a hill and tunnel there and was totally surprised. The one story brick building on the corner still exists today.
Here's the same general view today, courtesy of Google Earth:
That viaduct plan is interesting. I'm very sure it was a plan to make traffic flow more smoothly, especially considering back then, there were the PE streetcars going through that area. The viaduct looks similar to one that does exist a little west of this location, where 1st St., 2nd St., Beverly Blvd. and Glendale Blvd. all intersect.