Because of my post on the traffic study in 1926, The Otterman sent this copy of a postcard showing traffic at "Wiltshire" and Western, probably in 1924. Look at the signal device--the white stand with semaphore-style flags in the middle of the intersection. A comment to his post calls this an American Bobby. By 1926, the Bobby was gone and the intersection was clear.
Here's a cropped close-up. As the Otterman wrote, it looks like motorists treated the intersection as a roundabout:
In an October 1920 article from the Los Angeles Times, such a traffic signal is described. It had been tested on Broadway for a week "to howling success," so forty such signals were to be installed at congested corners (not intersections!) in the city.
The signal "behaves like a cross between a railroad semaphore and an alarm clock. The apparatus consists of a tall pole with a couple of semaphore arms which are operated from a central headquarters. As the arm moves them "Go" to "Stop" a bell jingles merrily in the crow's nest calling attention to the signal."
Huh? I got lost at crow's nest, but I guess that pillar is tall. This next paragraph tells us how traffic was controlled before these signals were installed:
"On Thursday the regular traffic officers left their posts in the center of the crossings and stepped off to one side to allow the things to run traffic all by themselves. Except for the inevitable fatheads who thought that the tinkling bells were part of our entertainment for tourists and proceeded on their way without looking at the semaphores the apparatus worked fine."
A police captain hoped these new signals could be combined with a device to control pedestrian traffic. Then, "we'll be far ahead of the rest of the country in traffic regulation." For the record, according to sites such as UC Berkeley's TechTransfer site, Detroit was way ahead of us and most other cities. They got their first "traffic tower" in 1917, and three years later in 1920, one of their officers, William L. Potts, invented the green-yellow-red four-way signals we obey today.
As for the "American Bobby," that showed up in a Proquest search too--in 1924, the first such signal was installed at Wilshire and Vermont. It stood five-and-a-half feet tall, and flashed red and green lights at drivers. The 1924 article is specific that lights are used instead of semaphore arms, and says one was to be installed at Wilshire and Western. Look at picture above: it definitely has semaphore arms. Was it the 1924 installation or an earlier model?