Before the late 1940s, automobile turn signals were an after-market item. At the Automotive Addictions blog, Bobby Green displays a wide variety of amazing accessory turn signals from the 20s and 30s that came in all kinds of cool shapes and styles.
From his post:
As necessity is the mother of all invention, you can only imagine how many model T's and A's smashed into each other due to the lack of information form one driver to another as to which direction you were about to suddenly go. Of course everyone could use hand signals, but you can imagine how many people actually did, I mean,… what if it's raining ?.. are you gonna roll down your window and stick your arm out of the car every time you want to turn? Probably not.