This excellent short essay by Jessica Gross can be accessed at http://www.nytimes.com/2013/
In preparing for my interview with Jessica I learned quite a bit about the history of the turn signal, admittedly at the periphery of history. Electric turn signals were first patented in the 1920s, with an important advance made in the 1930s that facilitated the "flashing" aspect of this indicator device. And by the late 1930s at General Motors Buick first, then other brands, gradually installed turn signal apparatus -- although still by the 1950s many folks actually bought aftermarket kits for their cars. In Europe, and particularly Britain and to a lesser degree Germany, the Trafficator -- a semaphore signaling device was favored to the 1950s. Quaint, but difficult to see in critical situations.
For me, a big question is why do people often not use their turn signals! Is it lazy, or screw you, or distracted. Or does your particular unit not cancel properly and you are afraid of going down the road like an old person. When I don't use them, it is a combination of all of the above, but clearly when I am miffed, it is because I feel like flaunting the world and its rules.