I have a theory. OK, some people simply can't drive, we all know it. But I noticed when I visited Nashville and drove a rental car, suddenly my ability to use turn signals went gone. And I'm religious about it, signalling is a reflex, don't think about it.

Around my little town, where no one is a visitor, everyone uses their signals. OTOH, go to the nearest city and visit downtown, especially on an event day like a graduation, people are goddamned maniacs! No signals, driving the wrong way on one-way streets, speeding, creeping, all that.

Next time you catch someone not signalling, have a look at their plate. Out of state? Hell, some states like Mississippi will even tell if they're out of county. See what you see. Note: Plates may not mean much if you're in a city that pulls unfamiliar drivers. Bet my theory's solid near airports!

My little idea holds up on country highways. Everyone knows exactly where they are and where they're going, be shocked to see a single missed signal. Worst you see is some hesitant on/off signaling if they're unsure of the upcoming turn. (Hey, it all looks the same from the highway.)

tl;dr: Drivers unfamiliar with the area are mostly the problem.