That's a very culture-dependent thing, it's much more unusual for me to see someone not using turn signals in Sweden than it is in the USA or Brazil. If no one around you uses them I'd blame driving licence requirements, enforcement, and training for it.

Everywhere I've ever lived or driven, the vast majority of people use them consistently - at least when appropriate. The most common place I see people not using them is when changing lanes, and then usually only if there's a vehicle in their target lane close enough that it would matter.

Do bad drivers exist? Obviously.

Actually, now that I really think about it, there are two places I've driven where I can recall seeing people merging and turning frequently without signaling: New York and Los Angeles.

In those cases, my guess is that it's partly cultural but predominantly driven by task saturation. When you're operating a vehicle at 80 MPH surrounded by other cars in rush hour traffic on a six-lane highway, sometimes you have to focus on staying alive.

Aviation has a saying that describes this: "Aviate, navigate, communicate". Basically, your first priority is to ensure the immediate safety of your vehicle. After that, you focus on identifying a clear path that doesn't conflict with the path of other vehicles. Only after those things are done do you worry about telling others what's going on. In the air that's the radio; in a car that's your turn signals and horn.

Here's the Wikipedia article talking about this in the context of aviation: https://en.wikipedia.org/wiki/Airmanship#Principles

If task saturation is actually what's happening here, then I would expect to see turn signal usage decrease as traffic density and speed increase. I bet, given the access to data like live traffic cameras that we have today and the current generation of multimodal generative AI models, I could whip up at least a first pass at measuring this without much time investment.

Great. Now I have yet another project :P

The problem with green on the front is that cars stopped at a light at night will be presenting a green light to oncoming traffic that should also be stopping, possibly overriding the red traffic light or stop sign.

A trick I learned years ago from a motorcyclist: pay attention to the front wheels of cars near you in traffic.

Very often, the front wheel turning is the first noticeable visual indication that a car is changing lanes or turning. That gives you a few more fractions of a second in a situation where they could be very important.

After a while, I noticed that I was instinctually able to tell more accurately if a car is accelerating or decelerating. After considering it, I realized that I actually could tell better than before, but it wasn't instinct. Because I was watching cars' front wheel, I was able to see the front suspension load and unload as they accelerated or braked respectively. It was just subconscious.