Almost everywhere I have lived, there is a small, frustrating percentage of people that don't use their turn signals to indicate what they are doing, so as to keep traffic flowing well and prevent accidents.
Here is Weber County, Utah there is an epidemic of folks that don't use their turn signals. No signal for lane change, merging, or turning.
This morning was the ultimate example. Two cars were at a stop light in the straight/left turn lane. Neither of them were signaling. The light turns green and they both sit there for twenty seconds looking at each other. Then both of their left turn signals come on and they proceed with their left turns.
It would've been a little humorous, if I was not stuck behind one of them, waiting to make a left turn myself. I shook my head in disbelief of what I had just witnessed.
Usually the people I see not using their signal are people whose lane is ending and they are merging into traffic. They appear to either be oblivious to the fact that it is their lane that is ending and not the lane they are merging into or they assume that people are so courteous here that they will just back away and let them in.
There may be some validity to their assumption, if they are indeed counting on the courtesy of others, but it is a dangerous assumption to make. Many around here do extend "courtesy" to those that are in front of them, regardless of how rude they are being to those behind them or what the law is. One should still signal and look before merging, in spite of this.
I saw a similar phenomenon is Indiana. People generally wanted to be courteous and would stop and let people into the flow of traffic. That is great except for in the case where one stops where they have no stop sign to let a person with a stop sign go, because the one at the stop sign is not getting a break to come out. This confuses everyone. The person stopped know they have a stop sign and yet are being motioned to move out into traffic. The people behind the "courteous" person are confused because they may not see what charitable act they are trying to perform, thus they might trying to go around them and then crash into the person that was let out when it was not their right of way.
It also isn't great when a "courteous" person let's multiple people merge out in front of them, while the line of cars that has been patiently waiting for a very long time get delayed longer and longer. If everyone let's in one car, traffic in both lanes with flow.
Another traffic thing I see here, which is either courtesy or the fear of being hit is letting a left turner with a green arrow turn into the far right lane, while a line of right turners coming from the other way sits and waits on them. The person at the front of the line generally just sits there and waits for the green arrow from the other way to go away, even though there are two lanes and the left turners should stay in the left and the right turner in the right. If they need the other lane once they turn, they should signal and keep the traffic flowing.
Instead what happens is that a line of right turners back up and it takes forever to clear them out, because there are too many of them to make it through the next light after they turn. In the meantime all of the left turners that drifted in to the right lane have cleared out because they are making a subsequent right turn.
If both would stay in their own lane, then the left turners want to make a subsequent right would still move out and the right turners making a subsequent left turn would be able to trickle through the next light and not back up.
Even though folks around here generally don't use their turn signals, they do make turns. Left turners often cut their turn short, drifting into the left lane of on coming traffic after the turn. If there are two turn lanes the right most lane will drift into the left lane and further encourage the left lane to head into oncoming traffic.