Have you noticed that fewer and fewer drivers use their turn signals when changing lanes on highways? Of course, one reason is that it's hard to hold a cell phone to one's ear, steer, and use the turn signals. And despite the ban on hand-held cell phone use in cars (in most states), this law is rarely enforced, so there are plenty of drivers who don't have enough hands to steer, talk on the phone, and use turn signals. And in the case of texting while driving, you'd need to be a juggler to manage all three.
Then there are those who really don't think turn signals are needed any more. They are also the ones that typically turn right on a stop sign without stopping. They're in a hurry, and what's the point?
For me, it isn't just a courtesy to let other drivers know what you're up to and where you're going, it's good defensive driving that gets you where you're going without mishap..
Modern cars are a great deal safer than they used to be, but they have more blind spots than they used to, too. The most obvious one is to the left rear of the driver's view, where a car passing you is temporarily invisible. But head-rests and smaller windows in most modern cars obstruct views to the right rear as well. For example, it's very difficult to see a car that's entering the middle lane from the right lane, while you're moving back into the middle lane just after passing a car on the left. For both situations, using a turn signal is critical. Since a car already in the left lane can easily see you in the middle lane, and a car moving toward you from the right lane has a clear view of you, if you have a turn signal on, there's at least an indication that you're about to have a collision, and the situation can be avoided. But since cars CAN be in your blind spots, you can never be certain when they are, and that's why using a turn signal is so critical (even when you don't think it's needed--in fact, that's exactly when it's needed).
I've noticed that a lot of people use their turn signals when they shouldn't. It's when they are in the middle lane and presumably know that you are coming up in the outside lane, so they put on their turn signal to let you know that they are intending to pull out, presumably after you've gone by. But how are you to know that? Maybe they didn't actually see you. Why don't they wait until they know (or presume) that the lane is clear, and then use their turn signal? In situations like this, I don't take a chance. If I see someone in the middle lane put on their signal and there really isn't room for them to safely enter my lane, I put my brights on and honk the horn (and typically get the finger from the driver). But at least I've avoided an accident if they simply didn't realize I was there.
I notice how readily police will stop people speeding, but I've never seen anyone pulled over for not using a turn signal. So the habit of not using them becomes more of a norm, and it's obvious from the skid marks on major highways that many accidents are indeed caused by drivers failing to use turn signals and sending other drivers into the median, or worse.
In the end, I doubt whether anyone can persuade drivers to always use turn signals, so I applaud efforts being made by car manufacturers to build devices into cars that warn if the car is about to enter a lane that's already occupied, either by vibrating the steering wheel or by a sound or visual alert. Eliminating the blind spots around vehicles should be a urgent priority among automobile manufacturers.
PS Is it just me, or have others noticed this? When I drive in my native country, England, in a hire car with right-hand drive, I don't have a blind spot on the driver's side. Or am I just imagining it?