WARNING: The following is not the conventional wisdom. It is reality. You will be asked to think about what is, not what you want to be. Thank You!
I think turn signals are evil, and should never be used. They were conceived long ago, in a kindler, gentler age, at a time when drivers of all kinds, including motorists, commonly waved their hands at each other to convey a wide variety of messages. Clever automotive engineers designed these little lights, hoping they might be easier to see and less ambiguous than hand signals. But, like many technology-driven innovations, turn signals are a bad mistake.
Let's look at the closely-related brake lights. These are connected directly to the brake pedal. You step on the brake, the lights come on. (In fact, in most older models, the brake lights and turn signals are one and the same, with two different controls.) If you change your mind, and take your foot off the brake, the lights go off. If you are a spastic fool, and pound on the brake pedal with your foot, then your lights blink on and off. At all times, the brake lights are a true indication of what you are actually doing with your brake foot.
Back to turn signals: Have you ever seen a fellow signal right, then turn left? I have. I've driven professionally for 10 years, and I've seen a sad truth. Drivers do not always signal their intentions accurately, nor do they invariably follow through on these intentions. In other words, turn signals sometimes lie. Drivers signal left and turn right or go straight. You cannot rely on turn signals to tell you the truth.
Why is this so bad? After all, people often lie in conversation, but that doesn't make it bad to speak the truth. The trouble is that on the road, you cannot afford lies. You cannot afford to trust other drivers (unless you live in a small community and you know every car on sight). You know nothing at all about the driver of the other vehicle; he has no reputation to make or lose. You try to grok the personality of the other driver based on make and model, and on your prejudices run wild -- and you guess wrong, and he kills you.
In the end, you must drive as if every other driver might be a suicidal madman hell-bent on killing you right this moment. You don't care what he says he is going to do; you don't even want to know what he says.
Every time somebody uses his turn signal and does what he said he'd do, he adds to your false sense of security. When the ugly moment comes that another driver signals right, and turns left directly in front of you, you'll be dead, because you "thought" he was going to turn the other way. The more often drivers signal and follow through properly, the more confidence you will place in turn signals, and the less ready you will be for that moment of decision.
I have another problem with turn signals: They displace responsibility. You signal, turn carelessly, I hit you. Whose fault? Mine, of course. After all, you signalled.
Do not signal turns, with one exception. When you want to turn (or change lanes), it is your sole responsibility to clear the way before you make your move. Don't signal, blow your horn, or pray to your choice of deity. Look and clear the way, then move.
Do not pay any attention whatever to turn signals, with the same exception. If you want to know what another car is going to do (and you certainly should), look. See which way it is moving now. Cars are physical objects with limited freedom. They can't move sideways, except on a very slippery road. They move in certain narrowly constrained paths. Before a car moves into your lane, he will have had to begin to move in that direction. That is the only "turn signal" you can trust.
Exception: There is one situation in which turn signals can do no harm, and may actually do a courtesy to other drivers. This is when you are stopped in a left lane which permits you to turn left or go straight. If you are stopped, you cannot suddenly swerve right; you will have to get moving again first. So, other drivers can trust your signal, at least to the extent of not having to fear you will kill them if they trust you this one time.
Nothing I have written here applies to four-way flashers. If you are moving, the message given is "I am going to do something weird"; nobody can be harmed if you lie, and do not. If you are stopped, four-ways just make you more visible, and there is never anything wrong with that.
Unfortunately, statistics are on my side. Way too many crashes are caused when one driver trusts another. There is a very thin and fragile layer of trust that you are compelled to assume, or else you could not drive at all -- you'd go insane with worry. But I tell you seriously: Drive every moment as if you really did not know what the other driver will do. You'll live longer.
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An Artisan has asked if, since some emails are spam, we should all stop sending emails.
Answer: If hundreds or thousands of people die from spam emails, Yes.