Despite its wickedly deceptive name, the turn signal (or turn indicator) is used to "signal" (or "indicate") that your car is about to turn.
Some people call them turn signals. Others call them turn indicators. Most people apparently have no word to describe their car’s single most effective safety feature, judging by how rarely they use it.
The first electronic blinking turn indicator was made available on 1938 Buicks (sold as a “Flash-Way Directional Signal”), and the first self-cancelling signal with a control on the steering wheel (featuring the same basic mechanism in use today) appeared in 1940. That means we’ve had nearly 75 years to learn to push a very simple lever every time before – and that’s the key word: before – we make a turn, change lanes, or are preparing to enter a parking space. You should use your turn signal when driving out of your driveway – because otherwise you’re not doing it all the time, and you need to do it all-the-time.
I may be able to tell many things about you from a quick glance at your car – that you’re politically naïve, that you have horrendous taste in music, or that you have several emaciated children and a couple of pets – but I can not read your mind, and neither can anyone else. So if you don’t want me to plow into the back of your car because I didn’t immediately realize that you had cut me off because you had to make a turn the second you were in my lane, and if you don’t want me to flip you off every time you force me to come to a complete stop for no apparent reason because you need milk and the store is right here, then I suggest you learn to use the single simplest mechanical device ever invented. It may one day save your life and it will definitely help lower my blood pressure.