I had yet another near-accident with someone because they didn't feel like moving one finger to flip their turn signal on. I'm so sick of this crap. If you were sitting on a bench somewhere and could save a person's car or life by just lifting one finger, why wouldn't you do it?
Reasons I've heard from otherwise intelligent people why they don't use their signals:
1) It gives other drivers information.
Duh. That's the freaking point. I can drive defensively and accomodate your every whim if you just tell me what you're going to do.
If other drivers are asses and close gaps because you're signaling, just signal at the last minute as you move on over. You still get benefits of signaling with opportunity for last-minute saves and you get your spot in the next lane too. I grew up driving in LA. I know this works.
2) Why signal when no one is near you or you can see the whole situation?
This is when it's MOST important. You just THINK you see everyone and know the entire situation. It's the cars you DON'T see that you really need to notify so they can avoid you.
Also, if you think about it, you also don't even really know what the other cars will do because maybe they don't signal either. It's insta-karma waiting to happen.
To summarize this rant, it just takes one finger to save yourself and others from crushed cars and maybe crushed body parts. If you don't signal, you're saying my safety isn't worth the trouble of moving your pinky. SCREW YOU, you selfish bastard.