6. The I-Don't-Use-My-Mirrors-or-Signals
The single most important thing you need to do while driving is stay attentive. Check your speed, check your side mirrors, check your rearview mirror, check your speed again, check the traffic in front of you, check upcoming signs, check your mirrors again. You get the point. People who have their heads in the clouds (or next to a cell phone) are a constant danger to everybody driving near them, and that's further perpetuated when they don't use their signals. They must have been on their phones during driving school, too.