Changing lanes as soon as you signal and not looking over your shoulder to check your blind spot is just as bad as not signaling, you have to check, make sure you're clear, then change lanes. Signaling doesn't mean that it's clear to move!
As I was exiting Route 3 South to get onto I-495 North, this gentleman (and I use that term loosely) in a gray/silver Toyota RAV4 cut me off to take the same exit at the last second and was likely inches away from hitting me, while I was driving a (thankfully empty) school bus minivan.
Changing lanes as soon as you signal and not looking over your shoulder to check your blind spot is just as bad as not signaling, you have to check, make sure you're clear, then change lanes. Signaling doesn't mean that it's clear to move!