You are in the fast lane and are going the exact same speed or slower than everyone to the right.
When you change lanes right in front of someone who is passing you.
When you reverse on a highway/interstate because you missed your exit.
When you merge or pass on a double yellow/solid white. (Friend was killed on his motorcycle because someone did this).
When you're stopping and accelerating erratically because you have to get that snapchat/text message out.
Stopping on green and going on red.
Pulling out into traffic when you have to take the next right turn in 300ft.
Going 45/55/84/35/64/52 mph all within 1 mile.
Drivers who continually merge and change lanes in a traffic jam. You're not going anywhere, just maintain your lane.
Approaching a red light where 1 out of 2 lanes has no one in it so another driver speeds up and cuts in front of you to get the open lane and when the light turns green they go significantly slower than the person who was in the other lane.
Making a U-turn in the middle of a road with no turn lane.
When you merge to pass someone and they speed up significantly to not let you pass.
Drivers on entrance ramps who merge out of turn and won't let anyone who was in front of them, merge in front of them.
I'm not a perfect driver myself, but I find it very important to signal your intentions on the road, not just for the sake of other drivers, but for yourself. It just seems that not a lot of people take driving on the road for granted and become very selfish. As if rules/laws/courtesy do not apply to them in the least. I wish police took "smaller" violations more seriously. I know if I was a cop, I'd pull people over left and right for not using their turn signal, and for impeding traffic (not passing in the passing lane).
Not to mention, when you communicate to another driver that the move they just made was absolutely ridiculous, they just flick you off and become more reckless to piss you off.
I can go on for days.