Mine is bad drivers. I'm on the interstate for about 2 hours a day and people just have no common freaking sense.
Right lane = slow lane, if you feel you must drive under the speed limit, please do so in that lane. At least get over for faster drivers. There are signs for this every so often (at least in South Carolina): "Slower traffic keep right". Related, don't ride it out side by side with another car so that both lanes are blocked. PASS THEM.
Turning signal. They are conveniently placed in front of you, but people are too lazy to flick a damn switch apparently. If you don't signal, you just may catch someone off guard and cause a wreck. Congratulations, asshole.
Tailing. Really? 2 second rule. Everyone learns this. But you're so close to me that your headlights are not visible in my low to the ground Honda Accord. Someone did this tonight, they rode my ass the whole trip back, and never even tried to pass me. Why? Sometimes when people get to close to me, I just take my foot off the gas and slow down until they get the message. But I was listening to Wilco and they always get me in a good mood so I didn't return an dick move for a dick move.
If you're following a car on the on-ramp, don't try to pass them in the small amount of distance you have until the lane merges with the rest of traffic. How dumb can you be?
Hold a speed. Most cars have this great thing called Cruise Control. Learn to use it. You can keep your foot near the break so you can actually react more quickly if you need to use it. You save gas by not slowing down and speeding up constantly. You don't disrupt the flow of traffic. And you don't play some cat-and-mouse game with me.
That is all...for now.