Impatient drivers when they don't have to be anywhere important. Over aggressive drivers. If you're going to to feel the need to ride everyone's bumper the whole way, maybe you should've left earlier. If I'm going 7-8 over the speed limit, which is about what I usually do, and someone's tailgating, I'll gladly slow down to the speed limit for them. (I'm talking about non-highways here.) When you're waiting to turn on to a street and someone who will be turning on to the street you're on doesn't put their blinker on. I can't believe how often his happens. The worst is when there are multiple cars behind them so it makes you wait quite a bit longer, when you could've turned much sooner.