The people driving in front of and behind me today on the expressway.
The expressway has a 65mph speed limit. I was in the middle lane (five lanes total on this expressway). The woman in front of me suddenly slowed way down, then slammed on her brakes and came to a complete stop because she wanted to get into the lane to the right, but traffic hadn't cleared enough for her to get over. Rather than simply adjusting her speed and waiting for an opening, she seemed to think just stopping and waiting for a gap to open up was the best plan. So I had to brake. The guy behind me wasn't paying attention (he appeared to be on his cell phone) and very nearly rammed me while doing about 70 mph. If he hadn't been able to swerve into the next lane, he would have hit me, likely pushing me into the woman who was still sitting at a complete stop in the middle of the expressway. He missed me by perhaps six inches.
When traffic moves that fast, everyone needs to pay attention, and coming to a dead stop in the middle of an expressway is an incredibly stupid idea.