If you want to spot cars you should keep your distance from, look no further than their bumpers. A social psychologist at Colorado State University conducted a study of drivers and found that those who displayed bumper stickers, window decals, vanity plates or other territorial markers were more likely to be aggressive drivers, which means that perhaps they don't respond well if you honk at them.
The study associated all kinds of poor driving behavior with these so-called "territorial markers," including tailgating, honking, flipping the bird and road rage. The funny part is that it doesn't matter what the marker is. A Prius driver with a "Kucinich for President" bumper sticker is just as likely to display aggressive driving behavior as the driver of a pickup with a Confederate flag decal.
This behavior is closely linked to evolutionary reactions to people's environments — or what we consider "public" and "private" spaces. A car is a private space within public territory and, according to researchers, personalizing that private space is a way for aggressive individuals to mark their territory.
In other words, steer clear of that mom who's so freakin’ proud of her honor student.
Got Bumper Stickers? You Might Be Prone to Road Rage (Washington Post)