A recent study conducted by Colorado State University's social psychologist William Szlemko found a correlation between aggressive drivers and bumper stickers. Drivers of cars with bumper stickers, window decals, personalized license plates, and other "territorial markers" (as the study refers to them) are more likely to express rage than those driving a car without any territorial markers.
One of the experiments related to this study featured a researcher driving a car sitting in a left-turn lane. When the light turned green, the researcher did not move the vehicle, blocking the car(s) behind. Meanwhile, another researcher examined the cars blocked behind the researcher, noting whether or not their car had any stickers or decals.
The experiment sought to find how long it would take for the driver of the blocked car to show frustration. Results found that cars with bumper stickers and other territorial markings honked nearly two full seconds faster than cars without any territorial markings.