Many years ago, I was leaving work after a particularly crappy day. As I was moving up the aisle of the parking lot, a car came cutting across the lot (where there were no cars parked) and nearly broadsided me. I snapped. I took off after the car and pulled up behind her at a traffic light. I slammed the car into park and got out of the car and went up to the other car and began to berate the driver. The girl behind the wheel would not even turn to look at me. It is fairly safe to say that I scared the bejeezus out of her. My tirade was so pronounced that someone coming out of a restaurant next to the traffic light encouraged me to get back into my car. Which I did. It was not one of my finer moments. I don't remember if her car had any bumper stickers.Flash forward to the present and I read this article in the Washington Post, recounting a study put out by Colorado State University about aggressive drivers. Evidently there is a link between the number of stickers you put on one's car and how aggressive a driver you can be. The study does maintain that the unstuck do get angry as well, but there was a definite correlation to bumper stickers and incidents of road rage.
The study, which appeared in the Journal of Applied Social Psychology, reported that drivers used the stickers to mark their territory (their cars) and they were more likely to defend that territory when they felt is was threatened.
An interesting side bar to this. As a federal employee, I work under the provisions of the Hatch Act, one of the provisions being (from an online training I took recently) that I may not have more than one partisan election sticker on my car. That's nice of them to look out for me in that way.