Mind Hacks reports a study that found a meaningful correlation between higher numbers of bumper stickers on a car and incidence of road rage by the driver of that vehicle.
The study authors postulate that bumper stickers function as territory markers, and drivers who use a lot of territory markers confuse the etiquette of public space (the road) with private (their cars).
Both number of territory markers (e.g., bumper stickers, decals) and attachment to the vehicle were significant predictors of aggressive driving. Mere presence of a territory marker predicts increased use of the vehicle to express anger and decreased use of adaptive/constructive expressions.
Hrm...we have a bumper sticker on our car...but only one!