The abstract below suggests that bumper stickers are potentially an expression of territorial markers and that aggressive people are more likely to use more, but I think we all know it’s just down to the fact that “my other car is a Ferrari” just isn’t funny any more.
Territorial Markings as a Predictor of Driver Aggression and Road Rage
Journal of Applied Social Psychology, Vol 38 (6) p1664-1688, June 2008
William J. Szlemko, Jacob A. Benfield, Paul A. Bell, Jerry L. Deffenbacher, Lucy Troup
Aggressive driving has received substantial media coverage during the past decade. We report 3 studies testing a territorial explanation of aggressive driving. Altman (1975) described attachment to, personalization of, and defense of primary territories (e.g., home) as being greater than for public territories (e.g., sunbathing spot on a beach). Aggressive driving may occur when social norms for defending a primary territory (i.e., one’s automobile) become confused with less aggressive norms for defending a public territory (i.e., the road). 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.