If we accept Adam Smith's argument from Theory of Moral Sentiments that affection is habitual sympathy, then the way we get strangers to treat us affectionately is to get them to sympathize with us, right? As long as I'm a stranger, I'm easier to hate, but as soon as you find out we have similarities, you start to temper your hatred. Even if we're not similar, making you know me better should make you more civil. This is why you tell the mugger holding you at gun-point, "I've got a wife and kids!"
If all of this is true, then bumper stickers can lessen road rage by making the other drivers less anonymous. The guy who cut me off is a bastard, until I find out he likes the Outer Banks or he has a stick-figure family. Then he's just a guy who sometimes makes bad driving decisions. If his favorite NASCAR driver happens to be the same as mine, well then maybe the near-accident was all my fault.
Of course, you have to reveal information that is likely to increase sympathy, not lessen it. The other drivers' reactions need to be either positive or neutral. Negative reactions aren't really helpful, so political bumper stickers aren't doing the trick. Tell me that "Virginia is for Lovers," not that you supported Gerry Connolly. Also, the correct information to reveal might be area-specific. A Dallas Cowboys sticker probably does well in Dallas, but not so much in DC. License plates are a start, as they let you know who is from your state.
This is the crap I came up with while I was merging down to one open lane on 66 this morning, behind a car driven by a guy who went to Indiana University of Pennsylvania and Johns Hopkins University, and who used two different stickers and his license plate to inform me that he enjoys visiting Emerald Isle, North Carolina.