At the risk of descending into silly subjective digressions, I'd say that the Yuppie BMW drivers and the SUV/Truck drivers are equally bad. Both are aggressive, rude and arrogant. In the case of the BMW drivers, they suffer from an overgrown sense of self worth and overconfidence in their own capabilities as a driver and that of their car (i.e. "I took a driving class and do SCCA Solo, I know I can cut into that lane safetly even if it scares everyone else to death").
In the case of the truck drivers, it's a feeling of physical invincibility and selfishness ("If I get in a wreck,
I'll survive, screw everyone else").
Again, let me emphasize these are just stereotpyes, I'm not name-calling anyone here, but there are enough "douchebags" out there that fit the mold to make the stereotype recognizable. I'd drive a BMW if I could afford one, but I can't so I bought a Subaru. Which makes me a granola-eating, kayak toting, sandal-clad, REI shopping wannabe outdoorsy type I suppose.
At least if you bought a Crossfire you wouldn't fit either stereotype.