Great comments here. I couple of ideas for consideration:

In mice, fice, apes and almost any animal, overcrowding leads to more aggressive and violent behavior, even when resources are not limited. (I.e. enough food and water and sex.) Suicide rates also increase. I think we can certainly apply this to human behavior when we look at urban crime vs. rural crime.

When we look at the fact that modern human is still the same physical animal it was thousands of years ago, I don't think we can make generalizations about being more or less prone to violence based of physiology. I think that a key component might be how our innate violence is directed. Our ancestors hunted, slaughtered animals, fought against the elements and defended themselves in a way that we don't generally need to today in a world with police forces to protect us and slabs of plastic wrapped meat in the supermarket. Their natural aggressiveness/violence was useful and had a satisfying purpose.

Today, we see people trying to direct their violent tendencies without getting the satisfaction. No matter how many aliens you kill in that videogame, it means nothing on an instinctual level. Does it stir up a feedback loop where we actively seek violence (movies, sports, etc.) because we can't direct our violent natures to something beneficial? Yet our natures drive us to compete at it anyway?

I couldn't say for sure -- but people who engage in mock violence almost react like it's an addiction that drives them to more. Think "football fan."

Maybe we're adreniline junkies, and violent criminal behavior is just an aspect of it.

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On the car thing -- people and their cars always remind me of that Bugs Bunny cartoon where the hats keep landing on their heads and changing their behavior. People DO act and react different based on the kind of car they happen to be in. I don't think it's unusual that the preponderance of larger tank-like vehicles might cause the driver to react as if THEY were bigger and better than the other drivers, nor that it might make the drivers of Geo's and Aspire's react in rage out of some misguided inferiority feelings.