In all likelihood, scores (if not hundreds) of graduate students have very probably written dissertations on the topic of the psychology of driving. Our brains seem to function in a completely different way when we're behind the wheel than when we're at the office or in our homes. At the risk of trying to reduce everything in life to Batman, there's a little bit of Two-Face lurking in all of us: when we're on the road, we tend to engage in language and behaviour that—in the rest of our lives—is off limits. What accounts for the disparity between our driving selves and our rest-of-the-time selves?
One major factor in the release of inhibitions on the road is anonymity—both of the driver, and of those whom the driver perceives. Although it isn't necessarily a conscious effort on the driver's part to think of themselves as anonymous, the very nature of the physical barriers of their car and of those around them automatically cue their brains to say “I'm invisible right now.” That anonymity can be dangerous: when you cease to realize the extent to which you are accountable to those around you, it's possible to become destructive and opportunistic if that seems to be a way to get what you want. (You see this phenomenon frequently in the way that people behave online as well).
The other way that anonymity plays a role in this scheme is the driver's perception of those sharing the road. Again, the physical barriers both of your vehicle and theirs function to suppress your ability to recognize them as a person, rather than a vehicle. This would account for why you might cut them off without so much as a shred of embarrassment, while you never dream of cutting in front of the same person in queue at the supermarket. (This is not to say that there are not people who would do both, but rather to highlight that most of us are more at ease with misbehaving when we can't be seen).
When we find that we are driving in a way completely discrepant to the manner in which we conduct the rest of our lives, it is worth pausing to remember ourselves, and behave like ourselves.