From Science Daily:
Sometime, when you are out on the road and looking for something to occupy you, try this little experiment: If traffic is heavy, pick out another vehicle. It can be anything, from a cute little Mini Cooper to a humongous Hummer, just as long as its driver is doing one thing: trying to get ahead of everyone else. After you have selected your subject, over the course of, say, the next 20 minutes, keep track of where that vehicle is in relation to your own. If the traffic is really solid, or if you are proceeding along a strip dotted by traffic lights, chances are that other vehicle will not move substantially ahead, and whatever energy that driver has been expending will be for naught.
Somewhere along the line, in the American driving culture, pushing to get ahead became the norm.
More here.