Frustrated in DC traffic, a friend of mine wondered why, in the States, people drive in the city about as much in the left lane as in the right while in Europe, the left lane is traditionally held exclusively for people going very fast and passing. This is an excellent question and I think I have the solution.
Americans have low car taxes, free parking, few tolls, and relatively cheap gas. Not surprisingly, we have more cars compared to our European counterparts and traffic is thus much worse. With crowded streets, drivers learn it's far better to drive in the left lane if they're planning to turn left, even if the turn isn't for miles. No one wants to miss a turn because they never could claim a spot in the correct lane. Thus people drive in one lane as easily as they drive in the other.
The same can be said of interstate traffic, though the story's slightly different. With so many cars on the road, there's greater likelihood that the right lane will have an unusually slow driver. Constantly passing in and out of the right lane to avoid these drivers might induce the faster car to simply stick with the left lane, even though an even faster car will likely be stuck behind it soon. Thus we see very fast cars weaving through traffic, blazing a sort of third lane.