There is a similar phenomenon in Chicago wherein the right lane is faster than the left. Why this is, is beyond me. Perhaps it is because of the many exits, so more cars are getting out of the right lane, thereby freeing up space in the lane. This is especially evident in the chaos after the O'Hare tollbooth. Every daily driver of this small, localized version of Hell knows that the right lane leading up to the tollbooth is relatively short, and you can speed past multitudes of idiots, as they could only be, in the left lanes.Once through the tollbooth, you have to use your judgement as to whether the right lane is moving fast enough to justify staying in it, or if you should try floating over to the left lanes which inevitably pick up and pass the right lanes. Although sometimes this doesn't happen and the right lanes blow by you.
All these reasons lead me to believe that either a)a very, very sick person designed the Illinois turnpikes, or b)if 25% of the dumbasses alone in their cars, myself included, would just carpool every now and then, this problem would not exist, traffic would improve, and gas consumption would decrease, thereby making the world a better place. I am frequently heard spouting off the latter theory.