In a multi-lane highway, when different lanes are moving at different speeds, drivers are more likely to change lanes.
Lane changing causes drivers to be more cautious.
Greater caution results in lengthening the distance between themselves and other cars.
This causes more lane changing (since there’s more space between cars)
Here is a picture that shows how one thing leads to the next. Note the positive feedback loop, which results in lots of lane changing and increasing distance between cars:
Each lane on the highway ends up carrying 10% fewer vehicles than if it were a single-lane road.
— Extracted from The Wisdom of Crowds by James Suroweicki