My interpretation is that gbadman did not mean the situation where two lanes merge into one, but rather where you have two lanes traveling in a single direction approaching an interchange where the lanes split such that the left lane goes one direction and the right lane goes another.

In gbadman's example one lane (let's say the right hand lane) goes to a road that does not back up, whereas the left hand lane goes to a road that will back up through the interchange. In this situation, you have a left lane congested with people waiting to go on the busy road, and a right lane that should be almost free flowing. By most understandings (including the execellent book, Traffic) people wanting to go to the busy road should wait in line in the left lane, allowing people going to the less busy road to pass freely. However, many drivers will stay in the right lane until the very last minute, then attempt to move into the left lane before the right lane splits off, backing up both lanes.