Interesting theories, my asphalt acquaintances, but I think there is a better answer. How can I say it in five words and stay true to the node title? Hmmm.
Not knowing how to merge.
There. Nothing drives me crazier while out in traffic than to get to an intersection with a merging lane and be behind some idiot (and, ladies, I hate to say it, but most of the drivers I see doing this have a front bottom) who sits there while an entire lane, going on forever and ever, lies before her unused.
A couple of times, I've actually gotten out of my car, gone up to the window, and discussed this with the driver. Well, there really wasn't much discussion, 'cause when they see me coming at them, they suddenly discover that there's a lane in which they can escape.