The theory is usually far from practice. The people zipping past the queue in one lane will wait till they can get no farther, slow down to a near stop (because an obstacle like the barrier makes a neat merge dangerous at normal velocity), and edge in instead of taking the gap provided by another driver a few car lengths behind. If they would do that, the entire row could move at a normal pace. Instead, they introduce this clog at the merge junction.

I always leave a slot and a half open so that I won't have to brake at the merge point, but no one takes it until they reach the very end, at which point they do the whole slow-down-then-carefully-turn-into-the-other-lane dance.