Phil sends along this funny sign from FAIL Blog: Lane Closed to Ease Congestion
The comments have a plausible explanation. There is debate about whether to force merges at the farthest upstream point, (before the queue starts, so that everyone is orderly) or at the farthest downstream point (to allow people exiting upstream of the bottleneck to have a shorter wait) and have a zipper merge. The answer is probably it depends.
This is discussed in a Times article

An agency spokesman insisted that the traffic would be worse with the lane open. “It may sound barmy but in fact it makes a lot of sense because, if it was left open, traffic from the two lanes would have to merge into one at the top.
“This causes a lot of aggro and a lot of stopping and starting which has been shown to delay traffic even more.”