What is a Zipper Merge?

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In construction zone lane closures, the zipper merge concept directs motorists to use both traffic lanes until reaching the defined merge area, and then alternately merge in "zipper" fashion into the open lane.
Zipper merge vs. early merge
Many drivers react to the first sign indicating “lane closure ahead” by prematurely moving into the traffic lane which will continue through the construction zone.  In many instances, the driver’s decision to abruptly change lanes may result in the vehicle slowing too quickly.  This unexpected behavior can confuse drivers in other following vehicles and result in severe crashes and/or road rage.

Zipper merging, however, benefits individual drivers as well as the public at large. Research studies show increased safety when motorists use both lanes until reaching the defined merge area and then alternately merge in “zipper” fashion into the open lane.
So I'm supposed to merge late?
Yes! As you see the “lane closed ahead” sign, stay in your current lane and be on the lookout for traffic backups ahead. Continue driving within your lane until the point of merge. Then take turns with other drivers to safely and smoothly ease into the remaining lane.  Each driver should practice zipper merge courtesy by allowing a single vehicle from each traffic lane to alternately proceed through the construction zone open lane.  When traffic is heavy and slow, it is much safer for motorists to remain in their current lane until the point where traffic merges in an orderly fashion.
When not to do the zipper merge
When traffic is moving at highway speeds and there are no backups, it makes sense to move sooner to the lane that will remain open through construction. The bottom line is to merge when it is safe to do so.
What does it look like?
A site using a zipper merge looks similar to a site using an early merge.  Roadside signs indicate the difference by notifying drivers when to merge.  In the zipper merge, signs notify drivers to use both lanes all the way up to the merge point.  Ideally, drivers will fully utilize both traffic lanes resulting in nearly identical lengths of backup or congestion.  By using both lanes, the length of traffic backups or queues will be reduced and result in improved traffic flow and safety conditions.  Drivers should not block or try to prevent other drivers from using both lanes since this interferes with the performance of the zipper merge concept.
 
 
Zipper merge signs instruct drivers where and how to merge.  The first sign will notify drivers to “Use Both Lanes During Backup” as indicated in the sign above.  At the merge point, a sign will indicate “merge here” by showing two traffic lanes converging into a single lane.  Additionally, the sign will instruct drivers to “Take Turns.”  Each driver needs to take turns with other drivers when merging to fully employ the zipper merge concept.  This will improve traffic flow conditions, prevent confusion, and reduce unnecessary congestion.  Drivers not allowing other driver to merge will ultimately cause more delay for everyone.