Montana Department of Transportation


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Public Involvement

East Helena Viaduct – Phase 2

Zipper Merge

Zipper Merge Benefits

  • Reduces differences in speeds between two lanes
  • Reduces the overall length of traffic backup by as much as 40 percent
  • Reduces congestion on freeway interchanges
  • Creates a sense of fairness and equity that all lanes are moving at the same rate


The Montana Department of Transportation recognizes the zipper merge as a traffic control strategy for work zones when applied appropriately.


The nature of work zones and lane closures are such that some level of delay is inevitable. However, when used appropriately, lane closure and merging strategies are able to improve traffic flow, reduce delays, and increase safety.

Lane merging tapers can create speed differentials between two lanes of travel as vehicles merge together. Lane closures can lead to long vehicle backups or queues in one lane as motorists respond to lane closure signage and move early into the indicated lane of travel. This queueing in advance of the construction zone creates delays, crash potential, and inefficient use of the roadway. The zipper merge is an effective traffic control strategy that can improve traffic flow and safety leading into and through the construction zone.

What is a zipper merge?

When a lane is closed in a construction zone, a zipper merge occurs when motorists use both lanes of traffic until reaching the defined merge area, and then alternate merging in “zipper” fashion into the open lane of travel.

Zipper Merge vs. Regular Merge

Diagram of the regular merge pattern compared to the zipper merge pattern
Regular Merge vs. Zipper Merge
Click to view a larger image.

Typically, when most drivers see the first “lane closed ahead” sign in a work zone, they slow down and move early to the lane that will continue through the construction area. This creates traffic backups which result in delays, crash potential and an inefficient use of the roadway.

The zipper merge encourages drivers to use both lanes until reaching the defined merge area, and then alternate merging in “zipper” fashion into the open lane of travel. This method of merging results in less delay, smoother merging of traffic, and safer conditions in the construction zone for both drivers and construction workers.

So I’m supposed to merge late?

Yes! Follow the signs. When you see the “lane closed ahead” sign and traffic backing up, stay in your lane of travel up to the “merge here sign.” Then take turns merging with other drivers to safely and smoothly ease into the open lane. Don’t worry about being “Montana nice.” When traffic is heavy and slow, it is much safer for motorists to remain in their current lane until the point where traffic can orderly take turns merging.