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Dynamic Zipper Merge



​​​​​​​​​In 2019, the N.C. Department of Transportation introduced new technology that has reduced backups in work zones where two lanes of traffic merge into one lane. The technology, called a Dynamic Zipper Merge​, also has improved safety by warning drivers of any traffic slowdowns or stops as they approach the work zone.

How It Works

​​The Dynamic Zipper Merge relies on a series of speed sensors set up on the side of the interstate for several miles before drivers must merge into a single lane. The sensors gather data about traffic congestion and then feed the data to message boards that change the signs that drivers see based on the traffic patterns approaching the work zone. In addition, the department's Statewide Transportation Operations Center also monitors the zipper merge system.

Depending on traffic conditions, the messages may indicate, for instance, "Slowed Traffic Is A Mile Ahead", or that "Traffic Has Stopped 2 Miles Ahead."

In certain conditions when traffic has slowed or stopped, the message boards will indicate “Use Both Lanes/To Merge Point." At the start of the work zone, the signs will display “Merge Here/Take Turns." As its name suggests, this type of merge is intended to work like a zipper. By taking turns at the merge point, drivers experience smoother merging conditions, which reduces traffic backups and the amount of time it takes to go through the merge.

The innovative merge system reduces the confusion between drivers who think they should merge early versus those who want to use the open lanes for as long as possible.

Where the Technology Is Deployed

As of 2023, NCDOT is deploying zipper-merge systems in these two work zones in western North Carolina:

  • I-40 in Haywood County (both directions)
  • I-26 West in Polk County

In 2019, NCDOT installed zipper-merge systems for the first time west of Winston-Salem at I-40 in Davie County and I-77 in Yadkin County.  

Although the zipper-merge systems required lengthy lane reductions during highway reconstruction, it dramatically reduced traffic backups. After the construction projects were completed in 2022, the systems were removed.

Future Considerations

The Dynamic Zipper Merge system does not work for every work zone. Because of the complexity of setting up the sensors, the system functions best for highways with travel lane reductions that will last for several months and cause significant delays. Engineers at NCDOT are evaluating where else to deploy the system in the future.​

2/1/2023 3:11 PM