If you’ve driven through a Minnesota construction zone, you’ve likely made a zipper merge.
It’s when drivers stay in both lanes as they approach a lane closure and then alternate, like a zipper, into the one open lane right before they merge.
Zippering reduces the differences in speeds between two lanes and cuts the length of the traffic backup, which can reduce secondary crashes, said Willy Sorenson, traffic & safety engineer for the Iowa Department of Transportation.
So why doesn’t Iowa do it?
“I’m a sworn believer in it, but it doesn’t matter what I think,” Sorenson said. “You need 50 to 80 percent of the people to believe in it.”
The practice in Iowa — and most other states — is for drivers approaching a lane closure to immediately get into the lane that will stay open through the construction zone. A few drivers remain in the other lane, zooming ahead of cars waiting in line, only to cut in at the end.
This situation can create hostility. Just search YouTube for “road rage cut in line construction” for some prime examples.
But educating the public about the zipper merge hasn’t been easy.
More than 60 percent of people who participated in online transportation forums sponsored by the Minnesota DOT said they were unaware the zipper merge is an acceptable driving technique included in the state’s driver’s manual, the Star Tribune reported in 2013. And that was more than 10 years after the state introduced the practice.
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Zipper merging doesn’t necessarily get drivers through construction zones any faster, Sorenson said. “But it feels like it’s faster because you’re always moving and the backup isn’t as long.”
Sorenson thinks zipper merges only work in areas with a long-term construction project with heavy traffic and a lot of commuters who travel the stretch every day.
“They’re the ones who learn what works, and everyone else will follow,” he said.
The Iowa DOT tried a zipper merge study in 2008 on Interstate 80 in Cass and Adair counties in western Iowa. It was a dynamic zipper merge, which means drivers were advised with electronic signs when traffic volume was high enough to zipper. Other times, a traditional merge was allowed.
The added signage and sensors cost a couple of hundred thousand dollars, Sorenson said, and results of the study were inconclusive because traffic flows rarely got high enough to trigger the zippering directives.
The Iowa DOT’s approach to avoiding long backups has been not doing long-term daytime lane closures on interstate highways, Sorenson said. Lane closures are more likely to happen at night when the traffic flows are lighter. “We do a great job at keeping a lot of lanes open,” he said.