Your Village: Zipper merges, how do they work?

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Thursday, May 31, 2018. — photo by Zak Neumann

Can you help explain how zipper merging works? I’m tired of getting yelled at for using the merging lane to merge. — Katie, Iowa City, via Facebook

Little Village can explain how zipper merging works, but that won’t do much to cut down on car-to-car yelling or mid-merge rude hand gestures. The only reliable way to avoid zipper-merge-related anger is to drive in Germany, where motorists fully embrace it.

The Germans even gave this style of merging its name, the Reißverschlusssystem (zipper-system), because the way the two lanes merge is supposed to resemble the way the alternating teeth of a zipper come together.

In a zipper merge, drivers in the merging lane follow that lane to its end, then merge into the neighboring lane. Each driver in that neighboring lane is supposed to let in one merging car. This allows for maximum use of the merging lane, which cuts down on traffic delays caused by mergers. Studies show zipper merges can reduce delays by as much as 50 percent, although most pro-zipper literature makes a more modest claim of a 40-percent reduction.

Traffic engineers love the zipper merge. It’s a simple and efficient system. At least it is until you introduce people into the system. Particularly Midwestern people.

The Associated Press asked a traffic psychologist for an explanation of Midwestern attitudes on merging for a 2016 story on the region’s reluctance to embrace the zipper.

Dwight Hennessy, a psychology professor at Buffalo State College in New York who specializes in traffic psychology, said Midwesterners tend to be polite and follow the rules — even unwritten ones — and get upset when others don’t.

“When a rule is being violated by someone else, it frustrates us, it irritates us, it makes us angry,” Hennessy said. “We expect everyone else to follow the rules, and when they don’t and we know they’re getting an advantage, it ticks us off.”

The unspoken rule in this case is, of course, the belief that a driver should merge as quickly as possible after spotting a sign indicating a lane closure. It’s the polite thing to do, people unpersuaded by traffic flow studies believe.

Around the country, the transportation departments of more than two dozen states are officially committed to the zipper merge. The Iowa Department of Transportation (IDOT), however, tweeted in April that it isn’t ready to commit.

This actually represents backsliding on IDOT’s part. In 2015, it was using social media to try to persuade drivers to merge zipper-style.


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Here's another post about the zipper merge we're implementing on Iowa 141 to I-35/80. It's been somewhat effective in other states. This post and web link comes to us from our friends at MoDOT.

Posted by Iowa Department of Transportation on Wednesday, July 22, 2015

The Missouri Department of Transportation (MoDOT) post to which IDOT linked contains a video of kids watching a video of adults wearing cardboard car costumes and simulating different types of traffic mergers.

“Why aren’t they using the other lanes?” asks Jenna, age 7, when the car-clad MoDOT employees don’t merge zipper-style. The not-so-subtle message is, if a second-grader can understand the superiority of zipper merging, what’s your problem?

Minnesota has been trying to get drivers to embrace the zipper merge for a decade and a half. After years of very limited success, the Minnesota Department of Transportation launched a major advertising campaign in 2011 to convince drivers to zipper. The next year, a poll found an impressive 73 percent of Minnesotans agreed that zipper merges were a good idea. But that doesn’t mean Minnesotans have actually changed how they drive.

In 2017, KARE-TV reported “80 percent of Minnesotans opt to be early mergers and two-thirds of early mergers would not be happy letting the so-called ‘later mergers’ into their lane.”

Zipper merge. Thursday May 31, 2018. — photo by Zak Neumann

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