Traffic was terrible on I-94 in Minnesota late last month. They were handling construction traffic that day the way Illinois usually does. (Irv Leavitt photo)

What is “zipper merge?”

If you live in one of several American states where it’s used, you may know the answer. If you live in Illinois, you probably don’t.

Once you know what it is, it may shake your confidence in one of your most cherished beliefs about the way to drive a car.

Here’s the deal. You know how when there’s highway construction up ahead, and a sign warns you one lane is going to merge into the other? You right away try to get into the lane that’ll stay open, like you were taught to in Drivers Ed. And you get real ticked off at the drivers who pass everybody in the short-lived lane, and then try to butt in to your lane before it disappears.

They’re cheating jerks. Aren’t they?

Not in some other states, including Minnesota.  And that state is full of people who are famously more congenial than you are. You never heard anyone say “Illinois Nice,” did you?

State transportation departments that use zipper merge post signs a couple of miles ahead of the lane closure saying “stay in your lane.” Then closer to the lane dropoff, they post another sign telling drivers to “merge here and take turns.”

The cars in the two lanes join together like the teeth of a zipper.

They get through the merge 40 percent faster than without it, at 40 to 60 mph, said Work Zone Delivery Engineer Chris Brookes of the Michigan Department of Transportation.

“We had a four-mile backup that was completely eliminated when zipper merge was functioning properly,” Brookes said.

“Really, it comes down to, ‘Will the drivers do it?’ You need 100 percent of the drivers for it to work.”

“If there’s one bad apple, it breaks the tooth off the zipper, you know?”

What happens, he said, is “some hero” blocks the cars from merging, or even pulls across both lanes to enforce what he thinks is the right way to drive.

Even so, Michigan uses the method at one or two construction projects per year. Minnesota does about 10 times that many, extending back over the last 20 years or more, said Ted Ulven, work zone standards specialist for the Minnesota Department of Transportation.

And zipper merging has been used in Europe for decades.

Illinois has done it just twice, on I-74 near the Quad Cities in 2016, and on I-55 near Pontiac last year, according to Guy Tridgell, the communications director of the Illinois Department of Transportation. He said IDOT is currently studying whether to make the practice more of a habit. But he didn’t sound impressed.

“We tried it on I-55, with somewhat predictable results,” he said. “It doesn’t necessarily speed them out of there.”

He said that the system doesn’t seem to work equally under all applications, and IDOT needs to learn how best to use it, if it does. Michigan’s Brookes agrees, saying the optimum conditions are 1,400 to 1,800 cars per lane per hour. Fewer, and people don’t need help to merge. More, and it may be just too clotted up. And if there are too many ramps in the wrong places, it may not be an option, Minnesota’s Ulven said.

Late at night, when cars are few, take-turns signs can come down, because people don’t need any help.

“Middle of the night, they may race up to the merge point,” he said.

States often use electric signs connected to sensors on the road to decide when to tell people to zip merge (though Minnesota has gone largely manual, Ulven said).

There’s a question in some places, like Iowa, whether it’s all worth it.

They don’t do the zip thing there, said Work Zone Traffic Control Engineer Dan Sprengeler.

“It would be great if you could get everyone to do it,” Sprengeler said. “It only takes one.”

Brookes said that even though it may fail, zipper merge is better than just letting cars stack up. He’s just still chary about doing it with more than two lanes, because of possible confusion.

In Minnesota, they have no such compunction. It usually works fine with up to five lanes, as long as only one of them is being closed, Ulven said.

When the zipper fails, it doesn’t get totally stuck, like on your pants when a tooth breaks. Traffic pretty much just returns to the way it would be without it.

The system works much better when the media explains it frequently, Brookes and Ulven say. Brookes tells a story about how a zipper broke down one morning, leading to extensive coverage on radio and TV. In the afternoon rush, it worked perfectly.

He said it’s hard to tell why it’s so hard to get the idea across: “They all take turns at sporting events,” when drivers work their way through parking lots — often without even being told.

And half those drivers are drunk.

But Michigan and Minnesota seem sold on zipper merge. Ulven said it cuts way down on accidents and road rage (Yes, they really have it in Minnesota). Would it work as well in Illinois?

Maybe not, said Kyle Froelich, who drives from Sunny Harvest Farms in Berrien County, Mich., to the Northbrook Farmer’s Market every summer Wednesday.

“No offense, but we drive better than you guys,” he said.

One day, perhaps, zipper merge will be second nature. Construction-area perils would drop, Brookes said.

“It’s a missed opportunity in driver training,” he said. “It’s not something to be selfish about. If we had a more courteous environment, the majority of our fatalities would go away.”

Eventually, we won’t have to worry about it, he said.

“In the future when we don’t have steering wheels in the cars, they’ll take turns and do exactly what they’re supposed to do.”

—- Other Midwestern states use “zipper merge” to manage construction traffic. Should Illinois? —-