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Zipper Merge Saves Time But Affronts Etiquette

By Aaron Crowe

If you’ve driven upon a lane closure, you’ve probably seen it: A driver zips up in the open lane to cut ahead of traffic merging from two lanes to one.

That person is either an idiot for cutting in front of the line, or a genius for legally following the merge directions and moving into the one lane at the last moment while other drivers slow down traffic by merging too early.

Called the “zipper merge,” it’s a method that transportation engineers use to get traffic running smoothly through construction zones by directing drivers to use both lanes to merge into one, like the teeth of a zipper, all the way up to the merge point.

Pulling into the continuing lane earlier when there’s a natural break or other drivers wave you over causes longer backups, according to engineering studies by the Federal Highway Administration, Minnesota Department of Transportation, and the Texas A&M Transportation Institute. But people who merge from the ending lane at the last possible spot can be breaking roadway etiquette and anger motorists around them.

“Most of us don’t want to be that jerk that’s bypassing the queue,” says Gerald Ullman, a senior research engineer and program manager at Texas A&M Transportation Institute.

How It Helps Traffic

Also called a “late merge strategy” and “dynamic late merger,” the zipper merge started about 20 years ago to help move traffic better miles before the merge point. It has been shown to improve capacity 10 percent, Ullman says. The total length of a backup can be reduced by up to 50 percent, though 40 percent is common.

With proper signage telling drivers to merge at the point of the lane closure and to take turns merging, zipper merging is a good idea, says John Bowman, spokesman for the National Motorists Association, a driver advocacy organization based in Wisconsin.

“Like many traffic related phenomenon, the zipper merge is counter intuitive,” Bowman says. “Most people are conditioned to merge as soon as they see the ‘lane closed ahead’ sign. But this ultimately slows down traffic flow because you’re essentially eliminating a full lane of traffic capacity. That is what really backs up traffic.”

A single lane of traffic flowing through a work zone may seem like it would move faster without a slowdown for merging traffic, but drivers slow down when they see a long line of cars for many reasons, according to the Minnesota Department of Transportation. Those include uncertainty of drivers’ actions ahead, poor visibility beyond, and signs, barricades and other obstructions.

MNDot found that by creating two full lanes of traffic, everyone is going about the same speed and is less likely to have road rage or other forms of bad behavior such as driving in the center of the lanes to prevent passing.

When Zipper Merge Isn’t Needed

But the zipper merge isn’t usually needed at low traffic volumes, when drivers finding gaps on their own and merging sooner will help keep traffic moving, he says. The zipper merge is best used when there’s lots of traffic, and when signs tell motorists to use all of the lanes and to take turns merging at the merge point, Ullman says.

The zipper merge also works well when it’s not done that often, and shouldn’t be used when people naturally merge on their own well before the merge point, he says.

“Most people seem to get it,” Ullman says. “The signs are pretty easy to understand.”

It’s also not needed much in rural areas, where there isn’t as much traffic and merging early is a more common behavior for people, he says.

While the method goes against everything you learned about driving etiquette, it does reinforce something that every kindergartener learns: Take turns.

As NMA’s Bowman says, “Most drivers resist the zipper merge because they don’t want to be seen as the person skipping line. They’re also afraid that other drivers will get angry and try to shut them out at the merge point.”

“If all drivers understood the principal behind the zipper merge, they might be more forgiving of drivers who practice it, or they might try it themselves,” he says. “But it takes cooperation between drivers to make it work.”


Aaron Crowe is a journalist who covers the auto industry for