The Zipper Merge: Convincing Motorists Isn't a Snap

By Matt Schmitz  on May 19, 2014

Global climate change is "undisputed fact" or "needs more study." Spiritual or secular. Coke or Pepsi. There are certain lines we draw in society that shan't be straddled. The totality of who you are, how you were raised, what you've experienced and what you know, or think you know, all converge to form steadfast beliefs. It's probably safe to add one's stance on the "zipper merge" to that list.

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Never heard of it? Lack of familiarity is central to the sometimes emotional debate that erupts when talking about the zipper merge, aka late merging. Say there are two lanes of highway traffic and because of construction a section of one lane is closed, narrowing traffic to one lane. It's rush hour and traffic flow has slowed dramatically. You see the sign well before the merge telling you traffic is reduced to one lane ahead. What's the safest, most polite course of action? A) Put your turn signal on and move from the ending lane to the continuing lane when there's a natural break or another motorist waves you over, or B) Continue in the ending lane all the way to the merge point, then make your way over.

Conventional wisdom or common courtesy dictates that you follow procedure A, getting over as early as possible to keep traffic flowing and be fair to everyone. And that presumptuous driver who motors all the way up to the front and then expects someone who got over early and paid their dues to just up and let them in? They're a jerk (or something saltier), right? Not so fast.

Studies by the Texas Transportation Institute, Minnesota Department of Transportation and Federal Highway Administration have shown that the zipper merge (thus named because both lanes are used and drivers take turns merging one car at a time, like the teeth of a zipper) improves traffic flow by as much as 15 percent, according to the Detroit News. MnDOT also found it reduces the total length of a backup by as much as 50 percent, and commonly by 40 percent.

"Although motorists seem to believe that a single lane of traffic flowing into a work zone should flow through unrestricted and much faster without a slowdown for merging traffic, this just does not happen in the real world of traffic hazards," MnDOT says. "Motorists slow down because of the uncertainty of the drivers' actions ahead, poor visibility beyond, signs/drums/barricades and (other obstructions). That slows down the rest of the line of traffic, and the longer the queue, the more it slows down and a longer time before it regains speed."

Instead, states that ask motorists to use the zipper method want all available roadway used up to the merge point. The zipper is not intended to be used when traffic is flowing briskly, with ample distance between vehicles to move over earlier without causing a slowdown. But when traffic is slow moving, using both lanes and taking turns one car at a time from each lane has proved effective. MnDOT says by creating two full lanes of traffic, the speed difference between the lanes is reduced. When everyone is equally "disadvantaged," incidents of road rage and other bad behavior — like playing "lane cop" and driving in the center to prevent passing — are fewer. Signs instructing motorists to use both lanes up to the merge point help take the pressure off people who fear scorn or retribution.

The problem is, the concept pretty much goes against everything we've learned about driving etiquette, which traditionally tells us to get into the continuing lane ASAP. Compounding that is a lack of consistency among states. While states like Minnesota, Illinois, Colorado, Washington and Pennsylvania use, or have used, the zipper merge, California, for example, does not.

Mark Dinger, spokesman for the California Department of Transportation, says the Golden State encourages early merging by posting signs at a half-mile, 1,500 feet, 1,000 feet and at the merge point letting the driver know a construction zone is ahead and which lane is ending. Citing Federal Highway Administration data, Dinger said sudden braking caused by late mergers results in rear-end collisions — the most common type of work-zone accident.

"Drivers who cut in at the last minute cause sudden stopping and lane changes, which cause direct collisions as well as delayed-reaction collisions by drivers further back in the queue who may not be paying attention or expecting traffic speed to suddenly change," Dinger said.

There's also a cultural attitude that late mergers are unfairly getting away with something. An informal poll of our street-savvy staff on's editorial team yielded considerable dissent, with 10 participants voting in favor and four against. Even among yea-sayers, a common caveat was that they'd need to feel confident drivers around them also understood the procedure. Meanwhile, dissenters' comments ranged from, "I understand the theory of zipper merging, but I'm a fan of getting over as early as possible" to "Zipper mergers deserve to be tarred and feathered."

Can zipper merging overcome such deeply rooted aversion? Proponents believe with enough public education and practice, motorists will get the idea. In the meantime — as always — it seems all we can do is stay alert, pay attention to signs, be courteous to other drivers and exercise caution when changing lanes. illustration by Paul Dolan

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News Editor Matt Schmitz is a veteran Chicago journalist indulging his curiosity for all things auto while helping to inform car shoppers.  Email Matt