Just do the zipper merge, for the sake of all things holy

·9 min read

Driving home from our cottage in Northern Michigan, we were warned by a sign on I-75 that the left lane would be closed in four miles. About a half mile later, traffic came to a sudden stop. It took about 15 minutes of sitting in stop-and-go traffic to figure out that there was someone a few cars ahead, blocking the left lane, which is clear for miles ahead of it. Some self-deputized wannabe traffic cop was sitting in the left lane, creeping at a pace even slower than the right lane to prevent people from using it. I moved over into the right lane. Eventually, I got past the guy and moved back into a clear left lane. Others fell in line behind me to make full use of the road. In a jam ahead of a temporary merge point, that’s what you do.

The "zipper merge," as I’ve come to understand, reduces traffic backups by making use of all open lanes. It seems intuitive to me. Two lanes full of 500 cars are going to be half as long as one lane accommodating those same vehicles. Duh.

As we creeped along in the left lane, another driver took umbrage at my presence there, waited until I’m just behind him, and, without warning, sprang into my lane. I swerved onto the shoulder to avoid him, my heart is racing like I just had a big moment near the wall at Sonoma Raceway even though we’re moving at maybe 25 mph, barely a brisk walk quicker than the other lane at that moment. My son asks what’s going on from the rear passenger-side seat, the spot that probably would have taken the blow from the other car if that driver had their way.

Apparently, my legal and expert-recommended use of an open lane is punishable by whatever happens when a man and his kindergartener are potentially forced off the road. Later that evening, I’d calm my mind for bed by closing my eyes and picturing a long, frustrating existence of unending inconveniences, incurable itchiness and terrible food for that driver.

I would also try to fathom why someone who would risk the life and limb of other drivers — and their own — by maneuvering 3,000 pounds of metal deliberately and suddenly into someone’s path. Or, more innocuously but no less illogically, why they’d do what the first guy did, and block miles of open road, puttering at the same speed as, or slower than, the lane everyone will need to end up in … eventually. (I often see drivers straddle both lanes to the same despicable effect.) Surely, they must know that they’re creating a traffic backup much, much worse than it needs to be, sometimes stretching far enough to create havoc at on-/off-ramps miles back. It’s something I see all the time. So why do they do it?

People who do this, what the hell are you thinking?

Do the zipper merge

First of all, here’s what you’re supposed to do when traffic backs up ahead of a merge point. It’s called the “zipper merge” (or, alternatively, the “late merge”):

When you come to a point in the road where a lane is going to end — for example, a sign saying “LEFT LANE CLOSED, 2 MILES” — stay in your lane. Be patient. Wait until the actual merge point and then merge, and give others the space they need to merge. It should be a simple, alternating feed at that point. Car in open lane moves forward, car in closing lane moves in behind them. Car in open lane moves forward, car in closing lane moves in behind them. A, B, A, B, like the teeth of a zipper coming together.

Cooperation is the secret sauce. The result is a faster, more organized traffic flow with the backup behind it being lessened as much as possible by cars making full use of all open lanes.

Here’s what a spokesperson at the Michigan Department of Transportation told me about the zipper merge:

“If traffic is light, then typically it doesn’t matter when you get over. But when things start to bog down and jam up, we want folks to use both lanes so the backups don’t extend far. So for example, if it’s congested with 1,000 vehicles, we would rather have 500 vehicles in one lane and the other 500 vehicles in the other so the backups don’t extend. This helps when there are curves, line of sight issues, or when other interchanges are within close proximity.”

In case you got your driver’s license from a cereal box, here’s what MDOT suggests not to do (emphasis mine, as it’s probably the most important words you’ll read this week):

“The motorists that block the open lane are only creating a dangerous situation and invoking road rage. They are accomplishing nothing more than that.”

Of course, I didn’t invent the zipper merge — though I feel as though any driver with any sense of spatial reasoning could come to this sort of conclusion on their own — but it is embraced and advocated by traffic engineers, transportation departments and driving educators. States like Arizona, Colorado, Michigan,  Minnesota, Missouri, Nebraska, North Carolina, Texas and Washington, among others, have tried to get drivers on board with the zipper merge. It seems like it should be common knowledge by now. When I mentioned I was writing this piece, a colleague said, “I thought there was an open-and-shut case in favor of the zipper merge.”

My reply: “Not in Michigan, man.”

So I ask again, what in the ever-living cuss are you drivers in your lifted pickups displaying multiple flag code violations thinking when you block an open lane? Does it make you feel powerful to obstruct traffic? Do you think you’re a cop? Do you take being passed as a personal insult that cannot stand? Do you think you’re helping somehow?

I won’t get a definitive answer from Dr. Bill Van Tassel, the manager of driver training programs at AAA, who was kind enough to answer my questions about the zipper merge.

The expert opinion

While Dr. Van Tassel hasn’t seen formal research on why people block the zipper merge, he has guesses. When I suggest maybe these drivers think they’re somehow contributing to some greater good, Van Tassel posits, “It’s probably just more, sort of, people focused on their self-interest as opposed to, ‘Oh I think this is going to help everyone if I block this lane so nobody can use it.’ I guess it’s probably closer to the former that, ‘This will help me get through faster, and if something else goes around me, then I’ll get through there slower, so I won’t let that happen.’”

That’s flawed thinking on the part of those drivers, which seems obvious to me, but you don’t have to take my word for it. As Van Tassel told me, the zipper merge demonstrably “speeds things up.” Using the zipper merge through temporary work zones, research suggests, improves traffic flow by up to 15%, which Van Tassel calls a “nice little bonus.” Furthermore, it can decrease the physical length of the backups by 40-75%.

Other benefits of the zipper merge

There is a safety aspect to it, too (beyond the obvious DON’T DART OUT IN FRONT OF SOMEONE IN THE NEXT LANE, PLEASE).

Dr. Van Tassel believes it can help improve safety for those working in those construction zones. “With people entering that single lane more calmly and more smoothly, we hopefully only enhance the safety of professional pedestrians, if you will. … They’re the priority.”

While he hasn’t seen a lot of research on this (lack of research on the zipper merge beyond its traffic flow improvements seems to be a theme), Van Tassel suggests the zipper merge improves driver safety as well. “The mere fact that you have to allow space for cars that need to move in front of you if you’re in that lane — or that you have to arrange for space to be there if you’re the car moving into that lane — that sort of gets everybody focused on space.”

“Space is your best friend,” he added.

Communication is also an aid to and a benefit of the zipper merge. It can be obvious what to do when cars are taking turns merging into a lane, but it never hurts to communicate with those around you with a wave of the hand or a blink of the headlights.

It’s not unlikely that there’s an environmental benefit to practicing the zipper merge (here's one study that supports it). It’s something folks like Van Tassel would like to see more studies on. “If we’re able to shorten the length of traffic and increase the traffic flow, then we’ve probably got fewer people just sitting there idling — pollution from a traditional-engined car. There might be some environmental benefits that could be quantified, and that might be strongly appealing to some people.”

Did I do it wrong?

I told Dr. Van Tassel about my earlier anecdote on I-75, and asked for advice and criticism. I was sure I’d get docked points for it. Here was his response:

“Generally, it is fine to pass by the slower vehicle, if the passing can be completed safely.  It would necessarily involve changing lanes twice, so safety is a significant concern and worth mentioning. It will also be critical to not cause any problems for other road users (including pedestrians, cyclists, etc.) during those maneuvers. One recommendation we make is to never cause another driver to have to brake, or steer around you. This could be quite applicable in this situation.

“It is also worth noting that some states may prohibit passing a slower vehicle on the right, so it’s important for drivers to be aware of their state’s laws regarding passing. Indeed, slower drivers should be traveling in the rightmost lane, but that doesn’t always happen. …”

Indeed it doesn’t.

Please, folks, drive cooperatively. And, whatever you do, don’t block traffic. Everyone’s trying to get somewhere, and we’ll all get there faster and happier if we use the zipper merge.

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