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Which is better: Merge early or merge late?
(Spoiler: Usually the latter, but it depends!)

This page last updated May 12, 2023

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Perhaps nothing can cause such a universal emotional response in drivers as the dreaded lane-ending-merge like the ones when a lane is closed for road work or a crash. We all know the drill-- most people move out of the truncated lane early while a few seemingly self-important people take advantage of the empty lane to cut in at the front of the line, causing incalculable road rage amongst those waiting patiently in the through lane. Even some of those people cutting in at the front feel the stress of that tense, last-second merge while knowing that they're now the target of disdain of dozens of people behind them. It's even worse for the innocent, well-meaning motorist who tried to get in earlier but was blocked by self-defeating road vigilantes.

Drivers properly executing a Zipper Merge
(Photo source: ABC7/WJLA-TV)

In Texas and most states, legally, there is no requirement to merge until the taper point. Therefore, this situation exposes a deep-seated philosophical argument: Is it better to merge early, like most folks do, or wait and merge "late", i.e. at the point where the lane ends?

German zipper merge signFortunately, recent research and experimentation is helping to inform the debate. Unfortunately, like so many things in life, the answer isn't cut-and-dried-- it depends on the situation.

Most drivers are either taught, or learn on their own (often instinctively), to merge early. That just seems to be the "right" thing to do on so many levels. And sometimes it is indeed the right thing to do.

But traffic experts have discovered in recent years that sometimes it's actually the wrong thing to do, and the late merge has now become the recommended procedure in a number of states including Minnesota, Wisconsin, Washington, Missouri, and Kansas, as well as in much of Europe, where in Germany it's actually the law. (German zipper-merge sign at left.) It's also supported and recommended by the AAA.

The "Zipper Merge"
The "late merge", known more descriptively as the "Zipper Merge", is when drivers fill up both lanes and then take turns merging one-at-a-time at the point where one lane ends. That one-at-a-time merge is where the "zipper" name comes from.

Research has shown that this method is safer and helps to keep things moving more smoothly through the merge point because drivers are cooperating and creating gaps, thus eliminating brute force injections into the through lane. It's those sudden, unexpected forced merges, especially from a standing stop, that causes shock waves in the traffic flow that creates the stop-and-go conditions often seen in these situations.

Here is why it works:

  • When people take turns and know that's the expectation, they can gradually open a gap earlier while still moving.
  • Drivers in the closed lane can better predict where they'll merge, align themselves, and merge smoothly and safely without having to stop.
  • Because everyone keeps moving, it eliminates shockwaves in the traffic stream, which reduces the risk of rear-end collisions, and drivers feel less stressed because traffic is still moving.
  • By using the full capacity of the truncated lane, the length of the backup is reduced correspondingly.

These circumstances together result in a smoother, safer, and more consistent traffic flow, and studies have shown it can reduce congestion by up to 40%.

(Source: Oregon Dept. of Transportation)


Here's maybe the best reason to use the Zipper Merge
One big benefit of the Zipper Merge that is rarely recognized or mentioned, but may actually be the most important reason for many early-merge adherents, is this: The Zipper Merge fills the lane that is closed, thereby preventing opportunistic drivers from taking advantage of an empty lane to zip past everyone and cut in the front of the line.
That alone makes it worth the price of admission to me. Furthermore, if both lanes are used, they will tend to even themselves out, which then makes it fair for everyone, i.e. the person you let in at the merge point will have been waiting about as long as you have.


But for it to work, everyone has to be on the same page
The biggest obstacle to implementing the Zipper Merge is that pretty much everyone has to buy into it and play along in order for it to work. If only a handful of people do it and the other drivers aren't aware of it, then it gives the appearance that the zipper-mergers are just taking advantage, and many of them may therefore feel like that's what they're doing and will abandon it.

Furthermore, it's hard to break such a deeply-ingrained habit and belief system. I myself know the benefits of the Zipper Merge, but I still often feel that visceral need to merge early.

Therefore, a significant public education campaign, along with signage in work zone merge area, is needed to make the program work. Studies have shown that when this is done, the benefits are realized.

This humorous description of the Zipper Merge is by a Canadian, but is still applicable to the US:


But there still is a situation when merging early is better
The Zipper Merge works best when traffic is already congested and moving slowly through a bottleneck, which is frequently the case. In that situation, merging early provides absolutely no benefit to anyone.

But in those instances when traffic is free-flowing through a lane drop (like in the photo below), then the early merge is the best thing to do. Merging early in this situation is safer and helps to maintain the free-flow of traffic because, as mentioned before, drivers who wait until the very last minute often need to slow considerably or even come to a stop in order to merge, or will sometimes just shove themselves into the through lane. Someone in the through lane then has to slow considerably or even stop to allow them to merge, which then causes the person behind them to slow or stop, and the dominoes fall from there and becomes the genesis of a traffic jam or, worse, a rear-end collision. Merging well in advance in that situation allows drivers to find and enter a gap when other drivers only need to make minor adjustments to their spacing while maintaining speed, thus preserving the traffic flow.

So to summarize (and because many people like things boiled-down to simple bullet points):

  • When traffic is already slowing or congested, use the Zipper Merge. Merging early is pointless and even counterproductive in this situation.
  • When traffic is free-flowing, merge early. Doing so in this situation is safer and can prevent congestion from forming.

This is an example of a time NOT to use the Zipper Merge
(Photo source: Oregon Dept. of Transportation)


Zipper Merge status in Texas
While the Zipper Merge is being promoted in other states and by AAA, and is even the law in places like Germany, it's still virtually unknown in headstrong Texas. TxDOT studied it over a decade ago, but it has not been officially adopted for widespread use. A blog posting by TxDOT's San Antonio office a while back was the first mention of it I've seen in Texas in recent years, and TxDOT's Waco district reportedly implemented it at select work zones in the Temple area for a time. Perhaps as more states implement it and have good results with it, Texas will join the party.


Other sites of interest
There are a lot of sites about the Zipper Merge, but here are a few good ones.

TxDOT San Antonio blog - Is the "zipper merge" rude?
http://txdotsanantonio.blogspot.com/2016/10/is-zipper-merge-rude.html
Minnesota DOT - Zipper merge
http://www.dot.state.mn.us/zippermerge/index.html
Minnesota DOT - Late Merge - The Zipper System (PDF)
http://www.dot.state.mn.us/trafficeng/workzone/doc/When-latemerge-zipper.pdf
YouTube - Minnesota DOT - Zipper Merge Traffic Camera Instructional
https://www.youtube.com/watch?v=ZcPby71TNC0&feature=youtu.benUtjs-jeJA
The beauty of zipper merging, or why you should drive ruder
http://arstechnica.com/cars/2014/07/the-beauty-of-zipper-merging-or-why-you-should-drive-ruder/
As States Fall in Line, Does Zipper Merge Still Push Drivers' Buttons?
https://www.cars.com/articles/as-states-fall-in-line-does-zipper-merge-still-push-drivers-buttons-1420688661722/
Will the "zipper merge" help traffic flow on the roads?
http://www.cbsnews.com/news/zipper-merge-helps-traffic-flow-engineers-say-kansas/


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