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

This page last updated April 8, 2021


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 impatient people take advantage of the empty lane to cut in at the front of the line, causing incalculable road rage amongst those waiting patiently. 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.

(Photo courtesy of Oregon Dept. of Transportation)

German zipper merge signLegally, 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 at the point where the lane ends? Fortunately, 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, 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.)

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. 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 predict where they'll merge, align themselves, and merge smoothly without having to stop. These circumstances together result in a smoother and more consistent traffic flow. And by using the full capacity of the truncated lane, the length of the backup is reduced correspondingly.

Finally, one big benefit of the Zipper Merge that is rarely recognized or discussed but may be the most important to many folks is this: filling the lane that is closed prevents opportunist drivers from taking advantage of an empty lane to cut in the front of the line. That alone makes it worth the price of admission to me. And 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.

The biggest obstacle to implementing the Zipper Merge is that pretty much everyone has to be on the same page 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 feel that visceral need to merge early. Therefore, a significant public education campaign along with signage 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 time when merging early is better
The Zipper Merge works best when traffic is already congested and moving slowly through a bottleneck. But in those rare instances when traffic is free-flowing, 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 just shove themselves into the through lane. Someone in the through lane then has to slow considerably or 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 and/or 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.
  • When traffic is free-flowing, merge early.

Zipper Merge status in Texas
While the Zipper Merge is being promoted in other states and is even the law in places like Germany, it's still virtually unknown in 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 few years ago (link below) is 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.

TxDOT San Antonio blog - Is the "zipper merge" rude?
Minnesota DOT - Zipper merge
Minnesota DOT - Late Merge - The Zipper System (PDF)
YouTube - Minnesota DOT - Zipper Merge Traffic Camera Instructional
The beauty of zipper merging, or why you should drive ruder
As States Fall in Line, Does Zipper Merge Still Push Drivers' Buttons?
Will the "zipper merge" help traffic flow on the roads?

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