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Merge early or merge late?

This page last updated June 29, 2019


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 an accident. 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.

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 at the point where the lane ends? Fortunately, recent research and experimentation is helping to settle 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, 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 Europe.

The "Zipper Merge"
The late merge, known more affectionately 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. Studies have shown that in congested conditions, this technique reduces the backup by 40 to 70% and that traffic generally moves better and 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. When people take turns and know that's the expectation, they can gradually open a gap 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 backup is reduced correspondingly. Finally, and maybe most importantly to many people, filling the closed lane prevents opportunist drivers from taking advantage of an empty lane to cut in the front of line, thus reducing everyone's anxiety. In essence, it disadvantages everyone equally.

The biggest obstacle to implementing the zipper merge is that enough people have 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 like 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 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.

When to merge early

The zipper merge works best when traffic is already congested and moving slowly through a bottleneck. When traffic is free-flowing, then the early merge is the right 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 stop to merge. Someone in the through lane then has to stop or slow considerably to allow them to merge, which then causes the person behind them to stop or slow, and thus you have the genesis of a traffic jam and/or rear-end collision. Merging well in advance allows drivers to find and enter a gap in the traffic and for other drivers to then make minor adjustments to their spacing while maintaining speed, thus preserving the traffic flow.

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 recent blog posting by TxDOT's San Antonio office (link below) is the first mention of it I've seen in Texas in recent years, and the Waco district recently announced they would be implementing it at select work zones in the Temple area. Perhaps as more states implement it and have good results with it, Texas will join the party.

Other sites of interest

TxDOT San Antonio blog - Is the "zipper merge" rude?
Minnesota DOT - Zipper merge
Minnesota DOT - Late Merge - The Zipper System (PDF)
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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This page and all its contents are Copyright 2019 by Brian Purcell

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