Zipper Merging

Monday, August 4th, 2014

Most people agree that merging early is polite, and merging late is rude, but traffic experts laud the beauty of zipper merging:

It works as follows: in the event of an impending lane closure, drivers should fill in both lanes in equal measure. Within a few car lengths of a lane ending, both lanes’ cars should take turns filling in the open lane and resuming full speed.

If roads are clear enough that everyone is already driving close to the speed limit, zipper merging isn’t as effective, but in the case of congestion, Johnson said that this method reduces backups by a whopping 40 percent on average, since both lanes approach the merge with equal stake in maintaining speed. “When the queue backup is reduced, the access points behind a work zone, like signals or ways to get on and off the freeway, those aren’t blocked,” Johnson pointed out. “People have a better opportunity to get off or on the system at that point.


  1. William Newman says:

    In principle this sounds plausible. But it’s hard to understand why those traffic experts who happen to have authority over roads have long used designs that seem to discourage it by making it unnecessarily nonobvious where this efficient merge point is supposed to be (and why the article doesn’t say anything about that). Wouldn’t it be easy to instead facilitate it not just by scolding drivers in general but by putting up a clear “Merge Point 1/8 Mi. Ahead” followed by “Merge Here”? The usual design has left me with the impression that authorities work on some other antithetical design principle, that somehow efficiency would be maximized by constricting things gradually and informally over very large multiples of a car length.

    Strategies and norms based on chains of “I know that he knows that she knows that I know that she knows that he knows…” logic naturally break down rapidly when they involve an issue which is significantly ambiguous for every participant.

  2. Roger says:

    I first experienced the “zipper merge” when I was working at an army base one summer when I forcefully learned what the signs “Traffic Will Alternate” at every parking lot exit meant.

  3. Rob De Witt says:

    Why is this difficult?

    You go, I go. “Merge,” get it? I figured this out over 40 years ago.

  4. William Newman says:

    “Why is this difficult?”

    Even easy things are easier when basic steps are taken to make things clear. Note that people seem to find it easy to figure out that we take turns at intersections, too. But even that easy case can be helped by practices like painting big white lines in front of the official stopping places to minimize ambiguity about concepts like “has arrived at the intersection”. And when tricky cases arise, like the judgment call about the threshold for transition to not taking turns because one road is slow and carries little traffic while the other is fast and carries a lot of traffic, it can be appropriate for one person to make the judgment call at leisure and then take a lot of care to communicate that clearly to everyone else.

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