Merge Overkill

The Oregonian’s Joseph Rose is wished dead by an irate reader after discussing the potential benefits of “late merging.”

This made me all nostalgic as it was this kind of vitriol that launched me on the Traffic road: How could this simple activity stir such passion? While I am delighted to see the issue receiving further elaboration and exploration, I should only clarify here that I was not advocating a Universal Late Merge plan. There are circumstances where this behavior would actually make things worse. But the point was more that in certain scenarios, it would work better (better traffic flow, shorter queues, etc.), and that of course it would be better if drivers were instructed what to do — so as to not set off anger against the minority late merger position by early merge vigilantes — and then, having been instructed as how to properly merge, people then actually left these old prejudices behind (which trials have shown does not always happen).

But this is admittedly complex, for what makes late merging a better overall system for some highway segment may depend on a change to a certain level of congestion (in which case you’d need real-time ‘dynamic’ signage announcing the late merge), or it may depend on the number of lanes on the highway, or it may depend on the volume of trucks on a particular ‘facility.’ The correct cure depends on the set of symptoms.

This entry was posted on Tuesday, February 24th, 2009 at 3:53 pm and is filed under Drivers, Traffic Culture, Traffic Engineering, Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

10 Responses to “Merge Overkill”

  1. Brad Templeton Says:

    As I read the analysis in your book, I came to the conclusion that late merge is not the best strategy (for the highway) when there is a “late merger minority” and an “early merger majority.” That, as you point out, was shown in studies to generate anger, lane straddling, and more stop and go, plus of course the feeling that the minority is cutting into line and thus selfish.

    Late merge provides the best flow (in a highway with too much traffic to avoid collapse to stop and go) when you don’t have a minority or majority, when they are split roughly evenly.

    I think this is a very important thing to point out when preaching late merge. People can see why late merge is fair if it’s 50-50, but they will obviously see it as unfair if it gets uneven.

    Perhaps the right approach (nudge) is to give it a better name which implies more fairness. “Late merge” implies, to most people, that somebody is merging later than somebody else, which is unfair, even though the real meaning is that the merge occurs late in the merge zone rather than early.

    So another name — “joinpoint merge,” “confluence merge”, “alternating merge”, “chokepoint merge” or something like it, might save the day.

  2. Jack Says:

    The American “melting pot” where our true identity is found, what vitriol. I definitely like “checkpoint merge”. What do you call truckers that team up to block merging traffic in this cultural melting pot?

  3. Dan Says:

    The solution in many, but not all, merge situations is to reeducate drivers that they are to alternate possession of the continuing lane. This would make it clear that cars in the continuing lane have no more right to it than cars merging into it. Once drivers realized this they would rationally occupy both lanes and better utilize available road space.

    And if I was to generalize I would say most traffic inefficiencies are a result of inadequate road sign information. And with GPS systems I feel this is becoming even worse. So when the GPS system says they will bear left in 500 yards drivers will cram all the way over to the left and not realize the left 2 or 3 lanes will suffice. Better road signs could educate drivers of optimal lane choices and better GPS systems could also make a difference.

  4. Joel Says:


    A related issue to the vitriol toward late mergers is the vitriol toward motorcyclists who “lane-split” or “filter” through traffic. As you probably know, it’s legal in California (and nearly everywhere else outside the U.S.).

    I live (and ride) on the East Coast, so I don’t lane split but I’ve heard many motorcyclists report hostility from drivers and traffic cops for doing it, even where it’s legal. Apparently a lot of them feel like it’s some sort of unethical form of “cheating traffic.”

    Also, have you seen any studies that consider whether it’s safer for motorbikes to lane-split vs. sit in the lane? This is a constant source of debate, and many riders feel more vulnerable to being rear-ended while sitting in a lane than filtering through traffic like they do in Europe.

    And really…you should do more blog posts on motorcycles/scooters. We’re a segment of the roadgoing public that is VERY attuned to traffic issues, and especially the hassles of dealing with “cagers” who seem to resent us (even though we take up such a small footprint on the road).


  5. MikeOnBike Says:

    Anybody who has ever stood in a ski lift line knows how to do an alternating merge. Sometimes there are multiple merges, with four lines merging to two, then those two lines merging to one.

    It helps that no line has priority. And it all happens at pedestrian speeds. This is different from the typical “lane ends, merge left” road design.

  6. Brad Templeton Says:

    I believe the best answer for the transition is metering lights, which we have been trained to obey. You need smart metering lights though, which only turn on when the traffic has collapsed to stop and go. Once people saw the light altering back and forth they would quickly fill both lanes.

    Only works if you have 2 lanes to 1. 3 lanes to 2 is hard because there is no easy place to put the light for the middle lane that isn’t a hazard when things are not stop and go, unless you can string them overhead.

    Power is of course an issue.

    For speed, it would be best to do a 2 cars per green metering light, though people seem to have trouble understanding those, even today when they have been around for a while at highway interchanges.

  7. mdf Says:

    Brad Templeton: Once people saw the light altering back and forth they would quickly fill both lanes.

    But wouldn’t that just force the solution some people feel is “right”?

    Suppose instead we just turn on the green for the continuing lane and leave the merging lane red. On the condition that everyone obeys the lights — that is, the merge lane would sit there and patiently for the light that would never come — wouldn’t the traffic flow equal the maximum possible rate determined by up-stream issues?

  8. Bruce Says:

    Truckers block the cheaters from merging late, because truckers understand efficiency and human nature like no professional writer ever could. Yes, I question the un-named “studies” that Mr. Vanderbilt claims have been done.

    On a limited access highway without obstacles, the maximum traffic capacity (vehicles per hour) of two lanes (3600 VPH) is indeed twice that of one lane (1800 VPH). However, in high density traffic, the maximum capacity of two lanes merged into one lane (about 1200 VPH) is significantly less than that of one lane (without a merge), because late merging invariably forces stop and go driving. (You cannot legislate human behavior.)

    At saturation, the maximum VPH of the entire stretch of highway between any two exits is determined entirely by the most restrictive bottleneck. “Using up” empty lanes does nothing to relieve the bottleneck itself.

    When truckers manage to block enough cheaters that actual merging is eliminated at the point of lane closure, the line that has already merged can and does proceed much faster (back as high as 1800 VPH), but of course only until the blocking truck (or car) arrives at the point of lane closure. Of course, some cheats are so self centered that they will even drive on the grassy median to get around a blocker. I have pictures!

    When the rate of vehicles entering a segment of highway is greater than the capacity of remaining lanes adjacent to a closed lane up ahead, the most efficient thing to do would be to move the point of lane closure back as far as necessary, to a point where traffic is not (yet) at saturation and can merge without forcing stop and go driving in the process.

  9. Dave Flynn Says:

    As a Traffic Engineer and someone who studied this at graduate school, late merging is best in most conditions. I myself took this to heart and was frequent effective late merger but my wife told me I wasn’t be fair so I had to knock it off. Kudos to Joel for his book and leading off with this critical driving question and taking such a subject on. Traffic management still has miles to go to get up to speed for an information driven society.

  10. Ed Foreman Says:

    A little late to the party, and I tripped over these reviews without reading the original article. BUT… I have been driving the LA Freeways since 1971. In my years of commuting I have had an average 1-2 hours each way. To put it mildly, I have seen one hell of a lot of merging. Barring all the odd scenarios proposed above leave 95% of real life on the road. I can tell you from experience that, one the rare occasion when intelligence wins out, the “choke point merge” or as my wife and I call it “the zipper effect” increases traffic flow and leaves everybody happy. When you merge early, the entry lane is contributing at a 2 or 3 or 4 to 1 ratio and creating the traffic jam everybody is trying to avoid. This does not apply to the idiots that go off roading to pass the head of the line and squeeze every last foot out of the approach lane. When the line ends the lane ends. At that point you should not pass. You are driving on the shoulder.

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