September 17, 2009

Late Merge Studies Indicate It Is Faster and Safer

When you are driving on a highway and your lane is closing, do you merge as soon as you see the signs or do you wait until the last second to merge?

It turns out that this is a very contentious question with strong feelings attached. When I brought it up the last time on my blog over 2 years ago I found that while I believe that it is more efficient to merge at the last second (in a zipper fashion with each lane taking turns) close friends of mine had contempt for late merge behavior. Unfortunately for them, I did not find early-merge arguments convincing. While I completely understood that perception of late merge behavior was at the least, a black mark against it, I am one who has trouble letting go of a better solution just because other people are not convinced.

Yeah, these things stay on my mind for a long time.

The issue came up on Reddit today. it prompted me to read some responses and I was happy to see some of my same arguments used. But there were also references to scientific studies done, so I tried to look some of those up for you. Here's some info I encountered, for you to read or ignore at your pleasure:

  • Here is the Reddit thread where you can see people mentioning that late merging eases a backup by making more use of the roadway. It appears that in light traffic it doesn't matter when people merge, but in heavy traffic, late merging is better.
    • The rules in Germany are cited: "Heavy traffic rules: Whenever traffic is heavily congested, normal right-of-way rules go out the window and the "zipper rule" (Rei├čverschlu├č) goes into effect. This means that cars feed one at a time alternating from each direction, regardless of who has the posted right-of-way. The "zipper rule" also applies when one lane ends and merges into another. Each vehicle in the through lane must allow one vehicle from the truncated lane to merge in." (The zipper rule occurs at the point of forced merge.)
  • Systems have been adopted in Minnesota and other states to referee the zipper system. [PDF] Based on what they refer to as traffic science, late merging is recognized as better, and so they are attempting to train people to expect late merge behavior and to make it even more efficient.
    • "When traffic is heavy and slow, it is actually much safer for motorists to remain in their current lane until the point where traffic can orderly take turns merging which is generally near the "MERGE" sign. Unfortunately, while the safer procedure is legal, it is not what has been taught."
  • This columnist at the Oregonian found people reacted quite violently to his suggestion that traffic science supported late merging. "I hope you die"
  • If you're so inclined, here is a research study on the development of a system to take advantage of the benefits of late merging. [.DOC format] It's got references to support a number of claims about late merging (known as "static late merge":
    • increases work zone throughput
    • decreases travel time
    • reduces road congestion
    • reduces queue length
    • decreases potential of rear-end collision
    • decreases lane-straddle conflicts

Perhaps this will convince nobody and states will have to continue to think up ways to force people to merge at the last second. In the meantime, at least I've got science on my side while the early merge folks are cursing at me. But instead of cursing, why not consider just staying in your lane and merging at the last moment, like a zipper? I promise to alternate and let you in if you're next to me.

(I will be happy to browse any research that supports early merging. I would like to know the rationale behind any such research.)

Posted by James at September 17, 2009 7:23 PM
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Sssh! Don't give away the secret. See, because lots of people do early merges, those of us who cottoned onto the late merge decades ago have saved ourselves countless minutes of our lives. We not only get the throughput benefits of the late merge, we also get the added benefit that people who merge early get out of our way.

The key is that when there's a lane that's ending, you get into that lane as soon as you can. And then you merge at the last sec. 'tis great.

Now, what I hate, and would like to see people fried for, is when a lane doesn't end, yet people try to use a continuing traffic lane to cut in front of a queue of people at the last moment. Most of the time, they wind up having to wait to be let in, and they block traffic in the other lane in the meantime. Grrrrr.

Posted by: Barry Leiba at September 17, 2009 9:39 PM

Yes - I agree, Barry. That is infuriating. As when people are queued up at an exit. In that case it's not merging and the regular rules of queuing apply.

Posted by: James at September 17, 2009 10:48 PM

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