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 > How to Merge

 The Three-Lane Merge

## How to Merge

Imagine a situation in which two lanes merge into one. I tend to think of an entrance ramp merging onto a highway; a lane being closed by construction, or by an accident; or simply a road becoming narrower, with fewer lanes. In any case, the cars traveling in the lanes also need to merge. How do you do that?

Well, there's an obvious answer, which is that you alternate, taking one car from one lane, then one car from the other. However, that's not a complete description of what's going on, and in fact it's not always true. So, let me come at it from a different angle.

Suppose the traffic is very light. In that case, the way you merge is that you all just move over into the remaining lane—the left lane, say—and keep on driving.

As the traffic becomes more dense, it becomes more awkward to just move over, so perhaps a bit of a line builds up. and you have to slow down a bit to get in line. And then—zoom!—somebody speeds by in the empty lane and cuts in right at the front of the line.

Then, eventually, when the traffic is dense enough, the two lanes will end up packed solid to approximately equal depths, with a classic alternating merge occurring at the front.

So, we have three different states the merging traffic can be in, and two transitions between states. The first transition is directly tied to the usual traffic phase transition—it takes place when the compression of two lanes into one pushes the traffic density past the threshold of condensation, or whatever you call it.

The second transition is more interesting. The final state, with both lanes packed equally, seems better to me. It certainly prevents the annoying defection of zooming to the front, and I'd bet that it's more efficient, too. My thinking there is that once you're in the stop-and-go wave propagation regime, it's better to have two lanes of waves than one … one that keeps getting interrupted, in fact. Somebody ought to make a model and investigate this!

So, anyway … if the final state is better, why do we bother with the intermediate state? I think there are two reasons.

The first is continuity. Suppose, again, that the traffic is light, i.e., that we're in the initial state. If the merge is marked well in advance, people will generally move into the left lane well in advance, too. Then, when the merged traffic condenses, everyone's already in a line behind it, so the first transition drops us right into the intermediate state.

The second reason is that most people want to be polite, that is, to cooperate rather than defect. (Are those really the same thing?) And, since zooming ahead of everyone else is clearly a defection, cooperation must mean forming up into a line. Right?

Now, at last, I can get to the point. If the final state really is better, how can we get there sooner? One possibility is education. If everyone understood what was going on, then perhaps moving into the empty lane would be seen not as a defection, but as an attempt to get to the final state. I don't see that as a practical method, but fortunately I know some other methods that are practical.

Once, in a merge situation, I saw two tractor-trailers driving side by side, at the same speed, even though the one in the right lane had empty road ahead.

It was immediately clear what the idea was—clearer, I think, than if it had just been cars doing it.

Since then, I've done the same thing occasionally myself. It does make me a little nervous, though. When you're first setting up, it's possible you will be sitting in an empty lane, moving slowly, so you have to keep an eye out for people zooming up from behind and make sure they don't rear-end you. And, of course, when you're blocking a lane, you are likely to make the people behind you mad.

(Speaking of which, did you ever wonder what happened to road rage? Did it go away? Is it no longer news that people shoot each other on the highway?)

All my “other” methods are really just variants of the same basic idea.

I don't think you need to maintain exactly the same speed; it's probably sufficient to keep your relative speed low. That way, the people in front of you have a chance to see what's going on and hop over into the right lane themselves.

If you want to prevent defection, but also want to make it clear you intend to keep your place in line, you can try driving down the middle, like so.

This isn't great, though—it has the same problem with people zooming up from behind, only more so, because people might start passing you on the shoulder, giving you the same problem over and over.

Finally, there's no reason the method can't be applied by several people at once. If you're in line, and see someone behind you blocking the empty lane, you can pop out and do the same right where you are. Then the original blocker won't have such a vast stretch of empty road ahead, and the people behind ver won't be quite so mad.

So much for variants. All I have left now are some random thoughts.

It's funny that I want to say education isn't a practical method. Those two truckers certainly educated me well enough!

That same story makes a good example of direct meme propagation, by the way.

It's also funny that in the intermediate state, the act of joining the line, which looks so much like cooperation, actually helps maintain the suboptimal status quo. There must be a lesson in there somewhere.