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Comment by nhaehnle | Hacker News Reader

dusing is probably talking about the (rather common in some places) situation where one lane has half a kilometer of slow-to-standing vehicles, while the other lane is entirely free.

In this situation, of course you're going to pass those other cars (please do so slowly and carefully if you pass them on the right). It would be ridiculous to slow to a standstill with half a kilometer of empty road ahead of you.

When zipper merging works correctly, you usually cannot pass cars in the other lane, because traffic automatically flows in the right way. So in a sense you are right that if you pass cars in the other lane, someone is doing the merging wrong. That someone are the people in the crowded lane, however, not the one of who is passing them in the empty lane.

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Thank you for your charitable interpretation of my comment, but in fact I don't think it is ridiculous to drive slowly in the right lane. I really do think that's the correct strategy.

Even if the second lane is free, it's safer and more efficient to match speeds with the first lane. And shouldn't we take actions that maximize safety and efficiency? Note that there is no global gain from speeding down the right lane, since the bottleneck is what controls the flow.

I suspect many of you will disagree with me, so let me address some possible concerns.

Is it safer? Yes. Generally driving is safer when all vehicles are moving at similar speeds. Relative speeds being low mean that there's plenty of time to react to danger. It's also safer because other drivers are more predictable when going at a single speed.

Is it more efficient? Yes. The bottleneck is constraining the flow, so gains before the bottleneck end up being useless. The important thing is to make sure that traffic is entering the bottleneck at a fast, consistent speed. Predictable and uncontested merges are the best way to stop the traffic lane from slowing. Going at the speed of the left lane offers two benefits here. First, it's easier and more predictable to merge when you travel at the same speed as other traffic. Second, if you travel at the same speed as other drivers, they won't get annoyed at you and attempt block you out (to everyone's detriment).

Is it weird? Yes. Driving below the speed limit with space in front of you is an odd thing to do. But in this case, you're matching the speed of traffic and there's no global opportunity cost because the bottleneck ahead is the constraint, not your speed. It's weird, but it makes sense.

The reason that the zipper merge is recommended by so many travel departments is not because it's straight-up more efficient than other merging strategies - it's because it's more robust to defection.

But just because it is robust to defection, that doesn't mean the optimal strategy is to defect.

For those of you who disagree, why? Could you point out where you feel my logic fell short?

Hmm, probably we don't disagree by much anyway. Perhaps it's clearer if we put some numbers to it: Suppose that the occupied lane is doing stop-and-go (due to the natural waves occurring in traffic), with a maximum speed of 20km/h.

What should the speed be for somebody in the unoccupied lane? Certainly 80km/h is far too much. On the other hand, it also doesn't make sense (IMHO) to come to a complete stop in the free lane just because cars in the occupied lane have to stop. I feel that a steady 30km/h is a good compromise.

Does that sound reasonable to you?

If the left lane is going an average speed of 10 km/h, then I believe the right lane should drive at 10 km/h. Doing otherwise is being selfish and unsafe, with no global benefit. It feels weird, but it makes the most sense as far as I can reason.

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