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Comment by justinpaulson | Hacker News Reader

Zipper merging is only optimal if traffic is already slowed down dramatically. If traffic is still moving at any kind of speed, then early merge when there is space available will clear congestion faster than everyone driving up to the merge point and hoping that it works out.

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No, it's provably optimal in flowing traffic as well. However, the "merge point" is variable depending on traffic speed.

Obviously when you're going 100 km/h, the ideal merge point is significantly farther back than when it's stop and go. Nobody is saying you should wait until you're 20 feet from the barrier and then swerve into the other lane. But it's not like that's unique to this situation.

Zipper merge is synonymous with late merge: https://en.wikipedia.org/wiki/Zipper_merge

If you move the merge point back, then by definition it becomes an early merge, right? I don't understand your disagreement with what seems like a totally reasonable statement by Justin.

My point is that what constitutes "late" is a function of the speed of traffic.

If you define "late" as "the latest point at which you can safely merge without causing negative traffic reactions (e.g. by forcing someone to brake)" then zipper/late merging is better than "moving over as soon as you can", even in light traffic.

In an early merge there is no real merge point, people just merge wherever they can. As traffic backs up further and further people merge earlier and earlier.

This is the commonly held belief that is false. It is false because traffic is caused by lack of capacity, not inefficient driving.

Road capacity and driver efficiency are related. Roads have a theoretical capacity that is rarely reached due to inefficiencies in driver's habits.

Exactly. Drivers as a class tend to think that they are smarter than everyone else, particularly the transportation departments, and think that e.g. speed limits are something you need to be above, otherwise you're an annoying driver. It's a huge cultural problem and one of the main reason I can't wait to see drivers displaced by self-driving cars.

> Zipper merging is only optimal if traffic is already slowed down dramatically.

I agree. But note that the idea of Zipper merging is when traffic has already slowed down, not when it is still moving.

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