Comment by wwweston | Hacker News Reader

Comment by wwweston | Hacker News Reader

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The benefits of zipper merge don't materialize fully unless merging lanes match speed approaching the merge point.

The parent is likely talking about situations like those presented where a relatively less dense (and ending) lane is merging into a fully-loaded thru-lane, and the traffic in the relatively less dense lane, rather than matching speed and finding the tooth-gap correspondence that's implied in the name zipper merge, zooms as quickly as possible to the last merge point, and then either cuts in without having negotiated a corresponding gap (causing the traffic column in the thru-lane to brake dramatically) or ends up having to slow/stop until an opportunity presents itself (causing a build-up in the ending lane). Or maybe even the latter and then the former.

So, yes, there is "no line" in the zipper merge. There are two lines. And yes, you are supposed to merge at the very end.

But if that's where the thinking/behavior stops, then the parent is right to complain. Drivers who are not matching speed before the merge point are not doing a zipper merge.

At the risk of sounding cold hearted: I get that it doesn't seem fair, but logically, it's the right call to drive to the end of the lane.

1) It keeps the rules simple. If we should merge at some arbitrary point before the lane is up, it will always be arbitrary, and different drivers will have different opinions.

2) By getting to the end and slowing down, the driver now has matched the speed of traffic. It's a self correcting problem.

3) Not going to the end of the lane is wasting capacity. If it's 4 lanes reducing to 3, how far back is it fair to expect that ending lane to go unused?

4) For on ramps, it gives more cars the chance to merge at once, which will either help the flow of traffic, or quickly reach equilibrium with the through lane.

5) And finally, if everyone always used the full lane, it wouldn't give "cheaters" the opportunity to pass everyone else and cheat.

>causing the traffic column in the thru-lane to brake dramatically

Theoretically, where the car merges doesn't have any bearing on this occurring or not. It shouldn't be any different at the beginning of the lane or the end. If traffic isn't moving, there's no breaking to be had. If it is, it should only give better opportunity to match speed and merge without disruption. Practically, the problem has as much to do with the aggression of the through lane drivers as it does the merging one.

They certainly have the right of way. I'm just saying that where the merging driver comes over doesn't really change any of that, unless the through traffic is behaving differently in response.

>Theoretically, where the car merges doesn't have any bearing on this occurring or not. It shouldn't be any different at the beginning of the lane or the end.

It’s a scheduling problem where people that join the lane early are starved by the people joining late.

In cases where there is roughly an equal exchange of vehicles between two lanes it doesn’t matter much, and maybe that’s the norm where you are.

Where I am, congestion is more localized to chokepoints where too many people need to use an exit or on ramp. In this case if time matters it’s usually better to be aggressive and merge at the last minute rather than be passive and get in line early.

People may think you are a jerk. For people where it’s important not to be thought of badly, they get in line first. For people who don’t care (or occasionally for other reasons) they skip the line and merge late.

> At the risk of sounding cold hearted: I get that it doesn't seem fair, but logically, it's the right call to drive to the end of the lane.

There's no dispute on this point. It's the right call to merge at the last workable moment.

One can also make another wrong call in failing to match speed and negotiate tooth-gap arrangement (still in separate lanes) before doing so.

If drivers don't do that, it isn't a zipper merge, even if everyone waits.

On paper, maybe the zipper is excellent. In practice, with prideful LA drivers, its a shit show. I know the maneuver the previous commenter described, it only leads to pain.

- everyone effectively stopped in the exit lane has to stop for even longer

- everyone in the thru traffic lanes has to deal with this person merging all the way from the left to the right with no blinker, then coming to a stop in the right most thru lane desperately trying to force themselves into the exit lane, usually driving over the gut in the process

- the end result is the entire freeway becomes even more backed up because this one person couldn't think more than 1 exit ahead, or worse, they convinced themselves that risking a brutal death like this is worth the 20 seconds it saved them

What you're describing is a problem, but a different one. Def not zipper merge.

For merging (on ramps), waiting to the last moment is collectively efficient. For diverging traffic (exits) it isn't, especially if there's a backup.

In times of congestion, the conditions rarely exists for very long which would allow both lanes to match speed before the merge point. The simplest rule is to use as much road as possible. It’s less than ideal 10% of the time, but 90% of the time it’s the inevitable result of too many cars.

> In times of congestion, the conditions rarely exists for very long which would allow both lanes to match speed before the merge point.

I see these conditions frequently in LA. Either two merging lanes are highly congested in which case they are moving at roughly the same speed anyway, or there's one congested lane and one less dense lane.

The latter case shows up at almost every onramp when traffic gets thick, so it's pretty common. It doesn't take any special skills to negotiate well: accelerate to the speed of thru traffic and proceed to the end of the lane looking for a gap to match with. If you feel like it or just don't want the people behind you who don't understand this to feel impatient, accelerate to maybe an additional 30% above matched speed, then slow down to matched speed and look for a gap as the end of the lane gets close.

Every once in a while you see the reverse situation, where for some reason the merging lane is thick and the thru lane is moving at speed. That is indeed a really difficult situation. Thankfully it's rarer.

> The simplest rule is to use as much road as possible.

Right. While matching speed.

They match speed here... that speed is stopped.

Yes, stopping is pretty much inevitable after a certain density of cars has been reached. In an ideal situation, where all cars have the same acceleration and deceleration, drivers are aware and allow zipper merging and don't tailgate, it would happen more smoothly.

But that isn't the world we live in, so at the end of the day, the simplest rule is to use as much lane as possible and take turns merging.

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