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> "Without enforcing proper merge behavior on both sides, zipper merge just becomes a big Prisoners' Dilemma game with a few defectors screwing all of the cooperators."

Except unlike prisoner's dilemma, it's not a zero (or negative) sum game. The entire traffic flow can speed up if zipper merging is done. The "late mergers" don't have to risk losing their game of "chicken" while the "vigilantes" don't have to deal with the reckless maneuvers of some of the former.

People don't do zipper merging because people are human and drive emotionally [1]. The stereotypes of the conservative early merger claiming the moral high ground and the reckless late merger doing anything to save a few minutes are the cultural results of this mentality.

[1] Another example is the special breed of driver who stops the traffic flow on a major road (which has the right of way) to let someone on a minor cross road or parking lot exit onto the major road, thus inconveniencing everyone behind them for the self satisfaction of having helped the one person they can see.

> [...] inconveniencing everyone behind them for the self satisfaction of having helped the one person they can see.

Or because you know that otherwise they'll be at that crossing for the next hour if nobody's willing to let them by. Perhaps there are some funny allusions to net neutrality to be made here...

Just because you want to turn left, doesn't mean you have to sit helpless waiting to do so. You're allowed to turn right and locate a safe place to turn around.

I have sympathy for left-turners in particularly busy intersections where there is no turn light. Driving in NYC, you pretty much have to nudge as close to the opposing lane as you can until the light turns yellow/red and you can scoot yourself out of gridlock. Furthermore, you must account for streets which are one way, too narrow, full, or active to stop and turn around. However, this was not the situation I was really talking about at all.

The scenario I had in mind while writing my post is that of the local road which crosses a major stretch of highway. Such highways might be an actual interstate, a busy two-lane highway, etc. One of the worst types of road to cross is the one that is far too wide for its own good– an ambitious 6-lane road with not enough traffic to ever be full (Trenholm Road in SC, ahem), but enough so that drivers spread out across all lanes and make crossing very annoying sometimes. There are parts of the Taconic (NY) that pretty bad to cross, though I'm usually the one on the highway. The local cross sometimes gets backed up 5-10 cars, 2-3 of which are stuck in the median. Getting on the highway or going around by going the next town can add 10+ miles. Letting someone cross by slowing down a bit and changing lanes is not, in my opinion, a smug assault on the efficiency of those driving on the highway... it's just something a few people do when they recognize that nothing is very efficient anyway. (Note that I haven't said much about safety because it was only a small part of their comment, and not the part I was responding to. I hope it's clear that I am not advocating for people to test fate by slamming their brakes to let someone curtsy across the pavement.)

WRT to your footnote, I also can't count the number of times I've seen that person on the side street come into traffic and get t-boned by the car that didn't stop in the next lane over. Letting people cross traffic like that isn't just illegal, it's downright dangerous[1]. As a motorcycle rider, few situations scare me more than seeing a lane of stopped traffic next to a clear lane.

1. The same thing applies to stopping to let people turn left across traffic, where the left lane of oncoming traffic is stopped and the right lane is clear.

[1] Another example is the special breed of driver who stops the traffic flow on a major road (which has the right of way) to let someone on a minor cross road or parking lot exit onto the major road, thus inconveniencing everyone behind them for the self satisfaction of having helped the one person they can see.

This is similar to the person on a grocery checkout line who lets someone cut in front of them; they are also putting that person ahead of every other person behind them the line.

It's an interesting situation; people feel they are doing the right thing yet apparently never stop to think how they are imposing their decision on others.

There's a blind-spot of sorts regarding the larger effects of their actions.

Zipper merging is the optimal solution. People early merge because they think it is the optimal/moral/better/'done my part' solution.

If everyone knew about zipper merging there would be no early mergers (and 'late mergers' don't exist within the context of zipper merging).

In simulations zipper merging is an optimal solution. In reality, relying on thousands of drivers to properly follow specific directions and be completely aware of their surroundings is a flawed solution. Telling people how to do something efficiently doesn't mean they'll do it that way. People are extremely loose when it comes to following driving rules, e.g. speed, texting, minimum distances, merging, etc..

I agree that telling anyone to do anything doesn't mean they will do it (change their habits).

However, educating people does allow them to make better decisions.

Rules themselves aren't always the best/optimal/safest solution either and being loose may be the best solution.

For example, in one country in Asia if you are "in-front" of someone you have the right to merge/enter a roadway/get on a free way/change lanes over the person already in the lane. This is just cultural and is so much better than what we have in the west (like the USA). If someone changes lanes and "hits you", it is your fault (unless you were in-front of them).

Makes it so much easier to get on the free way, change lanes, etc. as you only have to worry about what is in front of you. Not what is in front of you, behind you, next to you, etc. There are no "blind spots" etc.

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.

> 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.

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.

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.

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.

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 the optimal solution. People early merge because they think it is the optimal/moral/better/'done my part' solution.

It is optimal only when the local driving culture allows it to be.

I live in an area where drivers are extremely selfish and the police don't seem to spend a lot of time ticketing bad drivers, so zipper merging typically doesn't work here.

> It is optimal only when the local driving culture allows it to be.

Yeah, which is why almost the entire article was about the campaigns to change the culture, not about justifying zipper merging as optimal.

Fair point, but it's easier to successfully campaign a low density region (i.e, a state with around 7M people like Washington) than a high density metro area (the metro area where I live in has 6M people, so traffic is already a daily nightmare).

Those campaigns in Washington and Minnesota just wouldn't fly here.

When our metro population was around <=3M, people still zipper merged (in the mid 90s and earlier).

I live in an area where drivers are extremely selfish...so zipper merging typically doesn't work here.

This is backwards. Zipper merging doesn't work when drivers attempt to be unselfish by merging early, which results in vigilante lane blockage of those who weren't as considerate. In other words, I'm not being selfish when I block late mergers; I'm enacting justice on other people who were being selfish.

The system works when all parties recognize that the best thing for everyone is to merge late.

People used to zipper merge where I live... in the 80s. But the metro region I live in is much more dense than it used to be, and combined with the fact that the police don't spend a lot of time enforcing traffic laws, and that the public transportation system has poor coverage, traffic problems have skyrocketed.

One of the reasons why people early merge here is that many people don't let mergers in. And one of the reasons why people don't let mergers in is that they regularly get cut off by a second car after letting a merger in. It's a vicious circle.

Back in "the day", zipper merging wasn't even an issue. A lot of people early merge here because they'd rather be the one letting people in than hoping to be let in.

You're overlooking the situation where people merge out of the lane in a line without letting people who are already in the lane go through.

The problem is most people don't know how to properly zipper merge, so pretending that everything would be hunky dory as long as people play by a set of rules that you know they aren't likely to play by is an exercise in futility.

Don't know why you're being downvoted; I've had too many people deliberately block my merge when I merge late, to ever try it at full speed in the UK :(

People block your merge because they think you cheated and should have merged earlier. If they were educated about zipper merge they would not have the same feeling.

The solution is more late-mergers: if the other lane were already full, you wouldn't be "cheating" by passing all the cars who are "waiting patiently." You would just be waiting in a line the same as everyone else in your lane and the other lane.

I may be overly optimistic, but I think a sign saying "use both lanes and zipper merge at end" would do a lot.

Well, quite a lot of traffic is already regulated by people "playing by a set of rules", and that ends up being hunky dory for the most part. I think the reason why this one is the exception is that it's quite feasible for people to "fight back" to those they perceive as abusers.

The real problem is accountability. There is none of it(as cops can't be everywhere, all the time) so people end up exercising what little power they do have in order to punish those they view as the abusers.

If enough people are doing zipper merging, then there is no way to cut in line, because both lanes are equally backed up. Then once people get to the merge point, they won't have any sense of unfairness because the other lane was waiting just as long, so there will be less impetus for "vigilantes" to mess things up. I'm not saying there will be no problems, but I don't think that more zipper merging will automatically lead to more of these problems.

The question then is, do enough people zipper merge, and do people really have a proper understanding of fairness? A lot of the driving population may not be acting rationally, or even have enough awareness to comprehend the big the picture. Cars put people into their own bubble, where quite oftenly only their needs are being considered.

>> Cars put people into their own bubble, where often only their needs are being considered.

This is so true. At congested merge points in my city, you have people who don't really care about the alternating needed to make a zipper merge work. First, you have people who just won't let people in, and then you have people who will try to sneak into the lane with the merging car ahead of them.

They certainly do not. I always leave space for a car at the zip, and damn near every time 2 or 3 will try to push their way in there even though there is only space for 1 car. Infuriating.

Yes, of course many drivers will always try to cheat no matter what, but many more will cheat if they feel they have been wronged. My point is that the latter subset would be eliminated if most people used zipper merging.

I tend to start pacing for the zipper merge when I'm (reasonably) close to the end of the zipper. This way, I'm showing I am willing to wait while simultaneously giving myself the option of skipping forward past any particular territorial driver. When I slow down the merging lane to a similar speed as the non-merging lane, I can typically see the zipper start to form behind me when I check my mirrors.

In the same way one or two bad drivers can destroy a lane or two of functional traffic flow, a good driver can create a lane (or two) of harmony.

Hello, my name is Peter, and i'm a late-merger. But where I come from it's just called "Florida driving".

In Florida, we all have a non-verbal mutual agreement that we're all trying to screw each other out of the best spot in the road. We all want to be going faster than everyone else, we all want to get to the front of the line and skip all the chumps, we all want to move into the lane we think will move the fastest at a red light, and we all think turn signals will just warn someone when we're trying to get ahead so they can take our spot on the road.

It honestly wouldn't be so bad if we could keep the speeding and aggressive driving controlled, but there's an unusual number of highways with ridiculously reduced speed limits (55mph on I-95 in Miami, I shit you not.... right around a large stretch of road where 45mph is the minimum speed limit) and a lot of assholes who think they're more important than everyone else. Not much you can do about it, other than a "fast lane", which is basically what they've created for a large stretch of I-95 combined with the Sunpass system.

We have this problem in Texas but there is an additional problem; If you are not an early merger people tend to not let you in anywhere in the chain forcing you to become a late merger. I tend to grab a spot early because too often people would not allow me to merge at any point. Texas drivers are very aggressive, I imagine this scenario is true in other places as well.

In reality, the early mergers are the problem.

They are screwing up the system by merging too early, then adding insult to injury, getting all bent out of shape about people who aren't 'early mergers'.

So first they create the problem, then they get upset when someone does it the right way. Classic!