It's not the asshole lane.
May 11, 2016 1:14 PM   Subscribe

The Zipper Merge is the secret to reducing traffic backups, increasing safety, and minimizing road rage.

The Minnesota Department of Transportation (MnDOT) wants to teach you How to Zipper Merge.

[via]
posted by sparklemotion (179 comments total) 32 users marked this as a favorite
 
I heart the zipper merge and loathe the selfishness of my fellow humans.
posted by agregoli at 1:16 PM on May 11, 2016 [20 favorites]


I like to maintain room in front of me for someone to merge into. Its amazing how people will pass up the spot to merge stressfree in order to get to the head of the line and wait.
posted by museum of fire ants at 1:17 PM on May 11, 2016 [42 favorites]


The zipper merge when no one else is, on the other hand...
posted by michaelh at 1:17 PM on May 11, 2016 [11 favorites]


FTFA: Don't worry about being "Minnesota nice."

The hell is this!?

Did MnDOT hire a consultant from, like, Chicago to write this policy paper?
posted by wenestvedt at 1:21 PM on May 11, 2016 [14 favorites]


The zipper merge when no one else is, on the other hand...

...is a public service, in that you are educating people in the full lane about the availability of a whole 'nother lane, of which they were previously unaware.
posted by sparklemotion at 1:22 PM on May 11, 2016 [21 favorites]


I once posted that zipper merge link on my city's subreddit because there was a major construction project and everyone was merging early causing major delays.

I was quickly told by many Redditors how much of a selfish asshole I am for not merging early. Obviously my mistake was posting on Reddit.
posted by Deflagro at 1:27 PM on May 11, 2016 [29 favorites]


I feel like the zipper merge must only work in a just and equitable society. Here in Masshole country people will accelerate and basically kiss the rear bumper of the car head to prevent you from merging.
posted by backseatpilot at 1:30 PM on May 11, 2016 [19 favorites]


There's always some asshole who jets forward to skip a spot or so you can't get in, because being a couple car lengths ahead will change how fast they get somewhere.

I fantasize sometimes that their cars just start going up, up, through the sky and space and into the sun.
posted by kafziel at 1:31 PM on May 11, 2016 [57 favorites]


As much as I don't like being cut off or prevented from merging, nothing makes me wish my car was equipped with a tactical nuclear weapon more than that person who puts their blinker on and comes to a dead stop to wait for an opening with miles of empty space ahead of them
posted by prize bull octorok at 1:34 PM on May 11, 2016 [36 favorites]


This information can never be repeated too often.
posted by BlueHorse at 1:35 PM on May 11, 2016 [8 favorites]


The zipper merge is a great idea when the roads are set up to execute it properly, and if it's not executed properly an absolutely infuriating traffic clusterfuck. When traffic in the ending lane rushes forward to the very end at a much higher speed than the main lane, they then find themselves suddenly needing to simultaneously merge and brake before running off the road. This means they end up jamming into the pre-existing traffic and suddenly decelerating, which means the lane they merged into needs to jam brakes even more to prevent an accident, which fucks things up progressively more and more down the line. The best zipper merge structures i've seen have a stoplight on the onramp which lets cars through one or two at a time with a few seconds in between - this allows the cars to match the speed of existing traffic and merge in correctly without disrupting the flow of traffic.

And if someone is already in the main traffic lane and jumps out INTO the merge lane just to rush up and cut four carlengths forward in line, I will drive them off the road, over a cliff edge and wait for their car to explode just so I can piss on the burning corpse.
posted by FatherDagon at 1:36 PM on May 11, 2016 [57 favorites]


I ruthlessly zipper merge at the same backup spot every single day. I cannot understand why hundreds of people will line up for 20 minutes instead of everyone zooming to the end and merging. Unreal sheeplike behavior drilled into people from kindergarten.
posted by Cool Papa Bell at 1:38 PM on May 11, 2016 [7 favorites]


FatherDagon: And if someone is already in the main traffic lane and jumps out INTO the merge lane just to rush up and cut four carlengths forward in line, I will drive them off the road, over a cliff edge and wait for their car to explode just so I can piss on the burning corpse.

Found the Masshole.
posted by wenestvedt at 1:38 PM on May 11, 2016 [10 favorites]


FatherDagon: “And if someone is already in the main traffic lane and jumps out INTO the merge lane just to rush up and cut four carlengths forward in line, I will drive them off the road, over a cliff edge and wait for their car to explode just so I can piss on the burning corpse.”

Why? You have totally missed the point of zipper merges! The whole point is that everyone should do this! Then traffic would even out!

You can't say you think zipper merges are a "great idea" if you hate actually seeing them used.
posted by koeselitz at 1:43 PM on May 11, 2016 [9 favorites]


I feel like there's two things happening, one if for work slowdowns and things like that were it makes 100% sense, the other on a busy freeway where someone just is trying to skip the line and bringing a whole second lane of traffic to a stop.
posted by Carillon at 1:45 PM on May 11, 2016 [5 favorites]


My mom has been ranting about this since I started forming memories of riding in the car as a child. "MERGE LIKE ITS A ZIPPER, PEOPLE." I guess there's a name for it, then.
posted by Tesseractive at 1:50 PM on May 11, 2016 [4 favorites]


Prisoner's Dilemma on I-93.
posted by robocop is bleeding at 1:51 PM on May 11, 2016 [50 favorites]


I have slowly come around to the Zipper Merge, but I still consider changing into a clearly disappearing lane to be the similar looking Asshole Merge.

Other signs of the Asshole Merge: Not signaling, not letting a car in between you and the last merging car.
posted by ckape at 1:51 PM on May 11, 2016 [12 favorites]


The problem with expecting people to do this is that it isn't taught in most crappy driver's ed classes, which aren't universally required anyway and are only taken once, as apathetic teenagers, by most people. (See also proper use of turn signals.) And then there seems to be no good way to enforce it, and on top of that lane closures, at least where I live, are often set up in a haphazard fashion that encourages people to merge early because it is not clear where the lane actually ends (often that is unavoidable because of curves, hills, and other interruptions to the line of sight).
posted by TedW at 1:52 PM on May 11, 2016 [4 favorites]


I once emailed them about this regarding the east bound 394 to 94E/W lane mess that happens all the time (never got a reply). The exit to 94E is a right lane exit only lane. Next to it is the exit to 94W. So zipper merging means traffic to 94W ends up being blocked off, since most of the traffic is in the 94E exit only lane. Which makes for a messy mix of angry drivers who cannot get through to the 94W exit and a mess of angry drivers who have to deal with people who zipped through to the point where they either had to go the wrong way on 94 or manage to merge into the exit lane for 94E. Zipper merges work great on something like a two lane road reduced to a one lane road! (I think i first ran into this in the annual 35 road project). I have basically written it off as "I will be spending the next 20 minutes slowly idling, oh well, next podcast up to bat". Now let us talk about people who apparently cannot understand the giant signs saying "DO NOT CROSS DOUBLE WHITE LINE"
posted by jeribus at 1:52 PM on May 11, 2016 [10 favorites]


The Zipper Merge is the secret to reducing traffic backups, increasing safety, and minimizing road rage.

Other secrets include: not driving 95mph, building light rail, and taking the bus once in a while.
posted by j_curiouser at 1:52 PM on May 11, 2016 [43 favorites]


Zipper merges feel like traffic design fail; like the number of cars increased too much one year, and the resources needed to build more road just weren't there.
posted by ZeusHumms at 1:56 PM on May 11, 2016 [1 favorite]


Seems like a great example of engineering a system that works well if humans change their behavior. Good luck with that.
posted by Salvor Hardin at 1:57 PM on May 11, 2016 [36 favorites]


FatherDagon: And if someone is already in the main traffic lane and jumps out INTO the merge lane just to rush up and cut four carlengths forward in line, I will drive them off the road, over a cliff edge and wait for their car to explode just so I can piss on the burning corpse.

Found the Masshole.


If you think getting out of one lane just to rush forward and cut back into the same lane four carlengths later is somehow *improving* the state of traffic overall, you are insane.
posted by FatherDagon at 1:58 PM on May 11, 2016 [28 favorites]


I have never ever seen a zipper merge in real life.

It's one of those things where -- yeah, the solution works, except people are involved which means it doesn't work.

lane closures, at least where I live, are often set up in a haphazard fashion that encourages people to merge early because it is not clear where the lane actually ends

Even if it's clear where it ends, I merge early because I know that if I get to the place where the lane actually is, no one will let me in and I'll just sit there while people think I'm the asshole who is getting what they deserve.
posted by Kutsuwamushi at 1:58 PM on May 11, 2016 [22 favorites]


This sort of jumping up and down and stamping your feet and shouting "YOU'RE DOING IT WRONG!" seems to be the wrong approach to take if people are naturally inclined to merge early. Traffic planners should instead by designing traffic patterns with this natural human behavior in mind.
posted by indubitable at 1:59 PM on May 11, 2016 [16 favorites]


psychohistory, indeed
posted by j_curiouser at 1:59 PM on May 11, 2016 [5 favorites]


I once emailed them about this regarding the east bound 394 to 94E/W lane mess that happens all the time (never got a reply). The exit to 94E is a right lane exit only lane. Next to it is the exit to 94W. So zipper merging means traffic to 94W ends up being blocked off, since most of the traffic is in the 94E exit only lane....

Now let us talk about people who apparently cannot understand the giant signs saying "DO NOT CROSS DOUBLE WHITE LINE"


Here's the thing, there is a double white line there, and appropriate signage (streetview link). So people are zipper merging in the one place where it's inappropriate to.
posted by sparklemotion at 2:00 PM on May 11, 2016 [5 favorites]


I love the zipper merge, and it seems to be relatively common around LA.

If education and/or awareness is one of the issues... I'd love to see a designer come up with a zipper merge sign consistent with e.g. California Coded Sign Specifications.
posted by jjwiseman at 2:03 PM on May 11, 2016 [5 favorites]


I am about to get on the 405 freeway into the San Fernando Valley. Every day, the on-ramp (Wilshire NB) makes a zipper merge from 4 lanes to 2 lanes to one lane. Almost every day it works. The days it doesn't usually involves ambulances.
posted by Sophie1 at 2:04 PM on May 11, 2016 [4 favorites]


This sort of jumping up and down and stamping your feet and shouting "YOU'RE DOING IT WRONG!" seems to be the wrong approach to take if people are naturally inclined to merge early. Traffic planners should instead by designing traffic patterns with this natural human behavior in mind.

The problem isn't so much that people merge early (choose whichever lane you want, I don't care). The problem is that the early mergers think that the late mergers are morally deficient in some way, and do stuff like not letting people in, or attempting the block the ending lane.

I'm not sure what traffic planners could do better to confront that type of vigilantism, but more awareness can't hurt.
posted by sparklemotion at 2:10 PM on May 11, 2016 [21 favorites]


Why not just move the sign closer to the actual closure?
posted by lucidium at 2:12 PM on May 11, 2016 [7 favorites]


“And if someone is already in the main traffic lane and jumps out INTO the merge lane just to rush up and cut four carlengths forward in line, I will drive them off the road, over a cliff edge and wait for their car to explode just so I can piss on the burning corpse.”
Why? You have totally missed the point of zipper merges! The whole point is that everyone should do this! Then traffic would even out!


It's not really a zipper merge unless you're roughly matching speed with the lane you intend to merge into. "Merging late" is only half the process. You have to be able to smoothly move into a gap in traffic, which you aren't going to be able to do if you've raced up to the end of your lane and then have to wait at a dead stop. And the lane in which you're trying to merge isn't going to be able to provide smooth alternating gaps if people who were at dead stops keep trying to merge, it'll be all stop/go waves from the sudden jerky insertions.

Moving out into the merge lane and matching speed and merging late is... OK, I guess. In this imperfect world it's probably helpful since it enforces matching speed on the likely thoughtless drivers behind you who it'd never even occur to. In a perfect world where everybody had already internalized the zipper merge it's probably pointless more often than not.
posted by wildblueyonder at 2:14 PM on May 11, 2016 [9 favorites]


I remember reading about the zipper merge in Tom Vanderbilt's Traffic. He didn't have the catchy name for it, but apparently it's been common knowledge among traffic engineers for years. It's good that someone finally decided to do some public education about it.

It's counter-intuitive. I know I used to think "If we all merged now, half a mile ahead of the blockage, we'd have it done with and traffic would move faster." But non-zipper-merging simply extends the low-speed one-lane zone into what should be the higher-speed two-lane zone.
posted by zompist at 2:20 PM on May 11, 2016 [8 favorites]


Just one more example of why we need autonomous cars. Let the computers solve the problem. (But then someone will come up with an "asshole lane skipping" firmware-upgrade kit just to get FatherDagon's goat.)
posted by JimInLoganSquare at 2:21 PM on May 11, 2016 [7 favorites]


Maybe the solution is to erase the distinction between the "continuing" lane and the "merging" lane: redraw the dividing line so it precisely splits the narrowing road in half until its the width of a single lane. Then nobody is "cutting in" since everybody is merging.
posted by The Tensor at 2:21 PM on May 11, 2016 [11 favorites]


I've always thought the cause of zipper merging would be well served by construction companies using big signs saying something like "Lane Ends, Merge Ahead," followed by "Stay in Lane, Zipper Merge" followed by "Zipper Merge For Faster Traffic."
posted by fremen at 2:22 PM on May 11, 2016 [16 favorites]


We have autonomous cars. They're called busses. Just gotta use em.
posted by koeselitz at 2:22 PM on May 11, 2016 [11 favorites]


I once emailed them about this regarding the east bound 394 to 94E/W lane mess that happens all the time (never got a reply). The exit to 94E is a right lane exit only lane. Next to it is the exit to 94W. So zipper merging means traffic to 94W ends up being blocked off, since most of the traffic is in the 94E exit only lane. Which makes for a messy mix of angry drivers who cannot get through to the 94W exit and a mess of angry drivers who have to deal with people who zipped through to the point where they either had to go the wrong way on 94 or manage to merge into the exit lane for 94E.
I hated hated hated this interchange when it was part of my commute. Hard to zipper merge (because of 94W/double white line already mentioned + passive aggressive blocking) but also once you get on 94 you're at the tunnel entrance so most people don't want to change lanes so all the exiting traffic is still in the right lane so it stays slows and things back up more so then more people try to merge at the last second to avoid the line so more people have to hit the brakes so the line backs up more so it goes on and on and on.
posted by cnelson at 2:23 PM on May 11, 2016


They try to make you zipper merge in the checkout line at the local Whole Foods and it drives me crazy.
posted by capricorn at 2:24 PM on May 11, 2016 [1 favorite]


If traffic engineers really wanted zipper merges, they'd end *both* lanes. The fact is, merging early lets cars adjust their speeds and spacings with as much notice as necessary to merge while slowing down as little as possible. To run everyone to the end and hope that they suddenly just *mesh* is so not how humans work IMO.

(At least can we all agree that the assholes that use the shoulder as their own private zipper-merge lane are both wrong and ... assholes?)
posted by Popular Ethics at 2:25 PM on May 11, 2016 [8 favorites]


Route 163 north out of downtown San Diego effectively has a zipper merge, and it works perfectly, but only because it's 8 lanes shrinking down to 2 and traffic is basically already at a standstill.
posted by LionIndex at 2:30 PM on May 11, 2016 [2 favorites]


FatherDagon: “And if someone is already in the main traffic lane and jumps out INTO the merge lane just to rush up and cut four carlengths forward in line, I will drive them off the road, over a cliff edge and wait for their car to explode just so I can piss on the burning corpse.”

me: “Why? You have totally missed the point of zipper merges! The whole point is that everyone should do this! Then traffic would even out!”

wildblueyonder: “It's not really a zipper merge unless you're roughly matching speed with the lane you intend to merge into. 'Merging late' is only half the process. You have to be able to smoothly move into a gap in traffic, which you aren't going to be able to do if you've raced up to the end of your lane and then have to wait at a dead stop. And the lane in which you're trying to merge isn't going to be able to provide smooth alternating gaps if people who were at dead stops keep trying to merge, it'll be all stop/go waves from the sudden jerky insertions... Moving out into the merge lane and matching speed and merging late is... OK, I guess. In this imperfect world it's probably helpful since it enforces matching speed on the likely thoughtless drivers behind you who it'd never even occur to. In a perfect world where everybody had already internalized the zipper merge it's probably pointless more often than not.”

It does more than enforcing matching speed on thoughtless drivers. And traffic is hardly ever actually at a dead stop – we're talking about traffic that is merely moving slow. And why is it moving slow? Because we are ignoring a whole other lane because of a meaningless principle we adhere to.

Look, here's the rub: nobody can zipper merge alone. It's like the sound of one hand clapping: it isn't a thing that exists, because you need multiple cars for a zipper merge to be a thing. When everybody is stuck, bumper-to-bumper, going very slowly, and everybody is ignoring a whole other lane that is available for use at least for a while longer, there is no instant way for a magical zipper merge to appear. You know what it takes? That's right – one driver dipping out on her own into the merge lane, and then some other grudging driver willing to let the merging driver back in. Then, as people further up the merge lane see that happen, they decide to remain in the merge lane until the merge point. All of this can fall into place – but only if somebody is willing to do that thing which FatherDagon swears hyperbolically that he will happily cause you to crash and burn for.

Sometimes the world needs a hero. Heroes aren't always lauded to the sky, given a glamorous welcome by those they're helping. Sometimes heroes are despised, even when they're doing everybody a favor. This is one of those cases. There's an old saying: "be the change you wish to see in the world." Well – if you want to see zipper lanes in the world, working perfectly and smoothly, the only way that will ever happen is if somebody has the guts to take that burden on themselves, to be that asshole who dips into the inexplicably-empty merge lane, glides up to the merge point, and waits to be let back in. (Also the person gracious enough to say "eh, fuck it" and let them in again.)

Do it. Be the change. It's the only way. It's a sacrifice, I know, but we all have to make sacrifices sometimes to make society sane.
posted by koeselitz at 2:36 PM on May 11, 2016 [12 favorites]


"Lane Ends, Merge Ahead," followed by "Stay in Lane, Zipper Merge" followed by "Zipper Merge For Faster Traffic."

Absolutely. Clear signage is the key. I'd add: "Drive right. Pass left."
posted by jetsetsc at 2:37 PM on May 11, 2016 [5 favorites]


In the UK truck drivers control the merge. Two will drive side by side up to the cut in. Then the one in the continue lane will draw back to allow the shut lane truck to move in. The zip happens behind them. If left you get a bunch of BMWs, Mercs, and Audis race up the fast lane and bring both lanes to a halt.
posted by lilburne at 2:37 PM on May 11, 2016 [3 favorites]


I spent a lot of time SW of Davis trying to come up with a 3-lanes-to-1 zipper algorithm. Failed. So did everybody else.
posted by clew at 2:38 PM on May 11, 2016 [2 favorites]


"Lane Ends, Merge Ahead," followed by "Stay in Lane, Zipper Merge" followed by "Zipper Merge For Faster Traffic."

How about:
  1. Merge Coming
  2. But Don't Merge Yet
  3. ...wait for it...
  4. ...almost there...
  5. OK NOW MERGE NOW NOW NOW

posted by The Tensor at 2:39 PM on May 11, 2016 [51 favorites]


Well as the video shows, in light free-flowing traffic it makes sense to merge early when a gap appears, and not wait till the last minute. The main problem is with heavy, slow-moving traffic. That means no obvious gaps, and that is when the zipper merge is most efficient.

I am a convert to the zipper merge. Most of my fellow Torontonians are not.

Although for the record, jumping into an soon-to-be ending onramp lane to leapfrog ahead is still an asshole move (and I swear I've seen people pulled over for this in Ontario… no?).
posted by Kabanos at 2:41 PM on May 11, 2016 [5 favorites]


6. Burma-Shave
posted by lucidium at 2:43 PM on May 11, 2016 [13 favorites]


If you merge early, you should take responsibility for making a poor strategic decision and chill about the smart ones who stay in the lane until the correct merge point. Merging early is a strategy anticipating that you won't actually be allowed to merge by adjacent drivers when you need to. It isn't "polite".
I've observed another phenomenon here is Denver. People will actually do the zipper merge perfectly, but well in advance of the actual merge area. At some point, there is so much road ahead, it becomes and empty space that must be filled, and civilization breaks down. Again.
posted by Carmody'sPrize at 2:44 PM on May 11, 2016 [4 favorites]


I am pleasantly surpriseddumbfounded that the zipper merge is becoming more and more common in Northern Virginia.
posted by Johnny Wallflower at 2:45 PM on May 11, 2016 [4 favorites]


It's not really a zipper merge unless you're roughly matching speed with the lane you intend to merge into.... Moving out into the merge lane and matching speed and merging late is... OK, I guess.

I think this is where the confusion is. People who are mad about zipper merges are talking about the thing jerks in traffic actually do where they zoom to the end of the closed lane then have to slow tremendously to avoid crashing because there was no way for them to try to negotiate an opening while ignoring everybody else, and then they slow traffic in the continuing lane immensely by aggressively cutting someone off. Some of the people who are extolling the virtues of the zipper merge are talking about the thing where you match speed with the continuing lane and merge into it without causing people behind you in the continuing lane to slow down a bunch, but what they don't realize is that this is the thing all of the people in the first group are doing and they're just not calling it a zipper merge because they associate that term with the thing the jerks are doing.
And then it does appear that we have a couple of the jerks from the first example who think that anything that puts them two car lengths ahead of where they would otherwise be is justified. Yes, doing that is slightly faster for you. That's because it is slowing down all the people between where you would be if you had merged correctly and where you are now. It is not some kind of magic that creates earliness out of nothing and distributes it to all the people in line. It turns out that there is in fact a law of conservation of earliness, and there is only so much earliness to go around.
Finally, if you actually moved out into the merge lane while matching speeds with the continuing lane and then tried to merge later, you would just end up exactly where you started in the traffic line. There's only a personal benefit if you don't match speeds and in fact do the thing the jerks do, which is precisely why it is the thing that the jerks do.

If you think getting out of one lane just to rush forward and cut back into the same lane four carlengths later is somehow *improving* the state of traffic overall, you are insane.

No, didn't you hear? The most ruthless and selfish action in all circumstances is the one that is best for everyone! Do anything else and you're some kind of unreal sheep! If it works for economies it must work for traffic! It does work for economies, right?
posted by IAmUnaware at 2:49 PM on May 11, 2016 [21 favorites]


> You have totally missed the point of zipper merges! The whole point is that everyone should do this!

What FatherDagon is describing is the anti-zipper merge. It's where a driver pulls right out of the main traffic lane and hares up a few car lengths, hoping that the rube at the front thinks they're just some innocent merger. This is DIAF territory. It's really common on Toronto's DVP, usually accompanied by driving two wheels into the shoulder. It slows everyone down, as usually the re-merger forces cars to stop in a hurry.

Zipper merging is a very civilized thing. I suspect that wenestvedt and koeselitz live somewhere altogether far too polite to ever see such a thing as rude as the anti-zipper merge, so don't consider it possible.
posted by scruss at 2:49 PM on May 11, 2016 [13 favorites]


I wish there was a question about zipper merging on every driver's license test in the country, and if you failed the question, you couldn't get a license. It wouldn't educate those who already have licenses and never move to a new state, but at least eventually we would all learn about it. :).

Of course, I also wish for an end to hunger, global warming, and war.
posted by elmay at 2:50 PM on May 11, 2016 [4 favorites]


So my once mellow morning commute has been desecrated by lane closures for the last few months. The same two left lanes have closed at the exact same spot for two months. Every day, I drive in the left lane until I get to the first "2 Left Lanes Closed Ahead, Merge Now" sign, then I casually make my way in the slower right lane.

Then for the next half mile I leave tons of room in front of me and behind me (when possible) so that people can merge in around with room to spare. My fellow commuters blaze by this open spot to come to a screeching halt 3 to 4 car lengths ahead where the cones force them into the right lane. This is not a zipper merge, this is an "asshole thinks he found a fast lane in the middle of morning traffic and is trying to get one up on the rest of us merge."

Because I am a spiteful human, I will close that lovely gap I leave in front of me if no one has chosen to merge into it before the cones force them to. Because I am petty and I hate that I can't go as fast as I please because I followed the damn sign that told me to merge a MILE AGO!!!
posted by teleri025 at 2:53 PM on May 11, 2016 [4 favorites]


I just want to point out that on my commute home this evening I was honked and yelled at for exercising the right of way on a priority road and not letting the aggressor leap in to traffic in front of me, despite there being about twenty car-lengths of empty space behind me.

Proper zipper merge etiquette ain't gonna work around here.
posted by backseatpilot at 2:54 PM on May 11, 2016 [7 favorites]


My partner routinely gets honked at for driving only a mere 5 mph over the speed limit, heavens forfend.

Ban car culture IMO.
posted by tobascodagama at 2:57 PM on May 11, 2016 [2 favorites]


If it works for economies it must work for traffic! It does work for economies, right?

I'm pretty sure they get the researchers from the same place, at least, since a lot of what they propose makes you wonder if they've actually seen an actual driver in the wild, or if it's all just based on computer simulations using spherical cars.
posted by effbot at 3:00 PM on May 11, 2016 [4 favorites]


Merging early is a strategy anticipating that you won't actually be allowed to merge by adjacent drivers when you need to. It isn't "polite".

If wanting to give you the best chance to avoid an accident isn't polite, I guess?

Driving is inherently somewhat hazardous and one way to mitigate hazard is to both have and provide as much information about future positions of vehicles as possible. Having that information sooner rather than later gives you more time to react if a reaction should become necessary.

Being in the other lane in preparation to zipper merge introduces information uncertainty because it is an open question as to whether a given car will allow you to zipper merge and, if not, how many cars you will have to wait until you're given the chance to do so.
posted by juv3nal at 3:04 PM on May 11, 2016 [8 favorites]


Don't mind me. It's just the Puritan in me, I have to go directly to the longer, slower righthhand lane -- and then shoot little Hate Rays at the free spirits who roar ahead on the left.
posted by wenestvedt at 3:05 PM on May 11, 2016 [1 favorite]


scruss: “What FatherDagon is describing is the anti-zipper merge. It's where a driver pulls right out of the main traffic lane and hares up a few car lengths, hoping that the rube at the front thinks they're just some innocent merger. This is DIAF territory. It's really common on Toronto's DVP, usually accompanied by driving two wheels into the shoulder. It slows everyone down, as usually the re-merger forces cars to stop in a hurry... Zipper merging is a very civilized thing. I suspect that wenestvedt and koeselitz live somewhere altogether far too polite to ever see such a thing as rude as the anti-zipper merge, so don't consider it possible.”

So you're telling me you've never, in your entire life, seen a situation where a whole bunch of people ought to be zipper-merging, but somehow out of insane scruples refuse to even touch the merging lane at all? That's not a thing in your world?

In any case, let's just say it's a thing. What solution do you propose to this situation? Should everyone stay out of the merge lane, just to appear polite?

No. We should have zipper merges. And the only way to create a zipper merge in this situation is for someone to leave the full lane and enter the merge lane.

I guarantee you: they will instantly be viewed as an asshole, no matter how they do it, no matter how meekly they glide forward, no matter how quietly and calmly they attempt to merge. People will get pissed off. People will (as FatherDagon mentioned) try to run them off the road.

And I'll say this, too: for ever jerk who swerves around like an idiot, runs up on the shoulder, and just about slams into a car just to re-merge, there are ten thousand people who love to take every single pull-off-and-re-merge-er to be some kind of subhuman filth which they personally are allowed to punish. Why? Because smugness is a uniquely human weakness, and it afflicts all of us. And when we see somebody pull out into the merge lane, it's like a switch flips, and we're given a green light to go ahead and vent our smugness on everybody.

Even though pulling into the merge lane and actually using it – calmly and carefully – is, in fact, the right thing to do.
posted by koeselitz at 3:09 PM on May 11, 2016 [5 favorites]


If they want this to work, they need to signal it. Currently, there is a lane that is closing ("A") and a lane that is staying open ("B"). Lane A is diverted into lane B. Cars in lane B have to god given right to go as fast as they can because there lane is good. Cars in lane A have to find permission or opportunity to move into lane B. There is no my turn/your turn in this situation. Lane B has authority; Lane A requires cunning.

What they should do is have the guyse setting out the cones start farther back and have both lanes A and B merge into a center lane, which once established funnels into the open lane. That way there is not moral high ground of a lane that gets to "let" the other lane in, instead everyone has to merge.
posted by rtimmel at 3:13 PM on May 11, 2016 [9 favorites]


The correct thing to do, always and ever, is remain in the merging lane until you can seamlessly and effortlessly glide safely into an open space in the other lane while it's moving, joining the flow in perfect harmony. To do this requires faith in the other drivers and in the eldritch gods of traffic, and faithless drivers routinely fuck it up.

Of course, it happens sometimes that the gods never grant you that perfect merge and you end up at the cones and you have to wait for somebody to hold up and let you in, but someone will always let you in.

You all make this sound so fraught and complicated!
posted by prize bull octorok at 3:17 PM on May 11, 2016 [11 favorites]


I'm always amazed that I haven't smashed into the cones and electronic sign when I'm looking at traffic to the left of me and trying to merge. Sixth sense of spatial awareness or dumb luck?
posted by indubitable at 3:25 PM on May 11, 2016 [1 favorite]


I haven't even seen a zipper merge work properly in a multi-lane fast food drive thru around here, let alone in construction.
posted by jason_steakums at 3:28 PM on May 11, 2016 [3 favorites]


Well In Chicago we have zipper merges on the Dan Ryan as you pass by town, with left hand merges from local streets. It works as long as people understand driving is a social thing.

The inexperienced will break the flow by either stopping, like dead halt stopping, letting too many cars by and not being able to zip it up, because they STOPPED.
Or it's a dude from the suburbs with a Cubs hat on and a righteous " no one will EVAR get in front of me" attitude, while he's slowly freaking out because he's clearly in the wrong lane and all the tall buildings are now disappearing.

Learn to drive or learn to stay off the roads adults use.
posted by Max Power at 3:29 PM on May 11, 2016 [5 favorites]


Multi-lane fast food drive through!? You must be in California. Must be.
posted by indubitable at 3:31 PM on May 11, 2016 [1 favorite]


I unapologetically use the zipper merge about every day on the way home from work. If everyone did it there would be no backup on that particularly interstate junction.

I used to rail against those "idiots in the lane that's about to end" until I thought "how do they all go that way and it not back up?" Then I tried it. Cuts 5-10 minutes off my commute daily.
posted by Twain Device at 3:36 PM on May 11, 2016 [5 favorites]


Multi-lane fast food drive through!? You must be in California. Must be.

Iowa, actually. Are these not common? Is Iowa somehow ahead of the curve on fast food drive thru tech?

The inability of people to properly merge can be a boon in a drive thru situation, though. Learn to spot the difference between a person who isn't paying attention to merge order out of selfishness and someone who just spaced out and is now awkwardly flustered... waving the latter person ahead has sometimes gotten my meal paid for.
posted by jason_steakums at 3:40 PM on May 11, 2016 [1 favorite]


What FatherDagon is describing is the anti-zipper merge.

It is what my Mormon sister-in-law calls the "snake". Whenever my husband and I witness it — and we see it unsurprisingly often in Seattle, home of America's worst drivers — we hiss "SSSSSSSSS" at each other and laugh it off. I guess we'll probably stop laughing when the snaker ploughs into the car or concrete divider in front of him and kills another driver or pedestrian in order to gain a few seconds of advantage over the driver in front of him, but people gotta have their zipper merge. Give me convenience or give me death, as they say here in the United States.
posted by a lungful of dragon at 3:41 PM on May 11, 2016 [2 favorites]


when a half dozen cars in front of me successfully complete their zipper merge but then the donglord who is supposed to let ME in gives me a hard time about it i will immediately bully my way into the lane and then sit on the brakes to let the next five people in ahead of me because fuck YOOOOUUU

anyway i dont drive anymore
posted by poffin boffin at 3:41 PM on May 11, 2016 [18 favorites]


In a merging situation I always let one, and just one, car in front of me graciously. If everyone did this the zipper merge would result as an emergent property.
posted by Bringer Tom at 3:43 PM on May 11, 2016 [18 favorites]


A few years back the Mass DOT started putting messages on their electronic safety signs that read "USE YAH BLINKAH". They asked the public for more ideas and my suggestion, while not implemented (though I am sure much appreciated), was "MERGE NICELY LIKE A ZIPPAH".

I do admit that only works for me in the case of on-ramps and the like. When there's a merge due to a lane closure, I always feel the need to get to the open lane asap so as to not be That Guy
posted by Spatch at 3:43 PM on May 11, 2016 [6 favorites]


It's like you are just letting other people have things because you are nice.

This is so fundamentally unAmerican that the person behind you in traffic might just shoot you in the back of the head and have it considered justifiable.
posted by srboisvert at 3:50 PM on May 11, 2016 [3 favorites]


Iowa, actually. Are these not common? Is Iowa somehow ahead of the curve on fast food drive thru tech?

I have not once seen this, but my experience is mostly limited to the east coast.
posted by indubitable at 3:50 PM on May 11, 2016 [1 favorite]


My dad says that in Germany the zipper merge is just the normal way that people drive, and is puzzled about how we manage to balls it up so badly in north america
posted by 5_13_23_42_69_666 at 3:51 PM on May 11, 2016 [10 favorites]


I'm a former Masshole who has been living and driving in the UK and Australia for the last four years. (I even had to pass a UK driving test. Failed the first time. After 12 years of American law-abiding driving.) I had to be completely retrained on how merging lanes work. I grew up with my parents doing that thing railing against the people who drive all the way up to the end of an ending lane rather than merging the moment the "lane ends" sign appears, and they definitely would refuse to let them in. That was completely internalized for me.

Now pretty much everyone claims that their country/city/town has THE WORST DRIVERS, including Brits and Aussies, but everything being relative, American drivers, we suck. Imagine my surprise when zipper lanes work precisely as intended when driving in the UK and Australia! How gloriously efficient! And how much yelling from my husband was required for me to stop merging as soon as I saw that the lane was ending, or if I forgot to let in a merging car from the ending lane as was my responsibility! How much ridicule I incurred for trying to insist that the people in the merge lane waiting til the last minute were the assholes, not me!

I find that, like roundabouts*, the functionality of these (genuinely well-designed) systems comes down to the attitudes of the drivers, and their trust that the others understand how it's supposed to work and are following the same rules. Americans simply do not trust other drivers to be following the same rules, and as evidenced in the comments above, we're not. We don't even know the rules or how the systems are supposed to work, apparently, so we've been making them up for ourselves and passing them on to the drivers we teach and as a result we're all driving on our own rules and morals and getting furious at each other when the other drivers aren't following the same system. Some of it is definitely just our attitudes, but I really do think some of it is our really rather lacking driver education system.

* Mmm, roundabouts. Took me months to do anything other than brake and freeze until all other cars at the roundabout entrances were clear. I finally figured out that it was because, being a Masshole driver, I instinctively do not believe what anyone is or is not signalling. You have your left blinker on? Is that because you intend to turn left, or because you turned it on two miles ago and you just haven't noticed it's still on? You aren't signalling anything? Is that because you're going straight or because you just don't feel the need to let anyone know what you're doing? So anyway, the reason roundabouts work in the UK is because, by and large, everyone signals correctly, and you, trusting it, enter the roundabout accordingly. It was a genuine mental leap for me to get to the point that I could trust what other drivers told me they were going to do. Roundabouts don't work without the combination of comprehension of the design, proper signalling, and trust, but when they do work, they're effing awesome.
posted by olinerd at 3:54 PM on May 11, 2016 [19 favorites]


23skidoo: “If there's some big push for it to happen where I live and people start being aware that it's A Thing To Do Now? Awesome. If people want to just start LoneWolf-Zippermerging? That's just increasing people's road rage and increasing their unnecessarily aggressive driving.”

There is literally no difference between the two scenarios you are describing. You're basically saying: if a lot of people zipper merge, it's okay; if one single person zipper merges, that is terrible and they are bad. But there's no such thing as a lot of people zipper merging without single people zipper merging.

Incidentally, it's interesting to me how very many comments there are here that take the form: 'I think zipper merges are great, but anyone who does that on the X highway a mile from my house is a complete bastard and should be cut off from society. I guess it's hard to let go of daily aggravations, even if we know they're irrational.

Probably this is all a sign that driving is terrible and bad and we should all stop doing it.
posted by koeselitz at 3:56 PM on May 11, 2016 [4 favorites]


Yes, zipper merges are very standard in Australia. One thing that helps is that the cones are laid out to make the merging lane narrower-and-narrower-and-narrower, so there's psychological pressure to start merging well before the two lanes become one.
posted by Joe in Australia at 3:58 PM on May 11, 2016 [3 favorites]


The rage we see here, and on the roads, over people merging wrong is largely a symptom of inequity aversion. The deep dislike of unfairness, and the willingness to forgo personal and societal gains in order to punish inequity, is a challenge for any complex society to overcome. However, given the massive success of aristocracy and capitalism in getting people to swallow inequity for fucking bullshit reasons, I hold out hope that we will ultimately find ways to tolerate it for our own good.
posted by howfar at 4:10 PM on May 11, 2016 [14 favorites]


My risqué 1973 novel The Zipless Merge caused quite a scandal at DMVs and accredited driving schools across the country. I was just ahead of my time.
posted by Kafkaesque at 4:11 PM on May 11, 2016 [10 favorites]




The Tensor: ""Lane Ends, Merge Ahead," followed by "Stay in Lane, Zipper Merge" followed by "Zipper Merge For Faster Traffic."

How about:
  1. Merge Coming
  2. But Don't Merge Yet
  3. ...wait for it...
  4. ...almost there...
  5. OK NOW MERGE NOW NOW NOW
"

6. Burma Shave!
posted by Splunge at 4:14 PM on May 11, 2016 [4 favorites]


I'm reading a book about traffic, and there's an amusing bit in the section about zipper merging where traffic engineers, realizing that zipper merging is more efficient for everyone but also that most Americans are unwilling to do it, resort to tricking drivers into it by not having any signs about having to merge lanes until a couple hundred feet before the lane actually ends. Without any indication that the lane is about to end, drivers did pretty much what they were supposed to, but any situation where you gave them more information or warning just led to one incredibly backed up lane and one empty one.

I'd say that traffic engineers are about as aware of the basics of human psychology as anyone else, but it's hard to work yourself into a situation where the right thing to do when trying to help people safely maneuver deadly machines in unusual situations is to deprive them of useful information because you know they'll fuck it up.
posted by Copronymus at 4:16 PM on May 11, 2016 [15 favorites]


Serenity now!
posted by Carmody'sPrize at 4:17 PM on May 11, 2016 [2 favorites]


The World Famous: “In my own case, it's because I was talking about a road that's not appropriate for zipper merging, because there's no lane ending. It's a right turn only lane that gets backed way up, but some people decide to drive all the way up to the intersection in the through lanes and then try to merge into the right turn lane as it's turning.”

Ah, yes. Using a turn lane as a merging lane is not a good thing.
posted by koeselitz at 4:19 PM on May 11, 2016 [3 favorites]


The benefits of the zipper merge are over-stated. A zipper merge does not increase the flow of traffic past the obstacle. The flow of traffic past the obstacle will be the same whether there is an early merge with a two mile backup or a zipper merge with a one mile backup. The flow of traffic will be exactly the same -- assuming a perfect zipper merge. It the zipper merge is anything less than perfect, with some in the late merging lane forcing a slow down, then the flow of traffic will be worse for a zipper merge than for an early merge.

The only benefit of a zipper merge is reducing the length of the backup, not the time of the backup. The length of the backup only matters if the backup is blocking some intersection on the highway behind. Given that people are imperfect, late merging generally slows down traffic.
posted by JackFlash at 4:20 PM on May 11, 2016 [7 favorites]


Early merges can also be imperfect.
posted by howfar at 4:28 PM on May 11, 2016 [2 favorites]


And it's also more dangerous to have a more rapid deceleration of traffic, due to the greater risk of people crashing into it.
posted by howfar at 4:30 PM on May 11, 2016 [2 favorites]


Early merges can also be imperfect.

It doesn't matter to the flow of traffic. If the early merge is imperfect, it occurs long before the obstacle. An imperfect early merge does not impact the flow of traffic past the obstacle. An imperfect zipper merge does slow traffic past the obstacle.
posted by JackFlash at 4:32 PM on May 11, 2016 [2 favorites]


Oh my God!!!! JUSTIFICATION AT LAST!!!!! I have been saying this for fucking EVER! I guess it helps that I spent some of my early driving years in LA where you learn how to properly merge or die (either by being run down or shot by other enraged drivers).
posted by WalkerWestridge at 4:36 PM on May 11, 2016 [2 favorites]


An imperfect early merge does not impact the flow of traffic past the obstacle.

But it still slows traffic. That may or may not be made up before the obstacle, depending on what else happens, but discounting everything that happens before the obstacle seems to neglect the realities of traffic management. Delays matter whenever they happen, because they have complex knock-on effects. The actual data from actual traffic systems suggest you're not correct in your assertions that late merging doesn't reduce delays.
posted by howfar at 4:43 PM on May 11, 2016 [7 favorites]


Hmm, how about a sign that reads, "MERGE AHEAD. YIELD ONCE."
posted by The Tensor at 5:01 PM on May 11, 2016 [5 favorites]


I'm sorry, the potential of a cool graphic sign indicating a zipper merge is too great to let the sign be pure text.
posted by jjwiseman at 5:04 PM on May 11, 2016 [2 favorites]


This looks like a job for self-driving cars.
posted by rlk at 5:10 PM on May 11, 2016 [5 favorites]


This is one of the more civil discussions of zipper merging that I've witnessed and I get dragged into the discussion several times a week. I work for MnDOT and this issue is very much a thing for us. Well not me because I'm in IT and I take the bus and train, but wow it's a battle that's being fought.

As said many times above, it does make sense, it does reduce congestion and it does work great when people do it but that's nearly impossible. The only way it's worked a bit is to engineer around it like moving signage as mentioned in the article. The problem with that is if it's your regular commute you know exactly where lanes end so signs don't matter as much.

Anyway I know where you can get a bunch of "Do The Zipper Merge" bumper stickers if you're interested but I've seen many more of them on cubicle walls than cars.
posted by Clinging to the Wreckage at 5:13 PM on May 11, 2016 [10 favorites]


A successful zipper merge just makes me so happy. It's truly a delight. The only other driving thing that even comes close is when I have to drop off my daughter at school in the morning. There's this beautiful dance where a two lane road becomes a four lane, and the right turning cars, left turning cars, and exiting cars all go in order. One two three, one two three.
posted by Ruki at 5:18 PM on May 11, 2016 [6 favorites]


I always allow this to happen for those in front of me, but they don't believe it.
posted by turbid dahlia at 5:20 PM on May 11, 2016 [4 favorites]


I regularly have to make a merge from a fast-moving lane to a super-congested lane. (Seattleites: the west-bound HOV lane on 520 that ends right before the bridge. Or did until they opened the new bridge span a few weeks ago. But anyway. i don't have to do it any more, but I did it regularly for a long time.) I always do a zipper merge there, and you know what? It always works seamlessly! Somehow even when traffic is bumper-to-bumper, there's always enough of a gap for me to merge into as the HOV lane ends, calmly and smoothly. I was musing out loud as to why that is, and my nine-year-old daughter said "Mom, it's because of the diagonal."

"The what now?"

"As the lane merges in, it comes in at an angle -- you're not getting straight over, you're crossing on the diagonal line. Because it's a diagonal, it's longer, so if you have the same number of cars in the distance as measured on the straight, you have more room between them on the diagonal."

I think she might be right. I can't explain how that can possibly work, because the LANE is still the same length, but it certainly seems to make sense when I'm the middle of doing it.
posted by KathrynT at 5:21 PM on May 11, 2016 [2 favorites]


t's a right turn only lane that gets backed way up, but some people decide to drive all the way up to the intersection in the through lanes and then try to merge into the right turn lane as it's turning.

The reason I believe there is no god is that people who do that (always over the solid white line) are not struck down by lightning.
posted by jeather at 5:24 PM on May 11, 2016 [4 favorites]


Zipper merging is tolerable, but that person on the northbound Dan Ryan who waits to jump into the westbound IKE ramp exit lane, crossing the white line, ahead of the 1 block line of us who already got over, deserves to be DRAWN and QUARTERED in a tractor pull, then have his/her head publicly displayed on a spike as a warning to all the other assholes, right at the site of said exit.
posted by MikeWarot at 5:28 PM on May 11, 2016 [2 favorites]


there's another one
posted by koeselitz at 5:29 PM on May 11, 2016 [5 favorites]


the 1 block line of us who already got over

Seriously, this is terrible. People are merging an entire block before the merge point? If you do that, you're slowing everybody down by merging too early. The anger here is entirely misplaced.
posted by koeselitz at 5:32 PM on May 11, 2016 [8 favorites]


a lot of American politics only makes sense in the light of the fact that (almost) everyone here drives, usually on freeways, and for that reason we despise everyone outside our immediate families
posted by theodolite at 5:35 PM on May 11, 2016 [18 favorites]


When I worked in news I wished we could do a segment where you just park a camera at merge points, four way stops, etc and pull video of asshole drivers each day to publicly call them out, I always thought that would be a mighty fine public service for local news to take on.
posted by jason_steakums at 5:36 PM on May 11, 2016 [5 favorites]


Its amazing how people will pass up the spot to merge stressfree in order to get to the head of the line and wait.

while your generosity is admirable, you're advocating for the opposite of the zipper merge, and contributing to worse traffic.

Because the truth is that the zipper merge is totally counter-intuitive. Further, when you look at the video itself you'd be hard pressed to see the improvement between the two. They're both just... slow moving traffic. which is why educating people about this style of traffic merging... probably won't work.
posted by shmegegge at 5:38 PM on May 11, 2016 [1 favorite]


People are merging an entire block before the merge point?

That's not a merge point. That's when one lane is an exit lane and all the other lanes continue on the same road, and people block up the other lane because they want to (but can't) change lanes into the stopped up exit lane and have to park in the normal driving lane like sociopaths.
posted by jeather at 5:42 PM on May 11, 2016 [8 favorites]


People are merging an entire block before the merge point? If you do that, you're slowing everybody down by merging too early.

Not necessarily. If 1000 cars an hour are getting through the obstacle, it doesn't matter if they are doing it with one lane merging early or two lanes merging late. You still have 1000 cars an hour going through the obstacle. Using two lanes can reduce the blockage of intersections behind the obstacle, but it doesn't increase the speed of traffic through the obstacle or decrease your waiting time to get through the obstacle. It's still limited to 1000 cars an hour.
posted by JackFlash at 5:43 PM on May 11, 2016 [4 favorites]


Here's the thing, there is a double white line there, and appropriate signage (streetview link)

That photo is nuts. I've haven't seen those lanes so free and clear in the daytime for at least 15 years.

I hated hated hated this interchange when it was part of my commute. Hard to zipper merge (because of 94W/double white line already mentioned + passive aggressive blocking) but also once you get on 94 you're at the tunnel entrance so most people don't want to change lanes so all the exiting traffic is still in the right lane so it stays slows and things back up more so then more people try to merge at the last second to avoid the line so more people have to hit the brakes so the line backs up more so it goes on and on and on.

You're not supposed to change lanes in the tunnel, which sucks if you live in NE Mpls and you need to get in the far left lane pretty much immediately for your one and only exit onto 35W north. And there's the Hennepin/Lyndale exits going either way. And (from NE) the 35W south ONE LANE exit onto 94W just after the bridge! Jesus H. I could go on. I moved to this side of the river (NE Mpls) for the first time a few years ago and the road issues are the one and only thing that make me want to leave NE forever and move back to the south/west side. It's a nightmare that will only get worse when the stadium opens. The entire 394/94/35W area is just an epic clusterfuck of almost biblical proportions.
posted by triggerfinger at 5:44 PM on May 11, 2016 [3 favorites]


Ok, ok, can we talk about two-lane rotaries now, please?
posted by Salvor Hardin at 5:59 PM on May 11, 2016 [1 favorite]


I'm reading a book about traffic, and there's an amusing bit in the section about zipper merging where traffic engineers, realizing that zipper merging is more efficient for everyone but also that most Americans are unwilling to do it, resort to tricking drivers into it by not having any signs about having to merge lanes until a couple hundred feet before the lane actually ends.
I wouldn't say it's "tricking" as much as it is effective communication. Why would they put the sign up a mile beforehand unless they were trying to get people to merge earlier? An orange traffic sign should be actionable. If they want to warn people about stopped or slowed traffic ahead and to exercise greater caution, that would make sense, and they should use a sign that would indicate "slowed traffic". But early "lane closed" serve little purpose on their own except to confuse and make people think they should take some action in response.

Those super early "lane closed" signs are such an annoyance IMHO.
posted by Llama-Lime at 6:12 PM on May 11, 2016 [2 favorites]


The zipper merge will work if everyone agrees, but there's always that asshole who stays in the non-merged lane right up until the off-ramp and merges in at the front of the line. Not to mention that there's always that "good guy" who will let them in. I'm the one shooting them with my flamethrower.
posted by bendy at 6:39 PM on May 11, 2016 [2 favorites]


  the only way to create a zipper merge in this situation is for someone to leave the full lane and enter the merge lane.

No, a zipper merge is one from the left, one from the right, one from the left, etc. No crossing into the wrong lane. What you're describing, if tried where I'm originally from, is a followed all the way home appx 15 cm from your rear bumper, lights on full beam, leaning on the horn and if you're really lucky you'll only get your windows and lights stoved in with a tyre iron when you get there merge.
posted by scruss at 6:43 PM on May 11, 2016 [6 favorites]


What the hell, do people not know what a zipper merge is? A zipper merge is not merging at the last possible second. Zipper merging looks like a fucking zipper. Take all the cars in lane A and give them a letter, then give all the cars in lane B a number. The result of the merge should be A, 1, B, 2, C, 3, D, 4, etc... That is a zipper merge. A zipper tooth does not get ahead of other teeth, that makes the zipper jam. Merging is easy, match speeds with traffic, get over as soon as possible. The merge lane is as long as it is to accommodate the distance covered when you are going 70 mph and start merging when you are supposed to and you are moving laterally at a safe speed. When you're going thirty or fifteen that distance is much shorter.
When I merge one car behind the car that was in front of me when he merged, then you swing out around me and zip ahead of both of us and ride on the shoulder for a bit until some fuckwit lets you in, that's not zipper merging, that's being an asshole. Period.
And no leaving the lane to zip ahead in the merging lane, then trying to force your way back in doesn't make you any kind of a hero. It's makes you an asshole.
posted by MrBobaFett at 6:45 PM on May 11, 2016 [4 favorites]


You're not supposed to change lanes in the tunnel

Yes you are! You're just supposed to be careful. That's why it's a single white line and not a double.

Refusal to change lanes (safely) in the tunnel contributes to clusterfucks immediately past the tunnel, which can in turn lead to multi car accidents inside the tunnel which is very bad indeed.
posted by sparklemotion at 6:49 PM on May 11, 2016 [2 favorites]


People are merging an entire block before the merge point? If you do that, you're slowing everybody down by merging too early.
No, it's an exit lane... not a merge, an exit ... the OPPOSITE of a merge... and people are refusing to move to the exit lane until creating a traffic hazard.

Again... drawing and quartering, in public, and viral video, would be a fine punishment.
posted by MikeWarot at 6:49 PM on May 11, 2016 [2 favorites]


I was once waiting in a very, very long line at the border crossing back into Canada and some horrible person who deserved to have their car taken apart by border guards just drove by the entire line and went to the front and expected someone to let them in.

And some asshole was going to, until a border guard came up and made them reverse all the way back.
posted by jeather at 7:18 PM on May 11, 2016 [3 favorites]


The issue is often that people in the non-ending lane are tailgating each other ridiculously, so that cars from the ending lane cannot enter at the appropriate time, and they believe that tailgating is somehow morally superior to zipper-merging. If cars in the US would leave enough room for other cars to merge/change lanes in front of them, most of the angst about zipper-merging would disappear, I think.
posted by lazuli at 8:04 PM on May 11, 2016 [5 favorites]


Part of the problem is insanely shittily designed lanes.

So on a certain devil-highway, locals will all know - the HOV lane turns into "express lanes", that are closed half the time. If you are in the HOV lane, you WILL have to get over, because the lane is closed. But the next lane over is an exit-only lane. So to zipper-merge, you need to zipper merge into the exit only lane, only to then zipper merge onto the next lane of traffic. Thanks, asshole planners! And in each of these two lanes, there's a Righteous Man who thinks you deserve to be punished for being in the exit only lane and not exiting, despite the fact that you have to move over or you will crash, and he just plain won't let you in for love nor money.

HATE.
posted by corb at 8:07 PM on May 11, 2016 [1 favorite]


I do this every day on my morning commute without issue. Saves so much time! When I figured it out, it was game changer for me.
posted by oceanjesse at 8:29 PM on May 11, 2016 [3 favorites]


"So, just everybody drive in the same lane all the time, and nobody ever drives in the other lanes"

Firstly we're talking about merging, so we are talking about a lane that only exists for several hundred feet at most, the purposed of it is to transition into traffic.
Second, no. You stay as far to the right as possible and you move left as necessary to overtake. Then you move back as far right as possible.
posted by MrBobaFett at 8:39 PM on May 11, 2016


All I can say as a native Bay Area person who now lives and drives extensively across the the Northeast, the reality of zipper merging in the Northeast is a completely different animal than on the West Coast for a couple of reasons.

Over here (with the exception of Western NY, who I have found to be the sanest drivers generally speaking), people treat any sort of safe following distance like a personal affront. There also seems to be implied insult in letting a car merge in front of you for whatever reason. Also: people don't want to pass you, even if they're clear on the left--they expect you to move over. If there is no lane to the right, then they'll tailgate you until you move left so they pass, and then you merge right again. It's crazy.

To think I used to to think driving in LA or the East Bay was a mess...
posted by smirkette at 8:47 PM on May 11, 2016 [5 favorites]


on my commute home this evening I was honked and yelled at for exercising the right of way on a priority road and not letting the aggressor leap in to traffic in front of me, despite there being about twenty car-lengths of empty space behind me.

Oh good, a chance to wheel out favorite little pedantic niggle.

In traffic there is no pre-existing right of way. There are a heap of rules about when you must yield the right of way to other drivers, but failure to do so does not in any way absolve those other drivers of their overarching responsibility to avoid a collision.

In any circumstance where you are contending with another driver for space on the road, your right of way comes into existence only when the other driver yields it to you. If they fail to do that when the road rules say they should, and you consequently drive in a manner that attempts to enforce a right of way that the rules say should have been yielded to you, and an otherwise avoidable crash occurs as a result, that's at least as much your fault as that of the driver who failed to yield.

Simply being on a priority road does not, in and of itself, confer an automatic right of way. Nor does facing a green traffic signal. Nor does driving an emergency vehicle.

Driving with the mindset that you have a right of way that you're entitled to exercise might well kill you, or another driver, or another driver's children. It's a bad mental habit. Try to avoid it.
posted by flabdablet at 9:10 PM on May 11, 2016 [6 favorites]


Previously
Previouslier
posted by radwolf76 at 9:11 PM on May 11, 2016 [1 favorite]


If they fail to do that when the road rules say they should, and you consequently drive in a manner that attempts to enforce a right of way that the rules say should have been yielded to you, and an otherwise avoidable crash occurs as a result, that's at least as much your fault as that of the driver who failed to yield.

If someone runs a stop sign or a red light and crashes it means the other driver is "at least as much as fault"? Really?
posted by juv3nal at 9:21 PM on May 11, 2016 [3 favorites]


The best zipper merge structures i've seen have a stoplight on the onramp which lets cars through one or two at a time with a few seconds in between - this allows the cars to match the speed of existing traffic and merge in correctly without disrupting the flow of traffic.

Come to seattle. We have these on most on ramps!

...and somehow everyone has figured out how to still make them not work by going to the very end and STOPPING even when there was room to get in. So now your choice is either merge early and agressively to not have to brake, or brake hard and now... you're also stopped.

You can of course also just slow down and attempt not to stop, but that's barely better than stopping since now you're going 30 in a 60, or a worse differential.

Every time i get far enough away from seattle that things like this work i want to cry.

I could also write something about our left side onramps and exits, which are satan spawn. Especially because the lanes in seattle basically work like:
[60-90][55][55][50]

...And everyone merges at 40, or stops at the end of the ramp. EVERYONE.

If someone runs a stop sign or a red light and crashes it means the other driver is "at least as much as fault"? Really?

Nah, what i think they're saying is if you see them but don't stop or brake but you could have taken evasive action because you have the right of way and fuck them, then you're partially at fault.
posted by emptythought at 9:25 PM on May 11, 2016 [5 favorites]


My dad always loved (still loves) the 5-lane merge into the Midtown Tunnel eastbound. (A braid merge?) New York nice.
posted by anshuman at 10:23 PM on May 11, 2016 [1 favorite]


My favorite is when there's an on ramp and an off ramp connected by a bit of road and the people just going from one to the other and staying in that lane never go over surface street speeds while you try to exit at interstate speeds...
posted by jason_steakums at 10:44 PM on May 11, 2016 [2 favorites]


I'm tempted to start some computer models, but let me try ASCII diagrams instead.

A zipper merge looks like this: two lanes merging down to one, then opening out:

=====================–––––=======

A non-zipper merge looks like this:

======––––––––––––––––––––––––=======

And that can't possibly be better. It means maximizing the time spent going slower, and often extending the blockage back far enough that it catches people who aren't even taking that route (because they're going to take another exit, or whatever).
posted by zompist at 10:44 PM on May 11, 2016 [7 favorites]


"Early merge: in at-speed or nearly free-flow conditions, its best to merge early as space in the open lane allows."

Most of the merges I recall encountering in highway driving are more like that condition: people driving 55mph bumper to bumper through a construction zone. Only recently did I have the opportunity to observe LA highway traffic, which is quite literally six lanes of parked cars.

And yea, if you're trying to optimize for cars parked on the freeway, zipper merge is great. Does Minnesota have a lot of those situations?
posted by pwnguin at 1:10 AM on May 12, 2016


people get so angry over things that happen on the roads and in the great scheme of things, it doesn't really matter.

People all get angry at and want to punish unfairness in whatever context: this post is about driving as a context.

You know that puzzle -- you see four cards, one with a 4, one with a 7, one with a square, one with a circle, and the rule is "if a card has an odd number on one side, the other side has a circle", which cards do you turn over to see if they follow the rule? And people get it wrong most of the time.

But people get it RIGHT most of the time when you have "age 16", "age 30", "drinking soda", "drinking beer" and the rule is "you must be 18 (21) to drink legally".

Driving is part of life right now. Maybe that will change soon. But until then, people will want to enforce the social compact there (especially since screwing it up when a car is involved leads to a high risk of injury) as they do anywhere.
posted by jeather at 4:10 AM on May 12, 2016 [4 favorites]


Six weeks of driving across Canal St. has burned out eight years of hard-won highway driving zen. Sure, go ahead and rub the side of your car against my back bumper.
posted by whuppy at 5:40 AM on May 12, 2016


I never thought I'd say it, but I really miss Pacific Northwest driving. Sure, they get zipper merge wrong like everyone else in America, but at least their signaling was usually reliable and they'd let you in if you try to zipper. Here on the East Coast, the driving is so Fuck You Got Mine. No signaling because then other people would have the warning to NOT let you in and you don't want to give them that advantage. And they just simply will not let you merge in. It's terrifying, and dangerous.

I do blame the DMV at least in part for the terrible driving around here though: in the Northwest, when you moved from another state, you had to review the new state's driving rules and take a written test before they'd issue you a driver's license. Here, all you have to do is prove who you are and they license you. And there are a lot of transplants here, which means everybody is driving around without even knowing their state driving laws.
posted by rabbitrabbit at 6:07 AM on May 12, 2016 [4 favorites]


So wouldn't the logical way to do a "fair" zipper merge going from 2 lanes to 1 lane be to match speeds with the cars in the other lane, but wait to merge until the very end?

And how likely is it that someone wanting to race ahead would get super pissed off and ram you?
posted by Zalzidrax at 6:17 AM on May 12, 2016 [2 favorites]


Am still trying to figure out if Minnesotans are bad drivers because of road architecture, bad drivers in spite of road architecture, bad designers of road architecture, or just full of suppressed road rage.
posted by ZeusHumms at 7:32 AM on May 12, 2016 [1 favorite]


Just going to leave this here.
posted by entropicamericana at 8:41 AM on May 12, 2016


I will fite anyone that calls it the asshole lane. Idiots!
posted by Theta States at 9:02 AM on May 12, 2016


That's just increasing people's road rage and increasing their unnecessarily aggressive driving.

I have a reaction to blaming law-abiding drivers for the actions of hotheads who act-out and behave dangerously, but it would be deleted post-haste and probably get me a stern me-mail from cortex and possibly put on a government watch list. Let's say I feel more than a little strongly about it and I reject the classification of my use of my turn signal and merging lane decisions as the short skirt of traffic law.
posted by phearlez at 9:52 AM on May 12, 2016


So wouldn't the logical way to do a "fair" zipper merge going from 2 lanes to 1 lane be to match speeds with the cars in the other lane, but wait to merge until the very end?

But fairness isn't valuable in this context. That's the point. This is one of those situations, like the provision of welfare benefits, where the fact that some of the people benefiting are selfish dicks is irrelevant, in light of the overall good to society. Only more, because being a selfish dick will actually motivate optimum behaviour in this particular case. Your proposal wouldn't actually make anything better for anyone, and would eliminate most of the benefits of late merging for a point of principle.

What you're actually proposing is something that people do, in real life, in order to prevent zipper merging. That is, in order to prevent optimum traffic management. These people are not, I would argue, the heroes they consider themselves to be. Rather the opposite in fact.
posted by howfar at 10:14 AM on May 12, 2016 [3 favorites]


The zipper merge--yes, duh.

The problems start (as others have alluded to) when drivers use exit lanes (i.e. where you're supposed to get off the freeway) and then take those all the way until they end and expect to zipper merge.

It seems like the zipper would apply in that case as well, but it's more of an etiquette thing--no agreed convention.
posted by mrgrimm at 10:32 AM on May 12, 2016


This graphic shows pretty clearly what I've always said: Merging early and not taking advantage of all available lane space increases overall length of trip for everyone.

A====================–––––=====B
A=====––––––––––––––––––––––––=====B

The first - a proper merge happening towards the end - maximizes use of available lane space. If each dash is 100 meters, then there's 5500 meters of lane space used between A and B. In the second - early merge leaving 1500 meters of lane empty - there's only 3500 meters of lane utilized.
posted by jetsetsc at 10:35 AM on May 12, 2016 [1 favorite]


> So wouldn't the logical way to do a "fair" zipper merge going from 2 lanes to 1 lane be to match speeds with the cars in the other lane, but wait to merge until the very end?

I believe that is the ideal zipper merge, yeah. Matching speeds when there's space in front of you to zip along is arguably slightly less efficient, but on the other hand it might also encourage the ideal situation.
posted by lucidium at 10:36 AM on May 12, 2016


This graphic shows pretty clearly what I've always said: Merging early and not taking advantage of all available lane space increases overall length of trip for everyone.

No it doesn't. Using all available lane space does not shorten your trip. The length of your trip is determined by the number of cars per hour passing through choke point. Using more lane space before the choke point doesn't change that fact.

Look, zipper merge has nothing to do with early or late merging. Zipper merging simply means that cars merge as efficiently as possible with no one touching their brakes. Forget the name zipper merge as some people mistakenly think that implies a last second merge. It is better described as a moving merge. This is in contrast to a stopping merge in which cars alternately stop, left and right, to allow one or two vehicles to pass through the obstacle.

A moving merge doesn't matter if it early or late. It doesn't matter if you merge 20 feet, 100 feet, 200 feet or 400 feet before the choke point. The important thing is that everyone merge without braking so that traffic can flow through the choke point at the highest rate.

Given that traffic spacing is somewhat random and drivers are somewhat imperfect, the best strategy for everyone is to merge as soon as it is convenient to do so without braking. If is there is a space to merge, do so. It doesn't need to be at the last second.

What you don't want to do is gamble, press your luck and just hope that a space opens up for you at the very end. In that case, the best strategy is for people to block you out if it means they have to touch their brakes to let you in. You gambled, you lost, you can just sit in the stopped lane until the cows come home. You made a mistake by failing to merge in any available space earlier, waiting for the last chance to merge and trying to force your way in which slows everyone down. If you cause someone to touch their brakes, you have failed.
posted by JackFlash at 10:57 AM on May 12, 2016 [5 favorites]


A zipper merge can only be a zipper merge is traffic is moving at a reasonable speed before and after the merging point. If traffic has overwhelmed the infrastructure to the point that it's stop and go 500-1000 metres (not feet; think 1/4-1/2 mile) before the gore point, then it's no longer a zipper merge; it's an asshole dropping in at the last minute (and usually needing to stop in the lane that they're in screwing up that traffic).

A well done zipper merge is a beautiful thing, but too many planners (and news paper columnists) don't look at the actual circumstances of the road at the time. If people can't even do something simple like "green means go" or "get up to freeway speed *before* you merge, how can there be any realistic expectation of a well done zipper merge?
posted by nobeagle at 11:15 AM on May 12, 2016 [1 favorite]


"No it doesn't. Using all available lane space does not shorten your trip. The length of your trip is determined by the number of cars per hour passing through choke point. Using more lane space before the choke point doesn't change that fact."

Yes it does. In the first case the "choke point" is just 500 meters long. I the second it's 2,000. How does that not affect the overall travel time? It's like saying a one lane road is just as efficient at moving 1,000 cars 10 miles as is a two lane road.
posted by jetsetsc at 11:16 AM on May 12, 2016 [1 favorite]


A zipper merge can only be a zipper merge is traffic is moving at a reasonable speed before and after the merging point. If traffic has overwhelmed the infrastructure to the point that it's stop and go 500-1000 metres (not feet; think 1/4-1/2 mile) before the gore point, then it's no longer a zipper merge; it's an asshole dropping in at the last minute (and usually needing to stop in the lane that they're in screwing up that traffic).

And is mostly the result of people not having done a zipper merge well.
posted by Etrigan at 11:18 AM on May 12, 2016


Yes it does. In the first case the "choke point" is just 500 meters long. I the second it's 2,000. How does that not affect the overall travel time?

Answer this question. It isn't a trick question. It is quite simple.

You have two lanes of traffic merging into one lane. The single lane can handle 1000 cars per hour. How long does it take the 1000th car to pass through the final merge point?

Does it matter if the the zipper is 500 meters or 2000 meters?
posted by JackFlash at 11:25 AM on May 12, 2016 [1 favorite]


If the merge is very early, then the trip takes longer before the choke point.
posted by Monochrome at 11:33 AM on May 12, 2016 [1 favorite]


If the merge is very early, then the trip takes longer before the choke point.

No. This is mathematically incorrect. The determining factor for how long it takes to get to your destination is the limiting choke point. You don't change the time to get through the choke point regardless of the number or length of lanes approaching it.

There is a psychological fallacy that by getting closer to the choke point you are somehow getting through sooner. There is a tendency to want to push to the front. But all the pushing and shoving doesn't change the fact that you still can only get 1000 cars per hour through the choke point. If you line up single file or double file, it takes exactly the same amount of time to get through.

Think about the line for the check out at the grocery. Does it make any difference for how long it takes if you line up in single file or double file and alternate? The double file is more psychologically comforting because you stand closer to the cash register, but you don't actually get home any sooner.
posted by JackFlash at 11:47 AM on May 12, 2016 [1 favorite]


I'm not talking about the length of the merge, or arguing about the carrying capacity of a single lane. I'm talking about the length of the shut down lane. A choke point is indeed just a point, but what affects travel time is how long you are forced to use (in this example) one lane rather than two. if one lane can handle 1000 cars per hour, then two lanes should handle 2000, so it does matter if everyone has to use a single lane for 500 meters or for 2000 meters. Merging early takes away that resource.
posted by jetsetsc at 11:49 AM on May 12, 2016 [2 favorites]


We've got a well-understood problem with a proven theoretical solution that laypeople are unable to properly visualise and understand. Have we discovered the traffic equivalent of the Monty Hall problem?
posted by tobascodagama at 11:56 AM on May 12, 2016 [3 favorites]


The determining factor for how long it takes to get to your destination is the limiting choke point.

We're talking about two different things.
posted by Monochrome at 12:02 PM on May 12, 2016


There is a psychological fallacy that by getting closer to the choke point you are somehow getting through sooner. There is a tendency to want to push to the front. But all the pushing and shoving doesn't change the fact that you still can only get 1000 cars per hour through the choke point. If you line up single file or double file, it takes exactly the same amount of time to get through.

The grocery line analogy works if this is a closed system where there is no opportunity for exit between the end of the line and the choke point.

In the real world, getting all the cars to the chokepoint faster is more efficient because for at least some of the cars their goal is not to get to the choke point, but to exit at some point before the choke point. This is exacerbated by the fact that when people go single-file too early, they are actually moving the choke point back, and therefore involving more cars than necessary in the slower progression.

I'm relying on the pre tag here, so I hope this renders somewhat logically...

Example 1, car A can only exit as fast as cars G, H, and J can get through the choke
___| |________________ 
A B C D E F _G__H__J___
-----------/
Example 2, the back-up due to the chokepoint doesn't affect car a, benefiting car a and any cars behind it (for example car z, which was so far behind before you couldn't even see it.
___|a|________________ 
----- d e f _g__h__j___
------z b c/
This is also assuming a simplified world where merging "early" is as efficient as merging once at the choke point. When cars merge "early", you get people making their own decisions about what "early" means, so you end up with one car letting 4 or 5 cars in (slowing things down behind that car) and creating weird fast-slow-fast-fast-slow speed patterns in both lanes. Zipper merging means taking turns, one car yields for only 1 other car. My ascii art and ability to model traffic flows limits my ability to depict this appropriately.
posted by sparklemotion at 12:39 PM on May 12, 2016 [2 favorites]


Yes, I stated at the very beginning that as long as you aren't blocking intersections, it makes no difference how long the single-file line is. The solution to that is simple. If you are blocking an intersection and the other lane is open, then you should proceed in the other lane to unblock the intersection.

If you want to talk about the real world, the notion that a zipper merge consists of one car yielding for one other car at the last moment -- that's complete nonsense. As anyone has observed, real traffic consists of random clumps of cars -- two or three cars followed by a space for two or three cars, followed by another clump of cars followed by another space. Cars are not driven by robots -- yet. When you realize that you have to merge and you observe one of those spaces, you take it regardless of how far it is from the final merge point. If you wait for the last second and no space happens to occur when you need it, then you are forcing the line to brake in order to let you in. Requiring anyone in the moving line to brake is a failure. So better to merge when the opportunity presents itself than to merge late and cause a delay.

I hope we have gotten past the fallacy that late merging somehow magically shortens your trip time. Your trip time is exactly the same regardless. As pointed out earlier, if the length of the backup becomes excessive, it can affect intersections behind, but it has no affect on the trip time of people passing through the choke point.

It's very simple. If you know you have to merge, merge when a convenient space opens up. Early merge or late merge, it doesn't make any difference to the transit time. But if you gamble and wait until the very end and no space happens to open up, then you are causing a problem by forcing the moving line to brake for you.
posted by JackFlash at 1:28 PM on May 12, 2016 [4 favorites]


But if you gamble and wait until the very end and no space happens to open up, then you are causing a problem by forcing the moving line to brake for you.

You say "happens to open up" like it's a die roll versus human beings with agency. An opening always has the possibility to be there; when it is not - particularly in very slow traffic - it is the result of a conscious action.

You're also, IMNSHO, overlooking the zipper payoff of it happening at a predictable point. Traffic can come to a slow on a highway simply because someone taps their brakes. We've all see the mystery slowdowns with no root cause. Arbitrary early merge points rather than using the set traffic-planner location near the end of the merge lane exacerbates this.
posted by phearlez at 2:34 PM on May 12, 2016 [3 favorites]


Merging instructions:

In low-speed traffic, zipper merge.
In high-speed traffic, early merge.
On the Merritt Parkway, grab the steering wheel and hold on for dear life.
posted by breakin' the law at 2:36 PM on May 12, 2016 [3 favorites]


Okay so the key here is that the responsibility is not entirely on the cars in the lane that is ending; it's the responsibility of the cars in the continuing lane to let one car in front of them at the point the lanes merge. The logic is:

- Am I approaching the merge point? If yes, then:
- Is there a car in the ending lane? If no, proceed. If yes, then:
- Manage spacing between me and the car in front of me to allow exactly that one car in, and carry on our way.

This is why the "fuck you, last minute merger" attitude ruins everything. In heavy but not stopped traffic, there will almost never be a speed-matching way to easily merge in to a space that is magically available, so you get people basically stopping while the other lane continues at 30 mph and the ending lane gets super backed up waiting for a magic space to appear. The continuing lane must be part of the merging process too and should accommodate merging cars. The zipper thing is just like zipper teeth: one car from lane A, one from lane B, one A, one B... The attitude that the continuing lane has no responsibility to make the merge system work is the root of the problem.
posted by olinerd at 3:25 PM on May 12, 2016 [4 favorites]


TBH the "lane ends, merge right" sign should be over the continuing lane, letting them know to start adjusting spacing to allow in merging cars, rather than being read as instruction for the ending lane to get over ASAP.
posted by olinerd at 3:28 PM on May 12, 2016 [1 favorite]


As anyone has observed, real traffic consists of random clumps of cars -- two or three cars followed by a space for two or three cars, followed by another clump of cars followed by another space. Cars are not driven by robots -- yet.

Yes. This is why merging early, which increases the chances of traffic so slow it comes to a standstill, causes delays additional to those at the bottleneck. Your argument only works if you presume that traffic reaches and enters the bottleneck at a steady pace. But, as you yourself point out, that's not how things work.
posted by howfar at 3:53 PM on May 12, 2016


Exhibit A:
A zipper merge looks like this: two lanes merging down to one, then opening out:

=====================–––––=======

A non-zipper merge looks like this:

======––––––––––––––––––––––––=======

And that can't possibly be better.
Exhibit B:
This graphic shows pretty clearly what I've always said: Merging early and not taking advantage of all available lane space increases overall length of trip for everyone.

A====================–––––=====B
A=====––––––––––––––––––––––––=====B
These are both wrong, "obviously." The fallacy is that speeds where there are two lines are faster than the speeds where there is one line. I'd like to expand on what JackFlash has been saying.

There's a non-linear transition from smoothly moving traffic to congestion. These are two very different situations with completely different results. In smoothly moving traffic, the average speed in the two lanes and the single lane are identical. In the situation above. There, it doesn't matter there's a longer stretch of single cars other than to increase density of cars.

However, when there's congestion, and the speed of vehicles passing the chokepoint is lower than the speed of vehicles after the chokepoint, when it all opens up. However a very important distinction is the speed of an individual car, which we feel very intensely as drivers, and the number of vehicles that pass a particular point on the rode within a unit of time, which we don't really experience directly. This amount of traffic is the same immediately before, at the congestion point, and after the congestion point, even though each individual car drives at different speeds in each section. There's only so many cars, they're all going the same direction, and there's nowhere else for them to escape.

The traffic rate is the density of cars times the speed of traffic. In a congested situation, it looks like this:
Region
    X         Y             Z
========-----------------========
Traffic rate:
    t         t             t
Car Density:
  2*d         d           𝛆*d
Individual car speed:
 .5*s         s           s/𝛆
Where 𝛆 is less than 1, meaning that traffic speeds up after the congestion, and becomes far less dense.

Before cars reach the congestion, in region W, it looks like:
Region
      W           X         Y             Z
- = - = - = - ========-----------------========
Traffic rate:
      t'          t         t             t
Density:
      d'        2*d         d           𝛆*d
Car speed:
      s'       .5*s         s           s/𝛆
If t' > t, then the region X will expand, if t' < t, then X will decrease. The speeds and density in region W, before region X, have some amount of variability, but t' = d' * s'.

As traffic starts at nothing and increases throughout the day, eventually t' will get greater than the freezing point t, and then it all shuts down into congestion. There seems to be some sorts of behaviors where traffic can be "super-cooled" and where the chokepoint can at least temporarily support traffic greater than t, but if the right obstruction happens it goes back to the lower t. And there's no going from the lower t to a higher one until everything clears again. Which isn't going to happen until t' < t for long enough for X to go to zero.

So what's the tradeoff between the length of X and the length of Y? It takes twice as much length in Y to hold the same number of cars, since in congested traffic the traffic is the same. The backup is determined by the number of cars. For every length of distance of spent in Y, you'll spend twice as much time in the X region to cover the same distance. So for individual drivers, going through the obstruction, a longer or shorter Y translates into a shorter or longer X such that there's no change in the overall amount of time spent in traffic. The distance travelled is obviously the same as well.

Really, as all the sites say, there's no difference in the rate of traffic, or the amount of time that it takes for individual drivers to traverse the obstruction. The biggest difference is the length of the backup.
posted by Llama-Lime at 4:10 PM on May 12, 2016


Just out of curiosity, are there any actual civil/traffic engineers around here?
posted by TedW at 4:44 PM on May 12, 2016 [4 favorites]


*strokes beard thoughtfully*

Fuck math.
posted by Enemy of Joy at 4:45 PM on May 12, 2016 [3 favorites]


MetaFilter: LoneWolf-Zippermerging
posted by seyirci at 5:16 PM on May 12, 2016 [2 favorites]


The biggest difference is the length of the backup.

Which is what I said causes the problems. When you extend the blockage, you often extend it back behind an intersection or an exit, thus slowing down people who aren't even going to reach the actual blocked area.

Plus, cars are not peas on a chute. If everyone early-merged, you'd have an extended but smooth blockage; if everyone zipper-merged, you'd have smooth sorting. When people's strategies are mixed, then you get unpredictable behavior, moments when one lane is obviously faster, moments when one lane is blocked because someone decides to punish another driver. Drivers don't care about average speed getting past the blockage; they care about their speed, and with mixed strategies, that can vary in large (and infuriating) ways.

This is, you know, probably why the OP said zipper merging was for "reducing traffic backups, increasing safety, and minimizing road rage."
posted by zompist at 7:16 PM on May 12, 2016 [3 favorites]


I assert that the start of the single lane, beyond which nobody can asshole their way in is a choke point... and is the major limiting factor of the carrying capacity of the roadway.

Thus, speed at the choke point is critical, and any surprises (especially assholes) at that point slow it down for everyone else, and reduce the road capacity for all subsequent drivers.

The best way to reduce uncertainty, and increase safety, is to merge early, and not be a dick, that way, traffic enters the choke point at it's maximum safe value.

If it's a zipper merge, fine... but not at the actual choke point, well before it please.
posted by MikeWarot at 8:58 PM on May 12, 2016 [1 favorite]


not at the actual choke point, well before it please

...which means that some amount of road surface that could be safely carrying traffic (i.e. the empty lane between the early-merge point and the physical choke point) has none on it, which reduces the overall capacity of the road, which means that congestion will occur at a lower traffic volume than would otherwise be the case.

If the one-lane region past a choke point is very short then drivers will be willing to maintain what would otherwise be an unsafe following distance as they drive that region, which effectively increases its carrying capacity close to that of the two-lane regions before and after it.

On the other hand, if most people are early-merging, then the length of the one-lane region is extended and drivers will slow down in an attempt to maintain a safe following distance. This causes drivers behind them to slow down even more, and those behind them to slow down more again, and now you have a backward-propagating congestion wave that can bring the whole road upstream of the choke point to a standstill.
posted by flabdablet at 10:04 PM on May 12, 2016 [2 favorites]


So, wait. You are hypothesizing an exceptional case in which the choke point is very short? How short? 10 feet, 100 feet, 1000 feet, 1 mile? And you are advocating that people maintain an unsafe following distance so that they can get home quicker? "Here, hold my beer and watch this? If everything goes well we won't all die in a fiery death."

As has been pointed out many times, single-file or double-file makes no difference in the passage of people through the single-lane choke point assuming everyone is driving sanely. There is no "reducing the overall capacity of the road." The capacity is determined by the single-lane choke point regardless of its length.
posted by JackFlash at 10:33 PM on May 12, 2016 [1 favorite]


advocating that people maintain an unsafe following distance so that they can get home quicker?

No, merely observing that for short runs of merged traffic, this is what actually happens.
posted by flabdablet at 11:03 PM on May 12, 2016


But merged traffic runs are never short... they merge, and then go for miles during construction season. It works a lot better here in Indiana, away from the Chicago assholes, because we all follow the signs well before the choke point, and we're already impedance matched with the choke point, in Chicago, the same exact traffic flow would result in road-rage/shooting.

Illinois really needs to start some sort of formal driver education and testing program.
posted by MikeWarot at 5:48 PM on May 13, 2016


All y'all claiming that early merging is automatically better: cite?
posted by lazuli at 6:09 PM on May 13, 2016


surprises (especially assholes) at that point slow it down for everyone else

Why are you surprised at people trying to merge when their lane runs out? Why aren't you expecting them and preparing to let them in?
posted by corb at 6:22 PM on May 13, 2016 [5 favorites]


If you're actually paying attention while driving, it's obvious (even without the signs) that a merge is about to happen, sometimes it's because of an accident, a traffic stop, or a stalled vehicle... merging prior to the point at which paint gets swapped is the obvious right thing to do, but some people just rush right up to that point and try to swap paint anyway, deliberately, with ego aforethought.

Those people act in a completely different way from tourists and people who were actually surprised by the merge... and I give them slack.
posted by MikeWarot at 12:57 AM on May 14, 2016


Why aren't you expecting them and preparing to let them in?

Because at that point, letting them in means stopping, not slowing down slightly to widen the gap in front of me. And stopping will cause both lanes to grind to a halt, so most drivers will be happy to let the dude in the BMW wait a bit longer, not only because he has a tiny penis but also because they know that he'd never ever let them in.
posted by effbot at 1:48 AM on May 14, 2016 [1 favorite]


You know, after thinking about this for a few days, what seems to happen in my experience is that most people instinctively do use the zipper merge. It just happens fairly soon, right after the signs announcing the lane closure appear. People merge into the open lane in an orderly fashion with minimal decrease in speed. But there are always a few people left out of the process, either because they entered from a side street after everyone has merged, they weren't paying attention, or they are jerks who want to cut in line. How best to deal with them is the problem.
posted by TedW at 4:38 AM on May 14, 2016 [1 favorite]


Passage through a single point on the road may remain constant while average journey time changes, like the difference between water flowing through a 1m hose and a 5m hose.
posted by lucidium at 12:46 PM on May 15, 2016


Passage through a single point on the road may remain constant while average journey time changes, like the difference between water flowing through a 1m hose and a 5m hose.
The problem with this analogy is that traffic, unlike water is somewhat compressible.
posted by MikeWarot at 4:41 PM on May 15, 2016


You can have a longer journey without changing the throughput at a particular point. The rate of cars after a choke point might not change, but the duration of the choke can. Drive from A to B vs. Drive from A to B with a half hour traffic jam in the middle, people still arrive at B at the same rate.
posted by lucidium at 5:11 PM on May 15, 2016 [1 favorite]


Cars also pass any given spot on the road at the same rate, when measured over a long enough time span. That rate is traffic density, and it's independent of the number of lanes provided and, somewhat unintuitively, also largely independent of the average speed of the cars: to a good first approximation, drivers will follow the car in front with a spacing proportional to their road speed, so as to maintain a roughly constant following time (bastard tailgaters excepted).

Merging involves a forced step-decrease in following time, as the merging cars occupy what was previously free space.

If the average following time post-merge is within drivers' comfort zones, as it will be when traffic density is lower than the capacity of the narrowed roadway downstream from the merge zone, then traffic will continue to flow smoothly and the actual merging strategy (late zipper vs. ad-hoc early gap occupation) is almost completely irrelevant.

If the average following time post-merge is just a little bit shorter than is comfortable, drivers will mostly put up with that as long as it lasts for under twenty seconds or so. Much more than that and drivers start trying to back off from the car in front, and that generates a backward wave of slowing-down that quickly propagates back to the merge point and earlier.

If the average following time before the merge zone is still well above the minimum that drivers are comfortable with, then the backward slowdown wave damps out some distance upstream from the merge zone. It's under those specific conditions that late zipper merging is useful. If traffic is doing early merge, then the point at which the backward slowdown wave damps out is further upstream than it needs to be. There are plenty of road configurations where that might well make the difference between a slowdown wave having room to damp out vs. a phase change into congestion over substantial portions of the roadway.

If traffic density is high enough that the average following time before the merge zone is already about as low as drivers are willing to let it get, then the roadway upstream from the merge zone is at capacity, and downstream it's well above capacity. Under those conditions, congestion is completely unavoidable regardless of merging strategy.

Lots of the argument in this thread seems to me to derive from participants having one of the above scenarios in mind and forgetting that the others can and do exist for substantial portions of the day.
posted by flabdablet at 2:22 AM on May 16, 2016 [2 favorites]


I perform a clean zipper merge every morning at this onramp. It works because the people merging in from either side are going about the same speed, and automatically zipper merge. It's also a 'two lanes become one' situation rather than a 'one lane vanishes' situation. I run into someone who won't let me merge rarely enough that it's memorable when it happens. Before this ramp was redesigned a few years ago, it was the same two streams of drivers, but drivers turning left onto the ramp had to merge into the right, and traffic would back up for four or five blocks because nobody in the right wanted to let anyone in. I used to drive over one block and merge into the shoulder/turn lane and drive for about ten carlengths, just so I could be in the dominant lane and actually get through the bottleneck. It was silly. The same drivers would let me in from a side street, but not in a plain merge.
posted by Karmakaze at 11:20 AM on May 16, 2016


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