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this post was submitted on 01 Jun 2025
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Zipper merging.
If your lane is closing ahead, it is better for everyone in traffic if you drive all the way to the end of the lane and cut in at the last moment.
Note that this does not apply to exit lanes. The basic rule is if late merging blocks someone from going somewhere, merge early. Otherwise, merge as late as you can.
I really can't more strongly disagree with this take.
Zipper merging is to interleave two lanes of traffic when there's one lane of traffic available ahead.
It DOES NOT matter if it's done with 3 feet to merge or 300 feet to merge. There's no efficiency gain.
What does matter is some assholes trying to merge at speed at the last possible second.
The zipper point should not be the point where there's NO ROOM to merge SAFELY without EVERYONE going 3 miles per hour.
The handful of times I've seen a zipper merge actually start to work, someone rushes down to the end of the line where the problem is, nearly causes a second accident trying to get over, and then everything starts moving at a crawl again.
You don't need to zipper merge at the "physical barrier" causing the zipper merge to be necessary.
Merging early leaves unused road. Shoving the cars into fewer lanes makes the traffic jam longer and makes it impact more interesections far behind the actual hangup. If you can merge early without slowing down, sure go for it. I'm mostly talking about the scenarios where it's already slowed to a crawl and people feel like they have to merge early to not be seen as "cutting in line."
Edit, also to add, if everyone merges early even at speed, eventually, the car density in the reduced lanes will reach a point where people naturally slow down and you have bumper to bumper traffic.
I suggest Traffic by Tom Vanderbilt for more.
"Unused road" is ridiculous except in extremes. Unless people merge well over a mile back, 1 lane of traffic will make no difference. The only way "unused road" matters is for the people that haven't entered the traffic jam yet who are getting off before they reach it.
Very few people (from what I've seen) merge more than 30 car lengths out. 30 cars is not going to make a difference.
What does make a difference is the fact that we can't do a merge at speed because some people want to "zipper late." It's the zipper behavior that matters, the "at the very end" part never should've been added to that recommendation.
Looking at an actual research paper about this, the zipper merge demonstrated is not at the last possible point. A merge point forms ahead of that point and that's what should be used. The pictures from their study show the zipper occurring over a wide area with many of the zipped cars driving in the middle.
https://rosap.ntl.bts.gov/view/dot/35694
I don't know how studies like this have become the recommendations we have. They seem to me to miss critical bits.
Edit: based on my quick read, it's worth noting the study finds only minimal support for the zipper merge and only in contexts not involving trucks largely based on visual analysis from their video feed as the quantitative data was not statistically significant. We need better transparency on recommendations like this frankly and the research supporting them. We should be able to have an honest debate on the merits of the papers.
Zipper merge isn't a thing where I live. It's advised in the provincial drivers' handbook to merge early. Some folks from other provinces are trying to change things but it's too ingrained, been this way for as long as I can remember.
Merging early when at speed makes sense, because you still have a lot of lane left before you have to merge - less pressure, more time, less likely to make a bad decision.
Merging late during slow traffic makes sense, as it allows you to align with gaps in the traffic and for the traffic to make space for you without having to actually stop.
The problem with zipper merges as this person describes them is a zipper merge is SUPPOSED TO get traffic back up to speed. However, when your take on the zipper merge is "up there where the wreck is at the last possible spot I can merge" there's no time for a human to safely merge at speed. So everything has to continue at a crawl.
So the people jumping out of their lane and "zipper merging" at the last second instead of 50 feet out or so end up making things worse for everyone.
The zipper does not and should not be at the point of the physical problem on the road. Just like you should not just drive to the end of the on ramp and at the last possible second merge into the lane on your left without paying attention.
That's what I said?
Merge early at speed, merge late during congestion
I usually try to practice this (or at least make room for others to merge in) but last week the zipper merge took place right where an entrance ramp was also trying to merge creating a 3-way merge clusterfuck. Whoever put up the traffic cones should've been shot. Or... done a better job. All they had to do was complete the first merge before the entrance ramp, but no.
I was lucky enough to have already been in the left-most lane, at least.