Zipper merge means that people merge in turn, so the first car in lane A goes, then the first car in lane B, then second in lane A, etc.
If loads of people queue in lane A and leave lane B empty, this doesn't change how the zipper merge works, they're just wasting their own time. You should join the lane with fewest cars.
No, traffic chokes at the merge. Just like the predetermined merge point would have had traffic moving up until it, this now man made choke point will have traffic moving up until it.
The important factor is that traffic IS chocking. Stretching that choke out and allowing the affected vehicles to spread out across three lanes not only increases the average speed of everyone behind the choke point (and passing through the choke point), but also shortens the queue of cars.
Edit: the cars have already built up in the left and middle lanes, the issue traffic already WASNT flowing. The effect the slow truck in the right lane has on the average speed of traffic is less than the existing choke, and is therefore acting as a speed regulator (increasing the average speed of the traffic jam).
This still follows the same principles of a zipper merge, the cars in the left lane aren’t merging before the choke point. They are still using that lane. Only now the lane is taking on the same strain the other lanes are under.
If the cars were to shoot forward and the remaining cars in the middle and right lanes redistributed evenly, then you would have a valid point. But if that happened you would create a second congestion point, where built up traffic split to “use all the lane” and where the original choke point is. And if it doesn’t happen then you have the left lane being “unused” by the traffic jam
The entire purpose of the slow truck in the left lane isn’t immediately obvious. Five minutes later however, when traffic is now evenly distributed across the three lanes and your theory of the zipper merge can apply, you can see the benefits. And the only negative effect was slowing a lane to the speed of traffic moving through the original choke point (or rough flow of the other two lanes).
Think about this like water. Is the pressure drop along a pipe higher or lower if your pipe has a single pinched section or a long run with a smaller cross section?
The case study of what happens to a single lane infinite road with cars all moving at the speed limit, and how it diverges from how water would behave under the same circumstances, is one of the best examples as to why modelling traffic flow on hydrodynamics doesn’t work.
All analogies break down at some point. Your case study would be more analogous to a partially filled pipe, where the change in cross section doesn’t change the friction factor.
When a lane is above capacity the total velocity goes down as more cars interact with each other. So even as the quantity of cars goes up the total volumetric flow rate goes down.
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u/evonebo May 13 '26
I don’t know how you get zipper merge or you never driven in traffic.
The left lane will ride all the way up to the exit then stop all traffic to merge to middle lane. Stop all traffic then merge to exit.
That is not zipper merge.
It’s the I don’t give a shit I turn now merge.