Actual paper is here: https://www.science.org/doi/10.1126/science.add8091. Very cool result. This is the gist:

> We introduce a theoretical approach that uses temporal coarse-graining akin to Einstein’s kinetic theory of Brownian motion (34). Our averaging scheme is valid in the case of nonjamming mixtures of hard particles, where the dynamics is dominated by pairwise interactions, which is a good approximation for typical pedestrian flows as well as dilute colloids. We recover and unify in a systematic manner the fundamental insights of Helbing and Vicsek (22) as well as Vissers et al. (11) and Klymko et al. (23) by showing that undulation-induced drift and diffusion can both contribute to lane nucleation. We also demonstrate that diffusive processes suppress the formation of very narrow lanes, thereby providing a dynamical selection mechanism that favors the nucleation of lanes of a particular width. We provide explicit formulas for the propensity of a given system to nucleate lanes, and we present a simple approximate rule that lanes emerge at a rate proportional to the product of agent speed, density, and an effective parameter related to the average magnitude of lateral displacement in agent-agent collisions.

So, there are 2 main mechanisms proposed in the literature on lane formation: "drift" and "diffusion". Drift arises from the tendency of people to have a preferred direction to turn to avoid a collision (i.e. right- or left-bias). When facing an opposing flow, repeated conflicts will cause you to slowly drift sideways in your preferred direction. Diffusion is the random Brownian Motion-like jostling that arises from conflicts with opposing pedestrians, which has no directional bias.

In both cases, you're getting jostled around more when there's an opposing flow in front of you than when you're in a lane of people going in the same direction, so there will be a tendency for lanes to form spontaneously ("nucleate"), even from a completely homogeneous initial configuration.

The point of the paper is that they derive, with minimal assumptions, a quantitative relationship that incorporates both mechanisms, and makes specific non-obvious predictions about the shapes and widths of lanes. It also predicts that the rate of lane formation has a simple form (density * speed * average jostle displacement from conflicts with other pedestrians). They do some real-world experiments to validate some of their predictions.

As someone who works on pedestrian simulation software, I don't see how to use this to write the simulation code, but it could be an interesting way to validate the software (i.e. verify that lane formation obeys the predicted relationship).

Something that bothers me about a lot of these papers though is they often perform simulations with periodic boundary conditions. I get why they do that, but it seems to me that that will cause spatial correlations to show up that wouldn't happen in real life. Would the rate of lane formation be different in a system with different boundary conditions?

Another limitation mentioned in the paper that makes this model difficult to apply to crowds of pedestrians is that they assume the interaction between 2 pedestrians depends only on their relative displacement. However, it's known that they key parameter in pedestrian interactions is actually the Time To Collision--i.e., pedestrian collision avoidance is fundamentally anticipatory [1]. Presumably this would complicate the model too much though (now you have to take into account displacement and relative velocity).

[1] http://motion.cs.umn.edu/PowerLaw/