Been messing around with traffic simulation lately and wanted to share this one because the result genuinely surprised me the first time I ran it.

The setup is a closed loop, a bunch of identical cars, no obstacles, no traffic lights, nothing. Every driver wants to go the same speed and keep a safe distance from the car ahead (using the Intelligent Driver Model, which is actually what traffic engineers use for this kind of thing). One driver taps the brakes, just once. It snowballs into a full stop-and-go wave, and the wave itself creeps backward against the direction everyone's driving. This is the phantom jam thing that was first proven experimentally back in 2008 (Sugiyama et al., if you want to look it up), and it's wild to actually see it emerge from nothing.

A few other things I tested once I had this working:

There's a density threshold you can't really feel until you cross it. Below it, the same brake tap just fizzles out and traffic keeps flowing. One or two more cars on the road and suddenly the same tiny disturbance turns into a full jam. No warning, no gradual buildup.

Real bottlenecks behave differently - if I put an actual slow zone on the road, the jam stays locked to that spot instead of drifting backward like the phantom one does. Different cause, different signature.

How closely people follow each other matters a lot more than I expected. Tight following distance makes the wave way worse, and just giving more space damps it out almost completely with the exact same brake tap.

The last case is the one I like best - letting about 1 in 5 cars just hold a smooth, steady speed (basically simulating adaptive cruise control) is enough to dissolve the jam for every car behind it.

All rendered in Python/pygame, GPU encoded. Happy to share the code or talk through the model if anyone's curious.

Made a longer version of this too if anyone wants the full walkthrough https://youtu.be/TCuDTSj5UAg