THE PHANTOM JAM / LIVE MODEL
No obstacle.
Still a traffic jam.
Drag a car. Pass the slowdown on.
TEACHING MODEL · 5× MODEL TIME
Teaching model · how it works
22 identical cars on a 230 m ring. At model second 5, one car slows by 25%. Each car continuously adjusts to the gap and speed of the car ahead; there is no explicit reaction-delay memory. Dragging applies braking, not a change in position. Low acceleration makes stop-and-go waves easier to see.
Intelligent Driver Model: desired speed 11.11 m/s, desired headway 0.8 s, minimum gap 2 m, acceleration 0.3 m/s², comfortable braking 3 m/s², car length 4.5 m. Fixed 0.025 s numerical steps, with a non-overlap guard. These are teaching parameters, not a calibration to the experiment.
Inspired by Sugiyama et al. (2008). Model equations and integration method ↗ · Reading note
The puzzle
There is no crash, closed lane or intersection. Yet the cars slow, bunch up, and produce a stop-and-go wave. Where does the jam come from if the road itself contains no obstacle?
The experiment
In a 2008 study, Sugiyama and colleagues placed 22 cars on a circular course approximately 230 metres long. Drivers were asked to maintain a target speed. Small fluctuations grew, and a congested region emerged. The jam moved backward through the cars even though each car traveled forward.
The pattern is in the interaction
A driver responds to the spacing and speed of the car ahead. That response becomes part of the situation facing the next driver. Under the experimental conditions, these local adjustments did not cancel out; they amplified into a collective pattern. The unit of explanation is therefore more than any single driver.
What the evidence does—and does not—show
The experiment demonstrates that a jam can form without a physical bottleneck. It does not show that all jams arise this way or that road geometry, incidents and demand are irrelevant. A real diagnosis still needs measurements of the particular road and behavior.
A question to take elsewhere
When you see a collective failure, ask whether sensible local responses could produce it together. Then identify the actual response rule and look for evidence. The resemblance to a traffic jam is a prompt for investigation, not proof.