Why does a traffic jam move backward?

What will you explore?

Sugiyama and colleagues, 2008

A moving pattern is not the objects moving through it.

How does it work?

Watch a 90-second cinematic explanation, or explore a story that pauses so you can change its central mechanism. Inspect the source evidence, compare outcomes and resume the narration. This free Lesson Worlds adaptation needs no account or download; younger learners may want help from a parent or teacher.

Inside the cinematic story

1. A jam with no obstacle

   Cars forward · disturbance backward

2. The circular-road experiment

   22 vehicles · 230 m · reported experiment

3. Spacing changes the pattern

   22 versus 14 · original simulation

   Take control: How does this model respond to more room? 22 vehicles · 14 vehicles

4. Follow the car—or the wave

   Gold ring: selected car or slowest region

   Take control: Which object should the gold ring identify? One car · Slowest region

5. What was actually measured?

   Reported wave ≈20 km/h · not our model speed

6. The traffic is the system

   Explore the mechanism · inspect the source

Discuss the evidence

Before starting, explain what you think this claim means: A moving pattern is not the objects moving through it. After exploring, choose one stage above and describe what its model helps you see. Separate what you observed in the adaptation from what the original researchers measured.

Read the original publication for its methods, evidence and limitations. Ask which part of the explanation the interactive model leaves out, and what additional evidence you would need to test the claim yourself.

Original optimal-velocity teaching simulation, not experimental footage or fitted vehicle trajectories. Ring length 230 m; 22 or 14 cars of length 4 m; sensitivity 2.5/s; Euler step .02 s; one initial speed reduced 3%. Desired speed is a shifted tanh of headway, capped asymptotically at 16.66 m/s. A collision guard exists; validate its activation count before interpreting a run. Film compresses model time. The 14-car comparison and tracking controls are our illustration, not a reported experimental condition. This model does not reproduce the measured backward wave speed. The paper's short, dense circuit is not every highway or a universal threshold; no driving advice is implied.

Original research

Traffic jams without bottlenecks—experimental evidence for the physical mechanism of the formation of a jam

Yuki Sugiyama, Minoru Fukui, Macoto Kikuchi, Katsuya Hasebe, Akihiro Nakayama, Katsuhiro Nishinari, Shin-ichi Tadaki and Satoshi Yukawa · New Journal of Physics 10, 033001 · 2008

Educational adaptation by Lesson Worlds. Not affiliated with the authors or IOP Publishing. The original publication may have its own access terms.