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

TLDR

Traffic jams arise spontaneously when vehicle density exceeds a critical value, a phenomenon that can be understood as a non‑equilibrium instability of interacting particles, with bottlenecks merely acting as triggers rather than essential causes. The study aims to provide the first experimental evidence that traffic jam formation is a collective, dynamical phase transition rather than a bottleneck‑driven event. Using a circular road, the authors initialized vehicles at uniform speed and density near the critical threshold, creating a homogeneous flow that is susceptible to instability. The experiment showed that an infinitesimal fluctuation grows, breaking the homogeneous flow and producing a backward‑propagating jam cluster that behaves like a solitary wave, confirming jam formation without any bottleneck.

Abstract

A traffic jam on a highway is a very familiar phenomenon. From the physical viewpoint, the system of vehicular flow is a non-equilibrium system of interacting particles (vehicles). The collective effect of the many-particle system induces the instability of a free flow state caused by the enhancement of fluctuations, and the transition to a jamming state occurs spontaneously if the average vehicle density exceeds a certain critical value. Thus, a bottleneck is only a trigger and not the essential origin of a traffic jam. In this paper, we present the first experimental evidence that the emergence of a traffic jam is a collective phenomenon like 'dynamical' phase transitions and pattern formation in a non-equilibrium system. We have performed an experiment on a circuit to show the emergence of a jam with no bottleneck. In the initial condition, all the vehicles are moving, homogeneously distributed on the circular road, with the same velocity. The average density of the vehicles is prepared for the onset of the instability. Even a tiny fluctuation grows larger and then the homogeneous movement cannot be maintained. Finally, a jam cluster appears and propagates backward like a solitary wave with the same speed as that of a jam cluster on a highway.