The Phantom Jam Inquest: A Forensic Attribution of Singular Liability in a Provably Authorless Traffic Wave

The traffic stops for no reason. The road is open — no crash, no merge, no police — and yet the cars halt, clear a minute later, and no one is ever held to account. Physics has a name for this event, the phantom jam or jamiton: a stop-and-go wave that forms on its own once traffic grows dense enough, then rolls backward through the cars for as long as it likes. For forty years the field has filed it under emergent, authorless, no one to blame. We regard that as an open case, not a closed one. Treating each jam as a body owed an inquest, we re-analyse 400 spontaneous jams recorded on an instrumented ring road and, for each, establish a time of death, a manner of death, and a proximate cause. Three findings are reported. First, the jam is clinically dead a mean of $8.4\mathrm{s}$ before any driver perceives a slowdown: death precedes symptom. Second, causal responsibility — each car’s fair share of the total delay — is not diffuse but extreme (fleet Gini $=0.93$): a single vehicle, the index driver, carries a median $71\%$ of it. Third, the modal manner of death is not natural causes but homicide ($63\%$ of jams): an unwarranted leap up the Ladder of Inference, from one ambiguous brake light straight to a full-force stop, with no hazard present. The phantom jam has an author. We name them.