THE PHYSICS OF TRAFFIC JAMS. German scientists have shown that the changeover from free-flowing traffic to a traffic jam (in which cars on a highway are greatly slowed or halted, at least temporarily) conforms to the well-known physics of phase transitions, an example of which is the transition from water to ice. In other words, traffic jams are not random patterns, but are deterministic in nature; that is, when a parameter exceeds a threshold value---such as the flux of cars---then local perturbations can grow, possibly leading to jams, analogous to the nucleating effect of tiny ice grains in a body of water being frozen. Once formed the jam moves along the highway as if it were a kind of "solid," with identifiable edges and with a "vapor" of comparatively free cars in front of and behind it. The information gained in this sort of research, the researchers believe, might lead to more accurate traffic forecasts and could be used in future "intelligent" transport systems. (B.S. Kerner and H. Rehborn, Physical Review Letters, 3 Nov. 1997; contact Boris Kerner, Daimler-Benz AG, kerner@dbag.stg.daimlerbenz.com.)