How is automobile traffic like a gas? No, it's not because the collisions are inelastic. Researchers in chaos theory, especially Dirk Helbing and Boris Kerner, both theoretical physicists, have been working on traffic flow, using models similar to those of particle dynamics. Stephen Bodiansky has written a popular article about their results (that you can read online- for awhile, anyway- by clicking on the title above) that predicts that traffic jams can occur without a precipitating event, such as an accident or a mole crossing the road. Disruptions can occur well below the density that represents the traffic capacity of the highway. Interested readers may want to consult some of the original literature, such as Nature 1998 396(6713) 738 or Physical Review E 1995 51(6B) 6243.