Microstructure and microdynamics of uninterrupted traffic flow

In this paper, ''uninterrupted homogeneous traffic flow'' is viewed as a dynamic system of spontaneously aggregating and breaking clusters of cars. This approach gives qualitative and quantitative descriptions of density fluctuations and dynamics of free traffic flow in terms of car clusters, and their distributions with respect to the number of cars in a cluster. The distributions have a nontrivial shape and a nonlinear dependence on the density of cars on the expressway. Their shape and evolution as a function of car density are described adequately by a model proposed here. This one-parameter model based on a modified Smoluchowski equation predicts a critical density at which a phase transition occurs from free to synchronized traffic how for a multilane expressway: rho(cr)=16+/-3 vehicles km(-1) (lane)(-1), and that below this critical density traffic flow can be treated as a one-dimensional system. The model may also find application in the study of the low density pneumatic transport of granular and powdered materials.