Whether traffic displays multiple phases (e.g., laminar, jammed, synchronized) has been much discussed. Computational evidence is presented that a stochastic car-following model can be moved from two phases (laminar and jammed) to one phase by changing one of its parameters. Models with two phases show three states. Two of them are homogeneous and correspond to the two phases. The third state consists of a mix of the two phases (phase coexistence). Although the gas-liquid analogy to traffic models has been widely discussed, no traffic-related model ever displayed a completely understood stochastic version of that transition. A stochastic model is important to the understanding of the potentially probabilistic nature of the transition. If indeed two-phase models describe certain aspects correctly, predictions for breakdown probabilities can be made. Alternatively, if one-phase models describe these aspects better, there is no breakdown. Interestingly, such one-phase models can still allow for jam formation on a small scale, which may give the impression of two-phase dynamics.
Abstract