  There is discussion if traffic displays multiple phases (e.g. laminar,
jammed, synchronized) or not. This paper presents evidence that a stochastic
car following model, by changing one of its parameters, can be moved from
showing two phases (laminar and jammed) to showing only one phase. Models with
two phases show three states: two being homogeneous states corresponding to
each phase, and a third state which consists of a mix between 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. Having a stochastic model is however important to
understand the potentially probabilistic nature of the transition. Most
importantly, if indeed 2-phase models describe certain aspects correctly, then
this leads to predictions for breakdown probabilities. Alternatively, if
1-phase models describe these aspects better, then there is no
breakdown.Interestingly, such 1-phase models can still allow for jam formation
on small scales, which may give the impression of having a 2-phase dynamics.
