  In this paper, we propose a stochastic cellular automaton model of traffic
flow extending two exactly solvable stochastic models, i.e., the asymmetric
simple exclusion process and the zero range process. Moreover it is regarded as
a stochastic extension of the optimal velocity model. In the fundamental
diagram (flux-density diagram), our model exhibits several regions of density
where more than one stable state coexists at the same density in spite of the
stochastic nature of its dynamical rule. Moreover, we observe that two
long-lived metastable states appear for a transitional period, and that the
dynamical phase transition from a metastable state to another metastable/stable
state occurs sharply and spontaneously.
