We study the phases of the Nagel-Schreckenberg traffic model with open boundary conditions as a function of the randomization probability p > 0 and the maximum velocity ${ v}_{ max} > 1$. Due to the existence of "buffer sites" which enhance the free flow region, the behaviour is much richer than that of the related asymmetric exclusion process (ASEP, { v}_{ max} = 1$). Such sites exist for ${ v}_{ max} \ge 3$ and p < ${ p}_{ c}$ where the phase diagram is qualitatively similar to the p = 0 case: there is a free flow and a jamming phase separated by a line of first-order phase transitions. For p > ${ p}_{ c}$ an additional maximum current phase occurs like for the ASEP. The density profile decays in the maximum current phase algebraically with an exponent $\gamma \approx 2/3$ for all ${ v}_{ max} \ge 2$ indicating that these models belong to another universality class than the ASEP where $\gamma = 1/2$.
Categories: cond-mat.stat-mech