Effects of on- and off-ramps in cellular automata models for traffic flow

We present results on the modeling of on- and off-ramps in cellular automat a for traffic flow, especially the Nagel-Schreckenberg model. We study two different types of on-ramps that cause qualitatively the same effects. In a certain density regime rho(low) < rho < rho(high) one observes plateau for mation in the fundamental diagram. The plateau value depends on the input-r ate of cars at the on-ramp. The on-ramp acts as a local perturbation that s eparates the system into two regimes: A regime of free flow and another one where only jammed states exist. This phase separation is the reason for th e plateau formation and implies a behavior analogous to that of stationary defects. This analogy allows to perform very fast simulations of complex tr affic networks with a large number of on- and off-ramps because one can par ametrise on-ramps in an exceedingly easy way.