If the individual entities in a system are used as the main components of a traffic simulation, the simulation is called microscopic. Although the micro-simulations have simple rules such as car following, lane changing, gap acceptance etc., these rules can produce complex behaviors if the traffic density is high on a wide area. Such a large scale transportation simulation can consume more time and more computing resources.
Large scale simulations can be run on a cluster of PCs to speed up the computation. Using a cluster of PCs and partitioning the whole task among the computers in this cluster is economical in that such a cluster is affordable by most university engineering departments and by middle size companies. By ``a cluster of PCs'', we mean that a group of 10-20 PCs connected by a standard LAN technology runs Beowulf Linux. The other solution might be buying a supercomputer such as IBM SP2 or Intel iPSC/860 in order to achieve the parallelism but this solution is not cost-effective.