We define a model for a system of automatic vehicles (cybercars) circulating on arbitrary planar closed road-networks. The demands for transportation arrive according to a Poisson flow. The diffusion of cybercars along the network is influenced by Coulomb forces, determined by the distribution of clients in the system. This model is implemented numerically on a had-hoc graphical interface, using Monte-Carlo methods. In this way the the Coulomb forces algorithm, used to optimize the service, seems to be quite efficient. Preliminary numerical results are presented for this model, together with a phase diagram.