The road transportation system is complex, with different kinds of users interacting among themselves and with the infrastructure, resulting in a collective behavior which depends on individual motivations and actions. LEPSiS develops behavioral models and simulations, in particular with respect to perception, for ex-ante assessment of modifications in the traffic system. We aim at sustainably improving traffic safety and efficiency by implementing a holistic approach. It involves research in Artificial Intelligence, Complex Systems, Data Analysis and Mathematics.