The amount of GPS car data are steadily growing toward a relevant sample of the italian car population. From these data we are trying to reconstruct in real time the relevant dynamical fields. Our aim is to provide a now casting method, to evaluate its capabilities and also to suggest to the GPS manufacturers the quality and optimal frequency of the data needed. Discussion of the actual limits of the data will be provided. Some empirical laws emerging from the statistical analysis of aggregated data will be discussed. The laws open new challenges for the development, study and validation of traffic models.