This paper aims to present an intelligent system for tracking moving objects (such as vehicles, persons etc) based on a network of autonomous tracking units that capture and process images from one or more pre-calibrated visual sensors. The proposed system, which has been developed within the framework of TRAVIS (TRAffic VISual monitoring) project, is flexible, scalable and can be applied in a broad field of applications in the future. By the end of the project, two prototypes will be developed, each focusing on different traffic monitoring applications, such as the traffic control of aircraft parking areas at airports and tunnels at highways. First experimental results of the proposed system using two different data fusion techniques are also presented in the paper.