Wireless Sensor Networks for real time traffic surveillance system in an Intelligent Transportation Systems which is currently considered to be challenging application area for this emerging technology. The promise of an unmanaged infrastructure, with a continuous drop in cost per unit, attracts the attention of both final users and system integrator and thus, opening new business opportunities. This paper describes a Traffic Monitoring Wireless Sensor Network system, based on Wireless Optical Communication System. A practical case study is presented starting from a real problem and reaching the best architectural solution with particular focus on hardware implementation and communication protocol design. Finally, real experience results are shown to highlight the effect of climate over traffic surveillance using Wireless Sensor Network.