This paper shows the way to turn a defect inherent to single-mode fiber, namely birefringence, into a prime quality for a powerful and reliable sensor. The latter is entirely devoted to Weigh-In-Motion (WIM) applications extended to complete active traffic management systems. After a brief description of the sensor and its principle of operation, the theoretical model is developed. Then, a full characterization made in both static and dynamic conditions is presented. The results obtained illustrate how it is difficult to interpret a weight measured in dynamic conditions and correlate the value with the static weight.