Accurate traffic congestion detection methods are needed urgently to improve road traffic pressure and congestion, especially in metropolitan areas. This paper proposes TraD-VANET, a new road traffic detection system that employs RFID-based active vehicle positioning and vehicular adhoc networks (VANETs) to detect traffic congestion dynamically. The results can be used to making road traffic flow more fluent and effective. To validate the performance of TraD-VANET, we ran a simulation compared with cooperative traffic congestion detection (CoTEC). The simulation results shown that TraD-VANET has a good performance.