Recently, the explosive increase of vehicles on the roads has caused huge traffic congestion which leads various problems such as increase in air pollution and the frequency of car accidents. To address these problems, intelligent transportation systems (ITSs) - the convergence of information technology and transportation systems - has been studied. As an essential technology of the intelligent transportation system, traffic information acquisition technology plays an important role among the ITS technologies. Existing systems for traffic information acquisition have high cost and low scalability owing to their characteristics such as large size, wired power supplies, and wired communication. Recently, for the low cost and high scalability, wireless sensor networks (WSN)-based traffic information acquisition systems have been suggested. WSN-based traffic information acquisition system collects traffic data using low power sensors, and transmits it through wireless networks. However, WSN-based systems have many issues to consider, such as low computing power, limited battery capacity, and transmission delay. Existing studies on WSN-based acquisition systems have not considered all the issues together. Moreover, most studies have focused on theoretical problems rather than practical ones. Therefore, we propose a new system that considers all the issues. In our experiments, We installed our system on real roads for an accurate evaluation. The results show that our system has a high detection accuracy, low power consumption, and low transmission delay.