## 1 Introduction  
With advancements in semiconductor and embedded software technologies, wireless sensor networks (WSNs) have been applied in diverse industrial fields. Well-known WSN applicationsinclude consumer electronics, home or factory automation, personal healthcare, asset management [1], intelligent agriculture [2], and industrial control and monitoring [3]. Another application of WSNs is in intelligent transportation systems (ITS) where WSNs provide useful information such as real-time traffic information and statistics for both drivers and the transportation bureau [4, 5, 6, 7, 8, 9, 10, 11]. This paper proposes an application of a WSN evaluating deployed Vehicle Detection Systems (VDSs) for smart and safe transportation systems. VDSs are one of the basic building blocks of the upcoming smart transportation environment, and maintaining the performance of VDSs is crucial to providing a required level of service. The conventional way to maintain the performance of a VDS is to regularly compare the traffic information from the installed VDS with that gathered by a portable VDS (_i.e._, a reference instrument), and to adjust the installed VDS based on the evaluation results [12]. The existing portable VDS can be one of several types--a video-frame detector, tape-switch (or piezoelectric), laser detector [13], or loop detector--according to the technology utilized to detect a vehicle [14, 15]. The video-frame detector requires an expert to analyze the recorded images, which can be up to 30 frames per second. Video-frame detectors are one of the most credible VDS methods, but without a skilled expert, the method is error-prone [13]. The other types of VDS are wired, and it takes a long time to install them in the pavement. Vehicles can also easily damage them. In addition, the first three types of VDS cannot be used easily to detect multiple lanes of traffic at the same time. This scalability is becoming one of the important requirements for VDS evaluation systems with the increase of multi-lane roads [16]. Recently, wireless VDSs have begun to be used in the field, but they cannot monitor traffic in real-time. Therefore, to attack and eliminate the aforementioned defects of existing VDS evaluation systems, this paper proposes a novel architecture for a Portable Vehicle Detector Evaluation System (PES) based on a WSN considering the requirements of a new portable evaluation system (Table 1). Our PES consists of T-Sensor nodes, a T-Sink node, a T-BS-com node, a WSN, and the T-Mon host. T-Sensor nodes, enclosed in slim and hard cases, are taped to the pavement. Each T-Sensor node equipped with a magneto-resistive sensor [17, 18, 19] detects the disturbance of the Earth magnetic field caused by a vehicle and identifies the approach or the departure of the vehicle with a detection algorithm. It sends the event packet to T-Sink node. The T-Sink node relays the packets to the T-BS-com node, and the T-BS-com node bridges the WSN and the T-Mon host, which is the main controller of the PES. We describe the architecture and design of PES in detail. Through extensive experimental evaluation, we demonstrate the PES to be a feasible reference instrument, since it alleviates the problems of the existing evaluation systems and can measure traffic information with over 98% accuracy. In addition, PES shows better performance than a commercial wireless sensor network-based VDS. We may summarize the contributions of this paper as follows: (1) it proposes a novel application of wireless sensor networks, (2) it presents the architecture of an evaluation system for VDS, and (3) it demonstrates the feasibility of a wireless sensor network-based reference instrument through extensive evaluation of a real-world PES implementation. The rest of the paper is organized as follows: Section 2 surveys and compares the conventional methods or instruments used to evaluate a VDS. The system architecture and design of PES are presented in Section 3. The results of implementing and experimenting with our PES are analyzed in Section 4.