## 2 Related Work  
In this section, we summarize the existing VDS evaluation systems. Although there are stationary VDS evaluation systems, this paper focuses on portable ones, since most VDSs have already been installed at a stationary site, so we cannot move them to where the stationary evaluation system is installed. The existing portable VDS evaluation systems are based on several kinds of detection technologies. The most commonly-used evaluation system utilizes image frame analysis with a camera [13,19]. The video camera is installed at the same place where the VDS to be evaluated is installed. When a vehicle passes a known distance as shown in the video, the travel time is measured in frame units. The number of frames per second is used to measure the distance travelled by the vehicle (1 frame = 1/30 s). However, an expert measures the traverse time as an integer number of frames, and that expert must personally evaluate any fractions of frames. It is difficult for different experts (_i.e_., the video frame analyzers) to measure the fractions of the number of frames consistently and objectively, which adds variability to the results. In addition, image frame analysis tools sometimes experience mechanical failures. Another type of portable evaluation system is based on laser detectors [14]. This evaluation system sends laser light to a reflector, which is stuck to the pavement. The signal hits the reflector, which reflects it to the evaluation system receiver. With this transmitter and receiver set, the evaluation system can detect a passing vehicle. To measure the speed of a vehicle, the evaluation system needs two sets of transmitters and receivers separated by a known distance. A laser evaluation system can detect the traverse time of a vehicle very accurately without any room for subjectivity. However, since the evaluation system must be deployed on the side of the road and the reflector attached to the lane with the VDS to be evaluated, it is easy for any passing vehicle to interfere with the laser signal. In other words, only the traffic of the outer lane can be measured with this system and the measurement of multiple lanes (_i.e_., scalability) is very difficult. The third type of evaluation system is designed with a tape-switch or piezoelectric cable [13]. Passing vehicles press down onto the piezoelectric cable, which creates an electrical impulse. If two piezoelectric cables are separated by a known distance, traffic information including volume count, \begin{table}
\begin{tabular}{c l} \hline
**Requirements** & **Descriptions** \\ \hline Cost & Cheaper than the existing instruments \\ \hline Set-up time & Easy and fast to set up the system in the field to minimize traffic interference \\ \hline Accuracy & To evaluate the pre-installed VDSs, the accuracy should be very high \\ \hline \multirow{2}{*}{Objectivity} & By minimizing human interventions during the evaluation procedure, evaluation \\  & objectivity should be assured \\ \hline Scalability & The system should evaluate multiple lanes at the same time \\ \hline \multirow{2}{*}{Traceability} & To assist the video analysis of the information gathered for each vehicle, the recorded video should be traceable by a time stamp. \\ \hline Real-time & During the evaluation procedure, instantaneous evaluation results such as volume, \\ monitoring & occupancy, and speed can be shown in real-time \\ \hline \multirow{3}{*}{Fault tolerance} & Despite partial faults of the system, the evaluation in progress should be completed. Even after wireless interferences, the traffic information can be recovered by post-processing. \\ \cline{1-1} \cline{2-3}  & \\ \end{tabular}
\end{table} 
Table 1: Requirements for a new portable evaluation system. speed, and occupancy can be measured. Although this type of evaluation system can overcome the drawbacks of video frame detection and can measure traffic information very accurately, the piezoelectric cable is relatively fragile and can be easily broken.