## 4 Implementation and Evaluation
### Traffic Information  
To evaluate the performance of PES, we used the ITS performance evaluation center of the Korea Institute of Construction Technology (KICT), which is located in Young-in, Gyeong-gi, Korea. The evaluation center is equipped with video recorders, a laser-based traffic monitor, loop detectors, and other equipment, and it can monitor traffic in two lanes each direction at the same time. Two sets of PES were deployed, evaluated in the center several times (Figure 12), and improved based on the results. In this sub-section, the final evaluation done on 18 April 2010 is analyzed in detail. Figure 12: PES under evaluation in the KICT ITS performance evaluation center. The following table (Table 5) shows evaluation durations and traffic volumes used to evaluate PES. For 40-70 min, 940-1,455 vehicles passed through the four lanes. As recommended by national regulations and considering PES as a reference (or evaluation) instrument [20], every one-minute period traffic information measured by PES is compared to that of the reference instrument (laser detector) in the evaluation center in Table 5. The average speed was found to be biased at about 1 km/h, so we compensated the amount of the biased speed for every minute of speed. Each piece of traffic information such as traffic volume, speed, and occupancy was measured for the evaluation duration by the reference instrument and PES, and was averaged over every minute. Therefore, average traffic information for every minute measured by PES was compared to that of the laser detector. Table 6 shows the Mean Absolute Percentage Error (MAPE) for every minute of traffic information for each lane, and the average MAPE for all of the lanes. Traffic volume accuracy, speed accuracy, and occupancy accuracy of PES were measured to be 1.03%, 1.61% and 6.01% respectively. Since PES was designed to be a portable system, we evaluated it in other provinces at different times, and it still showed similar performance. As a reference, the performance of PES was compared indirectly with that of a commercial wireless vehicle detection system of Sensys Networks Inc. [39], which is a commercial WSN-based VDS. The accuracy assessment of a Sensys Wireless VDS is cited in [39] and some of the data is reproduced for \begin{table}
\begin{tabular}{|c|c|c|c|c|} \hline \multirow{2}{*}{**Lanes**} & \multicolumn{3}{c|}{**Mean Absolute Percentage Error for Every Minute of Traffic Information**} \\ \cline{2-5}  & **Traffic volume** & **Average Speed** & **Compensated Average Speed** & **Occupancy** \\ \hline N1 & 1.3 & 2.28 & 1.71 & 6.11 \\ N2 & 1.01 & 1.76 & 1.65 & 6.05 \\ S1 & 0.86 & 1.02 & 1.13 & 5.28 \\ S2 & 0.95 & 3.04 & 1.96 & 6.60 \\ \hline
**Average** & **1.03** & **2.02** & **1.61** & **6.01** \\ \hline \end{tabular}
\end{table} 
Table 6: Evaluation results. Figure 12: _Cont._ \begin{table}
\begin{tabular}{c c c} \hline \hline
**Lanes** & **Evaluation Time** & **Traffic Volume** \\ \hline Northbound lane \#1 (N1) & 13:18–14:59 & 1,273 \\ Northbound lane \#2 (N2) & 13:17–15:10 & 1,392 \\ Southbound lane \#1 (S1) & 13:07–15:16 & 1,455 \\ Southbound lane \#2 (S2) & 13:07–15:16 & 940 \\ \hline \end{tabular}
\end{table} 
Table 5: Evaluation durations and traffic volume. comparison with PES. Since the metric used to evaluate the Sensys VDS is based on average volume accuracy and average speed accuracy over different 15-minute periods instead of 1 min periods, the measured data for PES is rearranged to follow the same metric. The average volume accuracy is compared first in Table 7. The MAPE for the volume accuracy of PES is between 0.7 % and 1.0 % for all four lanes, while that of Sensys VDS is 1.7%. In addition, the variation of the absolute error for PES is also smaller than that for the other. The speed accuracy of both systems is compared in Table 8. The MAPE for the speed accuracy of PES is between 0.3% and 1.3% for all four lanes, while that of Sensys VDS is 1.4%. The variation of the absolute speed error of PES is overall smaller than that of Sensys VDS.