# Passive RFID-Based Inventory of Traffic Signs on Roads and Urban Environments
## 4 System Validation
### Experimental In-Motion Validation  
In this real scenario, three passive tags are attached over a real traffic sign and its metallic pole at three different heights (1.28, 2.13, and 2.35 m, as shown in Figure 13 (left)). The reader is installed in the car, with the antenna at about 1.5 m above the floor and pointed to the traffic signs from the open window of the vehicle. In the configuration illustrated in Figure 13 (right), the distance \(d\) between the traffic sign and the antenna varies from 2 to 4 m in the experiments. Some tests were performed at different speeds, whose results are showed in Table 6 and Figure 14. It is possible to observe that the results are satisfactory at any usual speed of the car without interfering in the traffic flow, as predicted by the simulations results. Moreover, from Figure 14, we can observe a general tendency to decrease the number of detections when the speed and the distance \(d\) are increased, where the field \"area type\" is 1, when the tag is placed over the metallic traffic signal, or 2, when the tag is over the metallic pole.  
Table 6 also shows that tags were always detected when the vehicle speed was 50 Km/h, considering a reliable detection when the tag is read at least two times. They are detected for any vehicle speed as well when they are placed over the traffic sign (2.13 and 2.35 m), instead of being placed over the metallic pole (1.28 m), even for a distance \(d=4\), which implies the system would work properly even in the case of having to maintain a certain separation from the traffic sign, which may occur in a multi-lane road scenario. This better performance at 2.13 and 2.35 m can be mainly attributed to the fact that, at higher heights, multi-path effects have less influence and the performance is more similar to the case where the antenna and the tag are faced, as was observed in Section 4.2 for the simulation results for \(h=2\) m. Moreover, the performance is worse when the tags' heights are lower than the antenna's height, which was also observed in the simulation results illustrated in Figures 7-12 for \(h=1\) m, where a higher time is required to reach \(P_{coverage}=1\), with this being more critical drawback at higher speeds.  
Figure 12: Simulation results: \(v=50\) Km/h, \(d=2\) m, \(f=2450\) MHz.  
Besides the effect of placing the tags at higher heights, another reason to explain this better performance at 2.13 and 2.35 m is that these tags are placed over the traffic sign, whose area is wider than that of the metallic pole, thus increasing the original ground plane of the RFID tag and improving the performance of its antenna [45].  
Note that tags may be also identified attached to urban facilities with small back-placed metal plates, since the performance is satisfactory in the most similar cases in Table 6, i.e., when the speed is limited at its maximum for these urban environments (50 Km/h) and the tags are attached to the smallest metallic surface (height of 1.28 m). Also, the obtained results can be extended to the inventory of other urban facilities with several tags placed very close, since all of them are detected in any case at 50 Km/h, as predicted in Section 4.2, leading to a reliable system for the applications where nearby items have to be detected without collision problems.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Speed (Km/h)** & **Distance d (m)** & **Height (m)** & **Detection** & **Number of Detections** \\ \hline  
\hline \hline \end{tabular}
\end{table}
Table 6: In-motion results.  
Figure 13: Tags over the traffic sign (**left**) and configuration setup (**right**).