## 5 Use Cases
### Accessibility Surveillance System at Stops
Study of the Performance of the Accessibility Surveillance System at Stops  
Prototype tests were performed to analyze four critical features of the system. The first feature is a low resolution image that provides sufficient image quality for the objective. The second feature is the method of image compression, which helps save storage space and bandwidth consumption in transmissions while maintaining sufficient image quality. The third feature is the amount of space required to store the OBC images that are acquired along a route. The fourth is the performance of the communications system. We present the primary results for each of these tests:  
* _Resolution_. The resolution must provide sufficient image quality to obtain the required information. In our case, the plate number of the vehicle must be visible. The camera offers several resolutions, which range from \(176\times 120\) to \(720\times 480\) pixels. A resolution of \(640\times 480\) is a suitable resolution for the tests. * _Method of compression_. Compression reduces the space that is required to store an image sequence and reduce the bandwidth that is required to transmit an image file over the wireless network. Three compression methods were tested: H.260, MPEG-4, and Motion JPEG. We opted for the latter because it is the latest method and offers significant savings in space, namely, compression was performed with 25 frames/s and a resolution of \(640\times 480\) pixels. With these parameters, the compression of a sequence of images with a length of 1 s requires approximately 34.13 Kbytes of storage space. * T0) \(\times\) 34.13 Kbytes. Table 5 lists the results for the selected route stops. This route has 28 stops in both urban and rural areas. The table lists the identifier of each stop, which coincides with its position in the route order (column 1), the maximum travel time for traversing 200 m around the stop (column 2), and the estimated space required for storing the sequence of images
acquired during this maximum time period (column 3). Estimation of the space required to store the image sequence of each stop in the line is 108.75 Mbytes. * _Performance of the communications system_. In order to assess the performance of the communications system we have only analyzed the time required to transmit the images. We have not analyzed how the mobility of the vehicles affects the transmission error rate because these transmissions are made when the vehicle is stationary. Image transmission is performed when vehicles are in places with Wi-Fi coverage, namely stations and garages. In general, these places are open-plan, _i.e._, they contain no obstacles that hinder communications. Considering the available bandwidth of the Wi-Fi infrastructure (802.11 g), with speeds in outdoor open environments ranging from 54 Mbit/s in a radius of 75 m to 6 Mbit/s in a radius of 400 m, the estimated transmission times for image files of various sizes are shown in Table 5. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Stop**} & \multirow{2}{*}{**T. Max (s)**} & **Estimated Maximum Required** & **54 Mbits/seg** & **24 Mbits/seg** & **6 Mbits/seg** \\  & & **Storage Space (KBytes)** & **75 m (s)** & **140 m (s)** & **400 m (s)** \\ \hline  
\hline \multicolumn{2}{c}{TOTAL} & 109,557.3 & 15.9 & 35.7 & 142.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Results for the study route.