## 4 Results  
Figure 5 depicts how the BBUNPROWS application performed on a smartphone-mounted vehicle when a worker wearing a PWS-beacon-mounted helmet was located in the study site along with a preceding vehicle. Until the worker entered the caution (20 m) or warning (10 m) zones near the vehicle, only the navigation image with vehicle position inside the tunnel was displayed on the smartphone screen, as depicted in Figure 5a. As the vehicle moved further and the worker entered the caution zone, a primary/caution alert was issued to the driver, as depicted in Figure 5b. Likewise, as the worker approached the warning zone, a secondary/warning alert was issued, as depicted in Figure 5c. It was also confirmed that when two vehicles approached each other, both primary/caution and secondary/warning alerts were issued to the driver in steps, based on the degree of proximity. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**Specifications**} \\ \hline \multirow{4}{*}{**Appearance**} & Dimensions (diameter \(\times\) height) & 45 mm \(\times\) 20 mm \\  & Weight (with battery) & 11.6 g \\  & Casing & Acrylonitrile butadiene styrene (ABS) plastic \\  & Chipset & Nordic nRF51822 (Nordic Semiconductor; Oslo, Norway) \\ Hardware & Processor & 32-bit ARM Cortex-M0 (ARM Holdings; Cambridge, UK) \\  & Antenna & Omnidirectional built-in antenna \\  & Wireless technology & Bluetooth® Smart 4.0 \\ Wireless Signal & Signal transmission period & Minimum 10 ms, maximum 2 s \\  & Transmission power & Minimum \(-16\) dBm, maximum 4 dBm \\  & Korea & Korea Certification (KC) \\
**Certifications** & USA & Federal Communications Commission (FCC) \\  & Europe & Conformile Européenne (CE) marking \\  & Japan & Ministry of Internal Affairs and Communications (MIC) of Japan. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specifications of RECO beacons (Perples Inc.; Seoul, Korea) installed on safety helmets, safety equipment, and vehicles. \begin{table}
\begin{tabular}{c c} \hline \hline
**Model** & **Galaxy S9 (SM-G960)** \\ \hline \multirow{2}{*}{**Operating System**} & Android 8.0 (Google; Menlo Park, CA, USA) \\  & Exynos 9810 Octa (Samsung Electronics; Suwon, Korea) \\ Wireless & Bluetooth 5.0 \\ Technology & 4 GB \\
**Battery** & 3000 mAh \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Specifications of Samsung Galaxy S9 (SM-G960) smartphone used for analyzing received signal strength indicator (RSSI) distributions. \begin{table}
\begin{tabular}{c c} \hline \hline
**Model** & **Galaxy S9 (SM-G960)** \\ \hline \multirow{2}{*}{**D Operating System**} & Android 8.0 (Google; Menlo Park, CA, USA) \\  & Exynos 9810 Octa (Samsung Electronics; Suwon, Korea) \\ Wireless & Bluetooth 5.0 \\ Technology & 4 GB \\
**Battery** & 3000 mAh \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Results of the statistical analysis of RSSI distributions. To analyze the accuracy of alerts issued to the driver by the BBUNPROWS application, the above experiment was repeated 50 times for the case in which a vehicle approached a pedestrian worker. Lengths of the caution and warning buffer zones were set in the range of 20-30 m and 10-20 m, respectively, ahead of the vehicle along its direction of motion (Figure 6). Alerts issued by the BBUNPROWS application when the vehicle approached the worker at a speed of 10-20 km/h in the tunnel were classified and analyzed as follows. * True positives: Number of times that the alert was issued correctly when it ought to have been issued. * True negatives: Number of times that the alert was not issued when it should have been issued. * False positives: Number of times that the alert was issued when it should not have been issued. * False negatives: Number of times that the alert was not issued when it should not have been issued. * Accuracy: Ratio of the sum of true positives and true negatives to the sum of all events (true positives, true negatives, false positives, and false negatives).