## 6 Conclusions  
The proposed study reports the development of a Bluetooth-beacon-based underground proximity warning system (BBUNPROWS) capable of receiving signals from Bluetooth beacons installed on the bodies of pedestrian workers, equipment, and at dangerous locations inside underground tunnels, which provide potential collision-risk alerts based on RSSI values of signals received. Field experiments were performed using the proposed BBUNPROWS inside an underground tunnel in Busan, Korea. The results of said experiments demonstrate that when a pedestrian worker or vehicle installed with a Bluetooth beacon enters a caution or warning zone, the extent of which typically starts from 20 m and 10 m upstream of the vehicle, respectively, the system successfully issues primary (caution) and secondary (warning) alerts to drivers. Furthermore, the said experiment was repeated 50 times, thereby demonstrating that the accuracy of issuance of the primary and secondary alerts corresponding to entry into the caution and warning zones equaled 93% and 95%, respectively.  
The proposed Bluetooth-beacon-based PWS possesses the following advantages over the existing PWSs employed in underground tunnels:  
1. Bluetooth beacons that transmit BLE signals do not require an external power source, because they receive power from a coin battery. Therefore, no additional infrastructure is required for supplying power to underground mines.
2. The system is cost competitive because Bluetooth beacons are inexpensive, and there is no need to set up a separate signal reception terminal, since smartphones are used as receptors.
3. Existing systems necessitate the installation of large and heavy antennas as part of production equipment and mining structures for installing wireless signal transmitters. However, Bluetooth beacons, being small and lightweight, can be easily installed anywhere inside underground mines. In addition, Bluetooth beacons are made of sturdy materials and are less vulnerable to humidity; this makes them especially suitable for use in underground mining sites.  
The proposed PWS system can be extended by adding new functions using the Bluetooth beacon software development kit (SDK) and Android application programming interface (API).
5. The BBUNPROWS application can be shared as open source. Therefore, it is expected that the proposed Bluetooth-beacon-based PWS can effectively be introduced at low cost for use in underground tunnels and safety management.  
Although the proposed BBUNPROWS application has been successfully tested for use in underground tunnels, the results obtained do not guarantee that it can be reliably used in underground mines. This is because operating conditions in underground mines are significantly different from those in underground tunnels. Therefore, further experimental research must be performed concerning the use of BBUNPROWS in underground mines.  
[PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON] and [PERSON] analyzed the data; [PERSON] contributed reagents/materials/analysis tools; [PERSON] and [PERSON] wrote the paper.  
This work was supported by (1) Institute for Information & communications Technology Promotion (IITP) grant funded by the Korea Government's Ministry of Science and ICT (Project No. 2018-0-01282) and (2) Korea Energy and Mineral Resources Engineering Program funded by the Korea Government's Ministry of Trade, Industry and Energy.  
The authors declare no conflict of interest.