Abstract
Sensing the crowds to understand crowd dynamics can be a challenging task. Passive sensing techniques such as camera-based sensing can provide flow detection, people counting, and density estimation, but they fail to provide accurate identification of individuals mobility patterns. Active techniques such as Radio Frequency Identification (RFID) tags given to people require expensive RFID readers deployed to perform sensing. In this paper, we propose to use Bluetooth low energy (BLE) tagging as an alternative method. When low-cost BLE tags are set in advertisement mode, they can be detected by smartphones. In this paper, we design an architecture for sensing the crowds by requiring a large population carrying relatively cheap off-the-shelf BLE proximity tags, and considerably fewer participants to run scanning application on their smartphones to collect data. We performed a large experimental deployment with 600 tags and ten smartphones conducted during the five days of the world largest annual gathering (The Hajj). We were able to achieve ~90% detectability rate while effectively reconstructing the routes of the participants.