Not use any device that has GSM/LTE, or Bluetooth.
Alternatively, broadcast a hidden SSID WiFi AP via an enabled RPi and use only devices that's have WiFi. Hand them out to people for free to increase the spread.
Attach magnets to the RPi's and go rogue by sticking them to buses, cars and trains et cetera to increase range.
Alternatively, broadcast a hidden SSID WiFi AP via an enabled RPi and use only devices that's have WiFi. Hand them out to people for free to increase the spread.
Attach magnets to the RPi's and go rogue by sticking them to buses, cars and trains et cetera to increase range.