The Waze Beacons themselves are just battery-powered microcontrollers that use low-powered Bluetooth signals to communicate with smartphones, allowing for Waze to provide alternative location services to drivers even when out of GPS range (relying on Google’s near-field Eddystone technology to communicate).

The Beacons are installed on tunnel ways and can be used to ferry information to drivers underground (for privacy reasons, the Beacons only relay information one way), making it possible to alert drivers of sudden changes in traffic or accidents even without GPS.
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