The concept is that there is this protocol called ITS-G5, which is a European profile of 802.11p. Vehicles and traffic infrastructure can transmit telemetry on 5 GHz. Other vehicles and traffic infrastructure can use it for situational awareness.
This website collects that data using local receivers and aggregates it onto a map, similar to what website like ADSB-Exchange do with ADS-B.
What is concerning is that vehicles appear to broadcast a MAC address. Does this mean that ITS-G5, 802.11p, and C-ITS could be used for persistent tracking?