So it is in principle similar to the many maps that show data from ADS-B (beacons the planes send). Or to aprs.fi that display beacons ham radio enthusiasts send.

Unfortunately, the vehicles sending C-ITS send them not from very high (like planes) or with considerable transmit power (like ham radio operations). And also in challenging areas. So an C-ITS receiver (e.g. based on a ESP32-C5) has a rather short receiving range.

So for a truly open traffic map we'd need millions of receivers!