Consider how many bits you transfer over wifi before hitting saturation.
Now consider how many bits of information is collected by these radars per second.
That gives and indication of how much free bandwith there is in the radar bands if the radars are built at the level of sophistication we expect from wifi. (At an OOM level, if not accurately).
Any congestion with current technology would be because the technology is far less optimized and standardized than wifi.
This also assumes the radars are using as much bandwidth as Wi-Fi, and that the noise floor is the same, and that Wi-Fi-like modulation works fine for radar, etc.
Keep in mind this was an OOM argument. How much info does one really need to collect from the radar? 1-10kb/s?
Even with 10 cars trying to scan the same region, that's about 5 OOM of headroom compared to wifi.
Also consider what modern military radiation radarss can do. The F-35 can actively track 50 targets, all in one direction. And it can do that while potentially 100s of aircraft all are sending radar beams into the same space, with some even activelly trying to jam the F-35 radar.
Obviously, really old and cheap radars can have interference issues. But any such limitation is not due to the Physics or even engineering, but rather on the cost of a radar sophisticated enough to handle its environment.
Now consider how many bits of information is collected by these radars per second.
That gives and indication of how much free bandwith there is in the radar bands if the radars are built at the level of sophistication we expect from wifi. (At an OOM level, if not accurately).
Any congestion with current technology would be because the technology is far less optimized and standardized than wifi.