Only skimmed this but didn’t see any mention of tracking, which is the only way (?) to get the true velocity when there’s non-radial motion.
> The velocity of the target also manifests as a frequency shift in the received chirp due to Doppler effect
If the detected object is moving tangentially with the radar, relative velocity would be zero, because it's staying a fixed distance from the vehicle while both are in motion, and would seem to not be in the path of a collision.