For automotive radar, I'm not sure the requirement is to determine the true speed of the detected object. The requirement is to determine if the radar vehicle is likely to collide with the detected object.
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.
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.