Chaff wouldn't be a fixed object: released from a moving car in response to a tailgater, the chaff would be moving at the same speed as that car, though it would presumably slow down rapidly in the air. Depending on how sensitive the tailgater's radar is, it might appear as if the followed car is braking rapidly.
However, I'm not sure it'd be effective: the whole problem with tailgaters is that they follow too closely, so there might not be enough space for the chaff to decelerate and trigger the tailgater's radar. It would be an interesting test, though. But I think an active radar emitter that detects the tailgater's radar and generates a matching signal that appears like a rapidly-braking car would be better, though obviously more technically difficult.
I've seen other brands say/do the same. I suspect these systems would have tons of false positives otherwise, because cars often drive in close proximity to fixed barriers with high radar reflectivity.
I'm sure the best way to not run into other cars on the road is to detect the other objects that are also moving at non-zero velocities. They're likely to be other cars, rather than road signs, poles, etc.
Non-directional doppler radars are cheaper than the kind of directed radars that would be needed to map the distance to all static objects.
Also a dopler radar can scan the whole sector with every emission, and simply measure the distance to the closest moving objects (and their speed) with each scan.
A radar that creates a 2d image of the sector usually needs to send a targeted beam in one direction at a time. Such a radar is not only much more expensive, it also adds latency.
Visible light is better suited for 2D imaging, anyway.
The radar detects them perfectly. Higher level logic filters stationary objects (using the ego-vehicle’s known speed) out to prevent nuisance detections.
If they wouldn’t do that filtering they would trigger a brake for almost all manhole covers and overhead signs and such.
Yeah, by 'detect' above I think we've been referring to the end behavior (e.g. what's in the owners manual) of the automated systems, not the raw RF received. Obviously, radar reflectivity itself is not dependent on relative motion of the object compared to the receiver.
It's not a problem of reflectivity, it's a problem of resolution. In order to detect something distinctly from other things (i.e. resolve that thing), you must be able to distinguish its reflected energy from that of other things by separating them along one or more dimension. Range is usually a good discriminator, but there are many things at (nearly) equal range to the radar. Azimuth is typically not great, because azimuth resolution requires a physically wide aperture, and real estate on the bumper is expensive. Doppler is great for moving things because it's easy to design a waveform with a small doppler resolution, and most moving things (cars, bikes, people) don't move at exactly the same speed as other moving things. However, nonmoving things have a very consistent velocity of precisely 0, and there are lots of them. So they can be very hard to resolve, and thus to detect.
