The "with almost perfect accuracy and very little false positives" part is not true.
If you look at euroncap data, you'll see how most cars are not close to 100 in Safety Assist category (and Teslas with just vision are among the top). And these EuroNCAP are fairly easy and ideal. So it's clearly not a solved problem, as you portray.
https://www.euroncap.com/en/ratings-rewards/latest-safety-ra...
Radar can absolutely detect a stationary object.
The problem is not, "moving or not moving", it's "is the energy reflected back to the detector," as alluded to by your second qualification.
So something that scatters or absorbs the transmitted energy is hard to measure with radar because the energy doesn't get back to the detector completing the measurement. This is the guiding principle behind stealth.
And, as you mentioned, things with this property naturally occur. For example, trees with low hanging branches and bushes with sparse leaves can be difficult to get an accurate (say within 1 meter) distance measurement from.
Non-moving vehicles are seen as approaching you at whatever speed you are moving at. Along with all the other things you mentioned.
So they all have doppler shift but the "stationary" things approaching your car at your speed actually have much higher shift than the traffic around you.
It's hard to tell something sticking off the back of a truck or a motorcycle behind a vehicle without false positive triggering off of other stuff and panic braking at dumb times, something early systems (generically, not any particular OEM) were known for, which is why they were mostly limited to warnings not actual braking.
And while one can make bad faith comments all day about that not technically being the fault of the system doing the braking allowing such systems to proliferate would be a big class action lawsuit, and maybe even a revision of how liability is handled, waiting to happen.
Earlier in the thread people were saying removing lidar was bad becuase multiple sensors types is good, presuming the camera stay either way, and one is not replacing camera with radar. I agree with this. It's usually trivially easy to corner case defeat one sensor type, as your example shows, regardless of sensor type. They all have one weakness or another.
That's why things like military systems have many sensor types. They really don't want to miss the incoming object so they measure it many different ways. Defeating many different sensor types is just way harder and therefore more unlikely to occur naturally.
And yes, control systems can absolutely reliably combine the input of many sensors. This has been true for decades.
Frankly I surprised more of these systems don't take advantage of sound. It's crazy cheap and society has been adding sound alerts to driving for a long time (sirens, car horns, train horns, etc.)
No, that's how lidar works. Lidars have a single frequency and a very narrow bandwidth. Automotive radars have a bandwidth of 1-5 GHz. They operate around 80 GHz, which is very well reflected by water (including people) and moderately reflected by things like plastic. 80 GHz is industrially used to measure levels of plastic feedstock.
Compare TSA scanner images, which are ~300 GHz: https://www.researchgate.net/figure/a-Front-and-back-millime...
You are correct that most automotive radars like Bosch units [1] are very low detail though. Most of them don't output images or anything- they run proprietary algorithms that identify the largest detection frequencies (usually a limited number of them) and calculate the direction and distance to them. Unlike cameras and lidars they do not return raw data, so naturally when building driver assistance companies instead relied on cameras and lidar. Progress was instead driven by the manufacturers and with smaller incentives the progress is slower.
[1]: https://www.bosch-mobility.com/en/solutions/sensors/front-ra...
For a long, long time automotive radar was a pipe dream technology. Steering a phased array of antennas means delaying each antenna by 1/10,000s of a wave period. Dynamically steering means being able to adjust those timings![1] You're approaching picosecond timing, and doing that with 10s or 100s antennas. Reading that data stream is still beyond affordable technology. Sampling 100 antennas 10x per period at 16 bit precision is 160 terabytes per second, 100x more data than the best high speed cameras. Since the fourier transform is O(nlogn), that's tens of petaflops to transform. Hundreds of 5090s, fully maxed out, before running object recognition.
Obviously we cut some corners instead. Current techniques way underutilize the potential of 80 GHz. Processing power trickles down slowly and new methods are created unpredictably, but improvement is happening. IMO radar has the highest ceiling potential of any of the sensing methods, it's the cheapest, and it's the most resistant to interference from other vehicles. Lidar can't hop frequencies or do any of the things we do to multiplex radar.
[1]: In reality you don't scan left-right-up-down like that. You don't even use just an 80 GHz wave, or even just a chirp (a pulsing wave that oscillates between 77-80 GHz). You direct different beams in all different directions at the same time, and more importantly you listen from all different directions at the same time.
(Also I wouldn't say it's _irrelevant_ that they don't have lidar, as if they did it would cover some of the same weaknesses as radar.)