ACC Really Scared me When

Not surprising. Minimal radar cross section was probably dismissed as an artifact of some sort.

 

No angles for good radar reflection, even stereo camera may have trouble being sure about that is shadows or darker conditions.
Future regulations will maybe require a vertical radar reflecting surface or thicker walls for laser or stereo am hits on such trailers.

 

I'll bet that with some basic radar geometry we accomplish this with a little license plate sticker.

 

I think AP1 has some kind of vertical radar at the center only. You know how AP1 will display a Cargo Truck on the screen when it detects a tall back on the vehicle in front? One time I was in traffic behind a truck carrying a porta potty. The truck kept moving to the left toward the left shoulder to peek ahead of the traffic, so the graphic kept changing from a truck to a car to truck to a car...