Namely, this user wrote:
To add even more detail, as there still appears to be a lot of confusion:
The radar systems in these vehicles send out a radio pulse in a broad approximate-cone forward. They get bounces back from everything that reflects radio in front of them. Distance from the object is calculated by time between pulse and response. Speed towards/away from the object is calculated from Doppler shift of the radio frequency.
There are two main things that these systems can't detect.
1. Speed of the object perpendicular to the direction of radio wave travel.
2. Location of the object within the approximate-cone the radio pulse travels in.
Note that thanks to the second, you can't calculate the first with higher-level object tracking, either.
So the data you get back is a list of (same-direction velocity component, distance) pairs. There's no way to distinguish between stationary objects in the road and stationary objects above the road, to the side of the road, or even the surface of the road itself.
Radar just doesn't provide the directional information necessary to handle obstacle detection safely.
Tesla may be trying to optimize costs and be aggressive as they have been in other areas, doing the whole MVP/iteration thing, but this is one place where that just isn't necessarily the greatest strategy. They should be aiming to be better than human drivers not just as-good-as, and the challenges and limitations of ultra complex software based systems development are hardly some new revelation. They should have been prepared for that too with more hardware backstop.