Recommending there to be no glass in the environment makes sense for liability, but I would have thought a few ultrasound detectors would be sufficient for gros

Recommending there to be no glass in the environment makes sense for liability, but I would have thought a few ultrasound detectors would be sufficient for gross collision avoidance e.g. to avoid walking into a pane. Similarly forward/down-looking sensors should be able to detect ground plane discontinuities. I guess the stereo isn't robust enough to do that reliably.