Yes I've seen it activate once already (only been on this build for a couple days). It was in city driving. Crazy traffic right now and while I noticed it, I didn't even have time to validate where the car was specifically lol. Useful if it works, although I've got full FSD with the visualizations, and my take on "is it safe?" is that the nav image with an immediate red glow of a vehicle is superior/safer than the camera image itself. The reason is that the nav screen is an instant go/no go. With the visual camera, it's a half second to activate, then a half second for me to see it, interpret the image and make a go/no go decision - and sometimes that's just too long. I will say the nav screen method seems a bit wonky, doesn't seem 100% reliable unless I'm on the highway with FSD engaged so haven't committed to it and turned off the blinker camera. Camera most useful to me when needing to cross multiple lanes of traffic particularly on highway. I would love a swipe or stalk control that engaged the side cameras. That combined with the nav screen functionality would be the "sweet spot" for me.
