Well first off I tether it to the bike. So if the suction cup were to come off, it shouldn't ever hit the ground. As for it getting knocked off, if a car were to get that close, the last thing I'd be worried about would be the camera. More likely major leg or other bodily injury would be my main concern at that point. It only sticks out about 4" from the mounting point, which usually isn't the widest part of the bike anyways. So no, I don't worry about it at all. Now if it were to hit the ground, it all depends on the hit. I've seen ones that have a few little scratches on the case and that's it. I've also seen them obliterated beyond recognition. Which is why mine is always tethered to the bike in some way.
Thanks. Those ones on Palomar are a GoPro HD that I borrowed from a friend. I have the GoPro standard def. Its what the first few videos are shot on. Not as high quality but still decent. You can see the setup in this picture.