Hahaha oh the good old bumslap... When I first started cycling I wondered what that was about too but it really is convenient to know when there's a massive pothole coming up... doesn't anyone wonder how quickly you'd be able to dodge a pothole if you're riding right behind the guy? maybe it just causes more trouble. anyway, i agree the bottle waving thing is more of a hazard than a precaution - if someone did that to me in a big group going 50 or 60kph I'd think a bird was flying into me or something!
