Originally Posted by
HumbleGrendel
The guy was a complete jerk for taking off and an idiot for not forseeing a potential situation after seeing your friends turn. With that said, I think it was much more important for you to communicate your intentions and hand signal your left turn than for him to call out "on your left".
Maybe that's what it takes for 5% of the cycling population to realize what would be obvious to the other 95%. By the way, I always use hand signals when riding in the street, which I do every day.