Well, you'd have trouble citizens arresting someone in their car when you were on your bike, but that's not the situation your man described, is it? I considered doing a citizens arrest on my neighbours son - he was off it on cocaine, had kicked their fence down, broken a window, attacked his dad, punched me in the face when I tried to calm him down, then tried to hit me with a piece of the fence. I restrained him (I'm big, like I said), and we tried to get him to go home. If he hadn't, I would've called the cops, and it would've been a de facto citizens arrest. I'm glad it didn't come to that, because as it ended, he was apologetic, whereas if I'd handed him to the scuff, he most probably would've hated me forever.