I cycle a lot around the town I live in.
On the way to work today, I come up to a junction and want to turn left. There's a car in the advance box, not signalling, so I pull to the side of the car. As the lights change, I go off, and the driver starts signalling to go left. By being over the line, and not indicating, she's causing a hazard. The driver overtakes me, and we meet at the next junction, again, they're past the line, not indicating. This time I hang behind them, thinking they may turn left. Lo and behold, as the light changes, they go to turn left.
I was sorely tempted to cycle on, tap on their window and explain how badly she was driving, but I thought better off it. As I cyclist, you're "outside" in a way that drivers aren't. Would I have been unreasonable to have explained to her the way she drove put people at risk.
PS: The point isn't to be a car v. cyclist thread, it's just that cyclists being outside are often freer to speak to other motorists.