Social cues, when is it okay to talk to strangers?

Back home in the US I'd tend to fill my day with conversations I had with strangers, really brightened my day. I know that's the American sterotype but I'm quite the extrovert and at uni it's how I'd make friends at various functions. I've kind of gone 180 on that here, but I really want to meet people from the city/conversate! Where is it socially acceptable to like talk to random people/mingle? Night out I suppose? Pubs? I was on the bus and these people had party horns and my instinct was to strike up a convo but I resisted lol