Always assume attraction. So yes, if she's looking at you, you have a great excuse to approach. Say "I have a rule. When a woman looks at me for too long, I have to go talk to her, so it's your own fault". Say it with a smirk and she'll laugh. If you misread, so what.
And if they did not look at you that's probably because she's in her own head thinking about the errands she's running. Or she simply didn't see you because she was focusing on traffic or something else in the distance. Or maybe she saw you but is too shy to make eye contact. It's not an excuse not to talk to her, if you like her.