Many of the images on my site are the result of engaging with the subject and asking permission. Street Photography purists eschew this approach with some justification on the basis that intervention by the photographer changes the dynamic of the situation.
However, I see no reason why one should not adopt both approaches. The photographs that follow, capture moments when the subject is unaware that they being recorded for posterity.
Inevitably, one comes across people who are less fortunate than ones self. This is when the perennial dilemma for the documentary photographer presents itself.- 'exploration' or 'exploitation'. If the young lady opposite takes 40 winks on the pavement, is she tacitly granting her consent? I think so.