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DorisPaoa street in Singapore ...Both 'a' and 'the' suggest that the identity of the street or road does not matter, that it has no relevance for your story. That has the effect of shifting the emphasis on the rest of the sentence: walking (somewhere in Singapore), driving (somewhere in Singapore).
DorisPaoHe was walking down the street in Singapore.This gives the impression that you're referring to one street in particular. It's odd because you have not, already mentioned which one.