When the thing being subsidized has no benefit to society.
'Subsidizing' the impoverished is helping another human being, and is always worthwhile. Healthcare for all is enlightened self-interest because I do not know what future illnesses I may have, and what they may cost, so I'd rather have everyone subsidized than risk myself not having access to future treatments.
Education and the public fire departments help build better societies.
But what benefit do car-based suburbs give to society as a whole? Can you least three benefits that we can discuss?