I’m also a big proponent of strong laws regarding dogs. Everything they leave must be cleaned up or owners should risk very heavy fines (think of 200 GBP / $300 or so, per time, that should work). And, have regular police enforce it – not just a city’s environmental inspectors (who are never seen).
I myself have three cats, but they don’t go outside alone, and do all their needs in a litter box. I don’t know why dogs can’t use a litter box (ok, it should be a little big, but still, it would make sense).