Complex sentence: Most external urban areas with high pedestrian traffic show high incidence of casual chewing gum discard and the Singapore ban on chewing gum sale may be felt to have some merit-Singapore's pavements are, perhaps uniquely amongst modern cities, free of gum.
Split into simpler sentences: Most external urban areas with high pedestrian traffic show high incidence of casual chewing gum discard. Likely as a consequence of Singapore's ban Singapore's pavements are, perhaps uniquely amongst modern cities, free of gum.