To calculate the number of pet dogs in a populated area, you divide the human population by 2.65 then multiply by .534. For New York City's 8 million, that works out to 1.6 million pet dogs. Let's assume 90% of dog walkers pick up their dog'(s)' daily shit. Assume some are crazy enough to pick it up with a newspaper. Some may use bio-bags as well. I could go on to calculate the number of unpicked up turds per city block in NYC, but that's not exactly the point. Point is, that's at least a million plastic bags a day. Banning plastic bags in this regard will be a tough sell. I can't imagine President Obama picking up Bo's poop without plastic.
Meet the Peepoople. They developed a biodegradable bag to help sanitation in Africa. The bag serves as a toilet but when buried, it provides organic fertilizer for improved soil structure. Perhaps this innovation could help NYC reduce trash and enhance urban farming and gardening.