A bus will be 4-6 mph, a taxi at great expense averages 8-9 mph. Sure, if you're not doing all 15 miles in city center but come in from outside, you can do moderately better than that, but for connections inside the city taking a bike is already the fastest transportation, even if you can only do 9 mph (and most people, in a flat city like NY, can do a lot more).
http://www.nyc.gov/html/dot/downloads/pdf/mobility-report-20...
A bus will be 4-6 mph, a taxi at great expense averages 8-9 mph. Sure, if you're not doing all 15 miles in city center but come in from outside, you can do moderately better than that, but for connections inside the city taking a bike is already the fastest transportation, even if you can only do 9 mph (and most people, in a flat city like NY, can do a lot more).