This is a chart of the average speed of Taxi cabs in NYC. As you can see they rarely get above 11 mph. Your average slow bike rider is going to easily be able to go an average of 12 mph.
I have found it to be true that bikes always win for speed in large cities, and while they might not win for speed in rural areas they for sure win as far as environmental concerns go. Plus riding your bike is fun, and driving in city traffic sucks.