American drivers (especially in places like New York) generally just ignore 20MPH speed limits (~=30km/h).
One good solution for the avenues would be to time their rolling green waves for an average of 15 mph. They just recently started doing this in Brooklyn on a particularly dangerous road.
I'm just pointing out that that's hardly indicative of the overall speed of traffic across Manhattan at all times. Vehicles go fast on the avenues when they can, definitely faster than you would want to try to keep up with on a bike.