Anecdotally, the times I see taxis most crowded/unavailable in New York is in the East Village / Lower East Side on weekend nights, headed towards the bridge; my guess is that most of the passengers are drunk and trying to get home, and I don't particularly want to share a normal-sized car with three drunk strangers. (Enhancing subway coverage between that area of town and Brooklyn would help; if someone vomits on a subway, it's easy enough to move to the end of the car or to the next car.)

Also it's not clear to me if this research assumes that every cab is carrying a single person. That's often not true, especially for the going-home-from-a-night-out crowd.