Need a Cab? New Analysis Shows Where to Find One

That doesn't actually tell you how easy it is it to hail a cab. Sure, there are lots of cabs at Grand Central at 9am, but that doesn't mean it's easy to hail one because there's tons of other people trying to hail them too.

Also, even if few cab trips originate from a certain point, that doesn't mean that there aren't a lot of cabs passing by there that will be happy to pick you up. For example, I bet if you were 2 blocks away from Grand Central during the morning rush, it would be pretty easy to pick one off on its way to Grand Central, even if few trips originate from that spot because few people find themselves there at that time.

http://www.metablake.com/m/home.html is the home of Blake Shaw who did this analysis; good papers on how to analyze spatiotemporal data... like this taxicab data.