In New York City, you can hail a cab but not a livery car, at least not legally - that may change if Mayor Bloomberg has his way. He is expected to propose a rule change in his State of the City speech that eliminates the 'hailing' distinctiion and also installs meters in livery cars.
"As things now stand, only yellow cabs can legally pick up street hails within the five boroughs, while livery cars can respond only to radio calls. Yellow cabs have meters, while liveries use a zone pricing system. It's worked this way since the medallion system was introduced in 1937.
The livery cabs that do ply the streets of Brooklyn, Queens, the Bronx and Staten Island wouldn't have many passengers to pick up on the street, because these boroughs have relatively little pedestrian traffic. This is what led to the rise of the radio-dispatched livery industry in the first place. What works in busy Manhattan doesn't necessarily work in the other boroughs."
The writer of this this column, who is an author as well as a taxi driver, explains why it is not only bad for cab drivers but customers who need to leave Manhattan as well - so you might just not get the cab you hailed. The rule will have to be approved by the City Council.