The main bridges in Liberty City in Grand Theft Auto IV connect the islands to each other, and most are modeled after bridges in the New York area. Player can obtain the Under the Bridge Achievement by flying a helicopter below all the bridges which can increase the running total flown under bridges that can be found in the "Player" section of the Stats found in the main menu. 

Bridge Name New York City counterpart Connects
Broker Bridge Brooklyn Bridge Broker and Algonquin
Algonquin Bridge Manhattan Bridge Algonquin and Broker
East Borough Bridge Triborough Bridge Algonquin, Dukes, Bohan and Charge Island
Dukes Bay Bridge Throgs Neck Bridge Dukes and Bohan
Northwood Heights Bridge Alexander Hamilton Bridge Bohan and Algonquin
Hickey Bridge George Washington Bridge Algonquin and Alderney
Leaper's Bridge Roosevelt Island Bridge Algonquin and Colony Island