Answer:
Broadway is the longest street in the world. It starts at the lower tip of west Manhattan, and goes all the way up near the Canadian border. It's not called Broadway the whole way (after awhile it's called Route 9, or the Albany Post Road) but it is the same street, nonetheless.
In Manhattan, Broadway is about 13 miles long, from the southernmost tip of west Manhattan up to the northernmost tip (the Inwood neighborhood). The total length of Broadway, from the southernmost tip of west Manhattan, past Inwood, through the Bronx and Westchester, and up past Albany to Champlain, NY (just south of the Canadian border), is approximately 400 miles or so.