German immigration to New York City was high in the years before World War I. The term “Dutch” was often used for Germans (for “Deutsch”).
 
Courtlandt Avenue in the Bronx had so many German immigrants that it was called “Dutch Broadway” by at least the early 1900s. A section of Elmont (Hempstead, Long Island) was also called “Dutch Broadway” during this period.  The German influence on Courtlandt Avenue is still reflected in many of the buildings, but the demographics of the area have changed and the name is of historical interest today.
 
 
Wikipedia: Melrose, Bronx
Melrose is a residential neighborhood in the New York City borough of the Bronx. It is north of Mott Haven and west of Longwood, and considered as part of the South Bronx. The neighborhood is part of Bronx Community Board 1. Its boundaries, starting from the north and moving clockwise are: East 161st Street to the north, Prospect Avenue to the east, East 149th Street to the south, and Park Avenue to the west. Melrose Avenue is the primary thoroughfare through Melrose. The southeastern corner of the neighborhood is served by the 2 and 5 line at Jackson Avenue, operating along Westchester Avenue. ZIP codes include 10451, 10455, and 10456. The area is patrolled by the 40th Precinct located at 257 Alexander Avenue in the Mott Haven section of the Bronx. 
 
Chronicling America
2 January 1901, New York (NY) Tribune, pg. 6, col. 3:
Hempstead, Long Island, Jan. 1.
(...)
Nearly all the slaughtering establishments are on what is known as Dutch Broadway, Elmont.
 
24 October 1909, New York (NY) Times, “Bronx All Stirred By Haffen’s Fight,” pg. 2:
In particular, Cortlandt Avenue, known locally as “Dutch Broadway,” is a mass of bunting in support of the ex-Borough President.
 
Google Books
Bronx in the Innocent Years: 1890-1925
By Lloyd Ultan and Gary D. Hermalyn
Published by Bronx Historical Society
1979
Pg. xiii:     
Another area heavily settled by the Germans was Melrose, whose main street, Courtlandt Avenue, was called “Dutch Broadway” because of the concentration of German shops, saloons, beer halls, and gymnastic and singing societies. (“Dutch,” a corruption of the word deutsch, was a popular term for Germans.)
 
Google Books
History in Asphalt:
The Origin of Bronx Street and Place Names
By John McNamara
Published by Bronx Historical Society
1993
Pg. 279:
DUTCH BROADWAY. This was Courtlandt Avenue from East 145th Street to East 161st Street. Due to its heavily Germanic population, this main thoroughfare of Melrose and Melrose South earned its nickname. The Haffen family, wealthy brewers, lived at East 152nd Street, and one son become the first Borough President. Beer gardens, gymnast clubs, German social halls and Lutheran churches lined Courtlandt Avenue, and the mile-long funeral cortege of General Franz Sigel in 1902 was of national note.
 
New York (NY) Times
Neglected Bronx Landmark Is Getting a New Life, Again
By DAVID GONZALEZ
Published: August 26, 2008
The red-brick building with the mansard slate roof and elegant windows at 614 Courtlandt Avenue looks like a Bronx take on the Bates Motel. For decades, the landmark 19th-century building had been left to decay, a home only to pigeons. 
 
Now, this stubborn survivor, among the first buildings built during the borough’s transition to city from farmland, is being renovated and has just been put back on the market.
(...)
The mistake is understandable. According to a report prepared before the building was granted landmark status in 1987, German immigrants who were leaving the cramped confines of the Lower East Side settled Melrose South in the Bronx in the mid-1800s. They established breweries, beer gardens and other businesses. Indeed, so strong was their presence, that Courtlandt Avenue came to be known as “Dutch Broadway.”