The Coney Island Boardwalk opened in 1923 and is officially called the Riegelmann Boardwalk, after Brooklyn Borough President Edward Riegelmann (1870-1941). Both the NYC Department of Parks & Recreation website and Wikipedia have identical text, stating:
 
“‘Coney Island’s Fifth Avenue’ opened with great fanfare on May 15, 1923.”
 
The reference is to Manhattan’s Fifth Avenue—a major thoroughfare lined with expensive shopping and the route of many parades. It’s not known when this Coney Island Boardwalk nickname was first applied, but it doesn’t appear to have been very popular. The full-text online Brooklyn (NY) Eagle does not contain the nickname.
 
   
Wikipedia: Edward J. Riegelmann
Edward J. “Ned” Riegelmann (c. September 5, 1870 – January 16, 1941) was an American Democratic politician from Brooklyn, Kings County, New York, best remembered for the Riegelmann Boardwalk.
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Riegelmann Boardwalk/Coney Island Beach
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The immense engineering project required 1.7 million cubic yards (1,300,000 m3) of sand to add another 2.5 million square feet (160,000 m2) to the beach area. Construction of the boardwalk made use of 120,000 tons of stone, 7,700 cubic yards (5,900 m3) of reinforced concrete, and 3.6 million feet of timber, including long leaf yellow pine for the flooring. From a height of 14 feet (4.3 m) above the beach, the 80-foot-wide (24 m) boardwalk stretched from W. 37th Street to Ocean Parkway and provided easy access to both beach and concessions. “Coney Island’s Fifth Avenue” opened with great fanfare on May 15, 1923.
   
NYC Department of Parks & Recreation
Coney Island
Riegelmann Boardwalk/Coney Island Beach
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Once the BMT subway line reached the area in 1920, the pleasures of Coney Island were just a five-cent ride away from the steaming city. Attendance on a hot summer day could reach as high as a million, causing extreme congestion on the beach. Making matters worse, private concessions (such as beachfront hotels, bath houses, and cabarets) controlled large portions of the beach. As Brooklyn Borough President from 1918 to 1924, Edward Riegelmann (1869-1941) took charge of beautifying Coney Island and ensuring public access to the beach and shore. After the city secured title to property along the beachfront, the $3 million beach improvement and boardwalk construction began in 1921.
 
The immense engineering project required 1.7 million cubic yards of sand to add another 2.5 million square feet to the beach area. Construction of the boardwalk made use of 120,000 tons of stone, 7700 cubic yards of reinforced concrete, and 3.6 million feet of timber, including long leaf yellow pine for the flooring. From a height of 14 feet above the beach, the 80-foot wide boardwalk stretched from W. 37th Street to Ocean Parkway and provided easy access to both beach and concessions. “Coney Island’s Fifth Avenue” opened with great fanfare on May 15, 1923.
 
Courier Life’s Brooklyn Daily
JUNE 6, 2014 / FEATURES / A BRITISHER’S VIEW
King and Queen Dante and Chiara need to get on pop’s case about restoring the Boardwalk
By Shavana Abruzzo
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The 80-foot-wide Boardwalk, nicknamed “Coney Island’s Fifth Avenue,” held court from 14 feet above the beach and yawned from West 37th Street to Ocean Parkway when it opened to an eager public on May 15, 1923 — just two years after breaking ground. By 1938, then-New York Parks Commissioner Robert Moses had placed the beach and the Boardwalk under his agency’s care, given it a $3 million makeover, and extended it to Brighton Beach.
 
These days visitors continue to flock to the sickly behemoth, but how long can a ghost keep up the allure?