New York State Route 878 (NY 878) is a state highway on Long Island, in the southern portion of the U.S. state of New York. The route exists in two sections, which both form the Nassau Expressway. NY 878's western terminus is the Belt Parkway and Conduit Avenue (NY 27) in Ozone Park, within southern Queens. Its southern terminus is Atlantic Beach Bridge in Lawrence, within southwestern Nassau County. NY 878 is discontinuous between Farmers Boulevard in Queens and the town of Inwood in Nassau County. The two sections are connected to each other by Rockaway Boulevard and Rockaway Turnpike.
NY 878 is maintained in part by the New York City Department of Transportation (NYCDOT); the New York State Department of Transportation (NYSDOT); and the government of Nassau County. The NYSDOT also maintains part of Rockaway Boulevard, which is designated as the reference route NY 909G. The 0.70 miles (1.13 km) of NY 878 between I-678 and the JFK Expressway is officially designated Interstate 878 (I-878), but not signed as such. This segment is instead signed as NY 878. The NYSDOT designated the eastbound lanes of the freeway as I-878 in January 1970, but the entire Nassau Expressway was publicly re-designated as NY 878 by 1991. The unsigned Interstate 878 is the shortest Interstate Highway in the United States.
NY 878, the Nassau Expressway, was originally planned in 1945 as a freeway between the Belt Parkway in Queens and Long Beach in Nassau. The expressway was supposed to replace Rockaway Boulevard and Turnpike in the vicinity of what is now JFK Airport, connecting to a proposed Long Beach Expressway south of Atlantic Beach Bridge. The short freeway portion in Queens was originally built as part of Interstate 78 (I-78) in the late 1960s, but the segment of I-78 through New York City was canceled in March 1971 due to community opposition. Through the 1970s, the rest of the freeway south of 150th Street was also canceled for various reasons. A scaled-down version of the road in Nassau County, a 4-lane arterial road, was completed in 1990. There has been an attempt to complete the section of the freeway in Queens, but it was deferred due to the early 1990s economic recession.
The 3.73-mile (6.00 km) northwest section in Queens is mostly built to freeway standards, except for a traffic light at the eastern end of the highway. It lies along the north edge of JFK Airport, just south of the Belt Parkway and Conduit Avenue (NY 27). Officially NY 878 starts at the interchange between the Belt Parkway, Conduit Avenue (NY 27) and Cross Bay Boulevard, and it stretches east to the intersection of Rockaway and Farmers Boulevards. The eastbound freeway does begin in the median of Conduit Avenue just west of Cross Bay Boulevard, but it carries NY 27 until the highways split at a point between the IND Rockaway Line underpass and Lefferts Boulevard. The separate NY 878 begins at that split, but the route only carries eastbound one-way traffic until it reaches the junction with I-678 (Van Wyck Expressway). There it becomes a two-way freeway. NY 878 continues east past the JFK Expressway, and the freeway ends at a traffic light at North Hangar Road. NY 878 ends soon after at Rockaway Boulevard and Farmers Boulevard.
The 0.70-mile (1.13 km) stretch from I-678 (Van Wyck Expressway) east to the JFK Expressway is designated but not signed as I-878 by the Federal Highway Administration. This makes I-878 the shortest three-digit Interstate Route and the shortest Interstate Highway in the Interstate Highway System. This section of NY 878 only has route designations for the eastbound lanes. The New York State Department of Transportation (NYSDOT) designates I-878 as the stretch of NY 878 from the Van Wyck Expressway east to the 150th Street underpass. The entire segment, including the unsigned I-878, is maintained by the NYSDOT.