Complex sentence: The Nassau Expressway was mapped as part of the Interstate Highway System in 1961, at which point the New York State Department of Public Works began purchasing land for both the Nassau and Long Beach Expressways.
Split into simpler sentences: The Nassau Expressway was mapped as part of the Interstate Highway System in 1961. At that point, the New York State Department of Public Works began purchasing land for both the Nassau and Long Beach Expressways.