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Orientation: The Southern State Parkway enters at bottom left, becoming the Heckscher State Parkway just before exiting at top right (the Heckscher State Parkway is signed as part of the Southern State Parkway). The Sagtikos State Parkway enters at top and ends at the Southern/Heckscher Parkway interchange. The Robert Moses Causeway, also part of the Long Island parkway system, enters at bottom and terminates at the Southern State Parkway. Bay Shore Road (Suffolk CR 57) is the surface street at bottom.
Exit numbers: On the Southern State Parkway, Exit 40 is for the Robert Moses Causeway, Exits 41N and 41S are for Bay Shore Road, and Exit 41A is for the Sagtikos State Parkway. On the Sagtikos State Parkway, Exit S4 is for the Southern and Hecksher State Parkways. The Robert Moses Causeway's exit numbering originates from the Southern State Parkway, but this interchange is not itself numbered on the causeway.
The interchange: Four parkways and one local highway intersect at three interchanges, each of which exhibits a typical interchange design. The primary interest here arises from the overlapping of two of the interchanges and the overall close proximity of all three.
The Southern, Heckscher and Sagtikos State Parkways are joined by a directional-T interchange. This is a type of three-way junction in which all left and right entrances and exits are made from the corresponding side of the roadway. This particular interchange might better be called a directional-Y, since it lacks the three-level crossover typical of the directional-T. Because of the left exit inherent in this design, a significant weave pattern exists for northbound traffic travelling from the Robert Moses Causeway to the Sagtikos State Parkway, which together form part of Long Island's only full north-to-south controlled access corridor.
The Robert Moses Causeway joins the Southern State Parkway at a "trumpet" interchange. This design is also common for three-way junctions on Long Island, but it is being phased out in favor of directional-T's and similar styles. This trumpet favors traffic between points west and south: the ramps for these movements have far gentler curves than the westbound-to-southbound loop ramp. The northbound-to-eastbound ramp, on the other hand, is extended through the Bay Shore Road (CR 57) interchange, joining the second eastbound entrance ramp to the Southern State Parkway. Because it bypasses the two Southern Parkway exits for Bay Shore Road, the causeway ramp has its own (unnumbered) exit for the surface street.
The Bay Shore Road (CR 57) interchange on the Southern State Parkway is a full cloverleaf, highly common on Long Island's parkways. However, the eastbound parkway onramp is joined by the Robert Moses Causeway entrance ramp before intersecting the mainline. Furthermore, a redundant left turn channel exists at the base of this ramp, opposite the causeway exit ramp to CR 57. Finally, the turnout (see below) on the left side of the westbound parkway ramp at Exit 41N is another common feature of Long Island cloverleaves.
One other feature in this interchange complex is found on the
Southern State Parkway westbound. Through Exits 40 and 41, the parkway
lanes split, passing either left or right of the bridge abutments. The
lanes are separated by a narrow grass median. This forms a sort of
miniature express/local configuration, where exits can only be made
from the right-hand lanes.