The incident happened in East Moriches at about 10:50 p.m. Saturday, Aug. 19.
The woman was driving a 2016 Dodge Ram onto westbound Sunrise Highway (Route 27) from the Exit 61 ramp, Suffolk County Police said.
The vehicle then left the roadway and overturned, ejecting her from the vehicle.
The victim, identified as Olivia Montgomery, age 20, of Farmingville, was pronounced dead at the scene by a physician assistant from the Office of the Suffolk County Medical Examiner.
Detectives are asking anyone with information on the crash to call the Seventh Squad at 631-852-8752.
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