The driver lost control of her grey 1999 BMW 328i convertible on the ramp from Interstate 287 to Interstate 95 just before 9:15 p.m.
The vehicle spun out on the wet roadway and crashed into a tree.
The driver, 17-year-old Margaret Radford, of Croton on Hudson was killed in the crash.
Her passenger, 16-year-old Celia Sprouse of Stamford, sustained massive head trauma.
She is in critical condition at Greenwich Hospital.
Investigators believe speed and the wet roadway were factors in the crash.
The vehicle was registered to Radford's father.
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