Rescuers were called to the scene on I-95 South at the ramp to Route 141 at 12:50 p.m. Monday.
There, medics found the woman, 29-year-old Kristy Long of Wilmington, dead. Her 18-month-old daughter, Aubry Carter, was found still strapped in a car seat and taken to the hospital with minor injuries.
Police say a preliminary investigation had determined that Long's car, a 1996 Honda Accord, was going southbound in the right lane of I-95.
For an unknown reason, police say the car went off the road, continued straight, and came to rest in the roadside brush.
Investigators say the car may have been in the underbrush off the exit ramp for as long as three hours before someone finally noticed the vehicle. The baby was out in the freezing cold temperatures the entire time.
Police are now trying to determine how Long died. The car was not badly damaged, police said. However, it was not equipped with an airbag and the driver's seat was pushed very close to the steering wheel.
At this point, police don't know if the driver suffered some sort of medical problem or if the car had mechanical issues.
The ramp was closed during the investigation but re-opened shortly after 2:30 p.m.