Police said the crime happened on the Parkway near Route 193 at about 2 a.m.
Investigators had to wait for the sun to come out so they could collect physical evidence from the scene.
Until that work was completed, southbound traffic was detoured at Powder Mill Road. Motorists can return to the Parkway at Greenbelt Road.
All lanes re-opened by about 7:30 a.m.