After flying south through the Cajon Pass at 6500 feet msl, the airplane turned west and encountered what the commercial pilot presumed was leeside turbulence from the mountain range. She turned back south to find smoother air but the turbulence became more severe and the airplane began to descend rapidly. As the airline transport pilot struggled to change frequencies in the turbulence, the airplane descended to 2000 feet msl (about 500 feet agl). The commercial pilot applied full power but the engine did not respond. After the airline transport pilot enrichened the mixture and applied carburetor heat, the engine momentarily regained power. At about 2300 feet msl, the engine again lost power, and the ATP decided to land on the westbound lanes of a freeway. As he attempted to avoid a vehicle, the airplane landed hard.