CHP officers Tuesday were attempting to learn what caused a Novato man to lose control of his sedan as he headed south on Highway 101 near Cotati and flip the car.
Driver Thomas Umphreys, 48, suffered life-threatening injuries and was being treated at Santa Rosa Memorial Hospital, said CHP Officer Jon Sloat.
He crashed at about 8:10 p.m. Monday.
The Toyota Corolla was moving at about 65 mph, south of Railroad Avenue at the top of the Cotati Grade when he crashed. The car veered to the right, ran up an embankment and flipped over.
It stopped on its roof, trapping Umphreys.
Passenger Angeliki Vartholomeos, 33, of Mill Valley and daughter Francis Daverm, 3, got out of the wreckage with minor injuries.
They were taken to Petaluma Valley Hospital.
Traffic investigators were looking into whether drugs or alcohol were a factor, said Sloat.