The crash occurred at about 3:15 a.m. on the shoulder of northbound Highway 101 just north of Metcalf Road.
The occupants of two cars, a Mitsubishi and a Toyota, were on the shoulder tending to a flat tire on the Toyota when the teenager, driving a Mercedes, veered off the roadway and crashed into the rear of the Mitsubishi.
The impact pushed the Mitsubishi forward into the Toyota, severing the leg of a 23-year-old San Jose woman who was standing between the two cars.
The driver of the Toyota, a 23-year-old Morgan Hill man, suffered major head injuries.
Both were transported to San Jose Regional Medical Center.
The driver of the Mitsubishi, a 29-year-old Milpitas man who had stopped to assist with the flat tire, suffered major head injuries and internal bleeding and was transported to Santa Clara Valley Medical Center.
The driver of the Mercedes, identified as Eric Nun, of San Jose, was treated at Valley Medical Center for a fractured ankle and other moderate injuries and later booked into Santa Clara County's main jail on a felony DUI charge, according to the CHP.
