The teen appears to have died on impact in the crash, which occurred around 11 a.m. on eastbound Highway 24 near the Pleasant Hill Road exit, according to California Highway Patrol and fire officials.
Several witnesses told the CHP that the teen was speeding at 90 to 100 mph and making unsafe lane changes before losing control of his car and crashing into two trees just off of the roadway, CHP Sgt. Joseph Johnson said.
No one else was in the car and no other vehicles were involved in the crash, which shut down the roadway's slow lane for nearly three hours on Thursday.
Johnson said Grosso was wearing his seatbelt at the time of the crash and did not appear to be under the influence of drugs or alcohol.
The incident remains under investigation and any witnesses are asked to call the CHP at (925) 646-4980.