Turner was lying, apparently asleep, in a landscaped area next to the regional route when a southbound 2006 Chevrolet Cobalt -- whose driver told investigators he fell asleep at the wheel -- veered off the roadway near Ninth Avenue shortly before 1 p.m.
The sedan hit several road signs, caromed off a large boulder and became airborne, CHP public affairs Officer Eric Newbury said.
When the vehicle landed about 65 feet down the sloping roadside, it smashed into several trees and rolled over Turner, killing him instantly, Newbury said.
The wreck left the motorist, a 19-year-old San Marcos man, with a broken nose and some bumps and bruises.
Turner is survived by his father, who lives on Tanglewood Lane, less than a half-mile from the scene of the fatal accident.