The Orange County Register reports Fullerton police Sgt. Andrew Goodrich said the boy climbed into the driver's seat of the SUV and put it into gear while his mother stood outside it in the parking lot of a restaurant about 4:30 p.m. Thursday.
The vehicle left the parking lot of the Summit House restaurant, sideswiping at least one vehicle, and traveled south along State College Boulevard through several intersections during rush hour, crossed into lanes of oncoming traffic, hit a tree and flipped over at least once, landing upright.
"As I looked, this little boy was standing on the seat, and he had this look of terror in his eyes," witness Danny Guerra told KTLA-TV, Los Angeles. "He was trying to control that thing."
Goodrich said the boy was taken to a hospital for treatment of what were believed to be minor injuries.
The Register said the Fullerton Police Department's traffic unit and family crimes detectives were investigating.