More likely than not. Without restraints it is one of the most lethal forms of auto accident. A Highway Patrol officer, in giving a safety talk where I work, related that the spookiest thing he would see is a car idling by the side of the road with doors open. He would have to look around to find the body of the driver.
According to the source, "Completely ejected occupants make up half of all fatalities resulting from rollover crashes. Compared to occupants who remain in the vehicle, the risk of serious injury is increased by a factor of 20 and the risk of fatality is increased by a factor of 91 in occupants who are completely ejected during a rollover crash. The influence of ejection on injury and fatality risk in rollover crashes dwarfs the effects of all other factors."
"In order to remain inside a rolling vehicle, an occupant must follow a curving path. Newton’s first law of motion dictates that centripetal force is required to make an object follow a curving path. For an occupant in a rolling vehicle, centripetal force can be provided by the seatbelt and/or various structures on the vehicle’s perimeter, such as the door, window glass, pillars, roof rail, or roof. If an occupant is not belted, the vehicle perimeter is often unable to apply sufficiently large centripetal forces to the occupant, and the occupant passes through a window opening and achieves free flight."
Farther down it indicates the risk of ejection, if unbelted, is about 10 to 20% if the vehicle rolls once, 10 to 60% if it rolls twice, and 80% if it rolls more than twice.