CHICAGO - Kids may be safest in cars when grandma or grandpa are driving instead of mom or dad, according to study results that even made the researchers do a double-take.

"We were surprised to discover that the injury rate was considerably lower in crashes where grandparents were the drivers," said Dr. Fred Henretig, an emergency medicine specialist at Children's Hospital of Philadelphia and the study's lead author.

Previous evidence indicates that car crashes are more common in older drivers, mostly those beyond age 65. The study looked at injuries rather than who had more crashes, and found that children's risk for injury was 50 percent lower when riding with grandparents than with parents.

The results are from an analysis of State Farm insurance claims for 2003-07 car crashes in 15 states, and interviews with the drivers. The data involved nearly 12,000 children up to age 15.

Henretig, 64, said the study was prompted by his own experiences when his first grandchild was born three years ago.

"I found myself being very nervous on the occasions that we drove our granddaughter around and really wondered if anyone had ever looked at this before," he said.

Reasons for the unexpected findings are uncertain, but the researchers have a theory.

"Perhaps grandparents are made more nervous about the task of driving with the 'precious cargo' of their grandchildren and establish more cautious driving habits" to compensate for any age-related challenges, they wrote.

The study was released online Monday in the journal Pediatrics.

Northwestern University Professor Joseph Schofer, a transportation expert not involved in the research, noted that the average age of grandparents studied was 58.

"Grandparents today are not that old" and don't fit the image of an impaired older driver, he said. "None of us should represent grandparents as kind of hobbling to the car on a walker."

Grandparents did flub one safety measure. Nearly all the kids were in car seats or seat belts, but grandparents were slightly less likely to follow recommended practices, which include rear-facing backseat car seats for infants and no front-seats. But that didn't seem to affect injury rates.

Only about 10 percent of kids in the study were driven by grandparents, but they suffered proportionately fewer injuries.

Overall, 1.05 percent of kids were injured when riding with parents, versus 0.70 percent of those riding with grandparents, or a 33 percent lower risk. The difference was even more pronounced - 50 percent - when the researchers took into account other things that could influence injury rates, including not using car seats, and older-model cars.

Kids suffered similar types of injuries regardless of who was driving, including concussions, other head injuries and broken bones.

The study does not include data on deaths, but Henretig said there were very few. It also lacked information on the types of car trips involved; for example, driving in busy city traffic might increase chances for crashes with injuries.

Schofer, the Northwestern professor, said other unstudied circumstances could have played a role. For example, grandparents could be less distracted and less frazzled than busy parents dropping their kids off at school while rushing to get to work or to do errands. Driving trips might be "quality time" for older drivers and their grandchildren, Schofer said.