Video captured by Chad Mock and posted to Facebook shows what happened.
The car seat falls out of the rear driver's side of a sedan as it takes a right curve.
Mock, who was driving behind the sedan stops his car and runs to the child's side. Waiving his arms in the air, he stops an oncoming van from getting too close to the child.
"If it didn't happen in front of me, I'd never have believed it," Mock said on Facebook.
The child was not injured, and the child's parents came back as soon as they realized what had happened.