Video captured by Chad Mock and posted to Facebook showed what happened.
The car seat apparently fell out of the rear driver's side of a sedan as it took a right curve.
Mock, who was driving behind the sedan, stopped his car and ran to the child's side.
Waving his arms in the air, he stopped 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 wrote on Facebook.
The child was not injured, and the child's parents came back as soon as they realized what had happened.
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