The driver who rescued the one-year-old boy Friday evening returned him to his mother and called police in Brigham City, Utah.
The unidentified mother told police she was resting and had put the infant in a crib to sleep.
Police say her daughter took the baby out to play and he somehow got out of the backyard.
They say the baby crawled less than a block when he was found in the lane closest to the sidewalk.
Police say charges are unlikely, but the case has been turned over to the Division of Child and Family Services.
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