The Royal Navy Petty Officer was with wife Claire, 24, when they saw the vehicle spin into an M5 barrier.
Martin, 26, pulled up and ran across the busy motorway in darkness to help the Khan family, stranded in the outside lane. Martin, of Yeovil, Somerset, said: "There was a lot of debris and smoke."
First he guided the stunned mum, standing with a baby in her arms, to safety.
Then he raced back to aid dad Darius, lifting his three-year old son from the back seat just seconds before another vehicle smashed into it. Then three more cars piled into the wreckage, completely destroying the Khans' motor and also crashing into Martin's car.
Grateful Darius said he will be forever indebted to Martin. He added: "I don't know what would have happened if he hadn't been there."