A young mom whose car flipped over on a Long Island highway Sunday miraculously emerged from the wreckage relatively unscathed — only to be struck and killed after she ran back to retrieve some belongings.

Brittany Leith, 25, of North Ba­by­lon had climbed out from under her vehicle after it struck the center median and flipped as she was traveling west on the Southern State Parkway near Exit 36 at about 3:54 a.m., state police said.

The single mom managed to walk to the right shoulder of the roadway with the help of other motorists who stopped to aid her.

But after getting to safety, she told the good Samaritans she needed to run back across the busy highway to retrieve her property from inside her car, according to police.

They begged her not to go, to no avail.

As Leith was making her way to her upended car in the left lane, a passing vehicle fatally struck her.

A state police car arrived a short time later — and was struck from behind by another passing vehicle.

A police officer who was inside the rammed cruiser was taken to Good Samaritan Medical Center in West Islip, but officials did not release any additional information on the injured cop’s condition.

Anna Leith, Brittany’s distraught grandmother, said she now has to face the difficult task of telling the woman’s young daughter, Zoey, that her mom isn’t coming home.

“She has a 3-year-old daughter . . . who doesn’t have a mother anymore,” said Anna Leith, 75.

“She was the best mother in the world. She loved Zoey more than anything,” she added.

The grandmother said Brittany and Zoey had been living with her since the young mom got divorced. Brittany was on her way home from a friend’s home when she was killed, the grandmother said.

Anna Leith says she thinks the people who were on the scene with Brittany should have done more to keep her from running back into traffic.

“I was told someone was there to help her out of the car, but she could have hit her head and been out of it,” the grandmother said. “Somebody should have held on to her and not let her run back into the street.

“This should not have happened,” she added.

The parkway was shut down for more than seven hours as police investigated both accidents. They finally reopened the roadway around 11:30 a.m.