Samuel Berkley was found unconscious on the hard shoulder of the M67 eastbound at junction 3, Hyde, Greater Manchester, during Friday afternoon rush hour.
He was taken to hospital but died early on Saturday from what police called “unsurvivable” injuries.
Samuel attended Audenshaw School and was a “talented footballer” who had started playing for Hattersley Football Club, Greater Manchester Police (GMP) said.
The teenager, who lived at home in Denton, Greater Manchester, with his parents and 20-year-old sister, had recently become an uncle to his 21-year-old brother’s daughter.
The force said in a statement: “His family said they are completely heartbroken by their loss and although nothing will ever replace losing Sam they hope everyone remembers him for the fun, outgoing and friendly boy he will always be.”
Police were called at around 5.25pm on Friday to reports of an unconscious boy on the side of the motorway, GMP said.
Samuel was found in a critical condition and taken to hospital.
The driver of the grey BMW stopped at the scene and spoke with police.
Sergeant Lee Westhead, of GMP’s Serious Collision Unit, said: “My thoughts are with the victim’s family who are being looked after by specially trained officers at this difficult time.
“Whilst we work to uncover how this has happened and piece together the moments before the collision, I am asking the public to help in any way that they can.
“If you were in the area at approximately 5.25pm on December 1 and saw a pedestrian on the motorway, or have dashcam images, please get in touch and share this information.”
Anyone with information should contact GMP on 101 or Crimestoppers anonymously on 0800 555 111.