Daniella Tunmore, 23, was driving with a friend along the A505 between Baldock and Royston in Hertfordshire on Saturday when she saw a shadow blot out the sun.
It was a convertible BMW 330, which had collided with the central reservation before tumbling through the air and crash landing on the carriageway.
The driver, Leigh Windsor from Letchworth, had been taking his recently-purchased car out for a spin with the top down in the spring weather.
He was thrown out of the car and left seriously injured on the road, but miraculously his friend in the passenger seat was able to walk away from the crash.
Daniella, who was on her way from her home in King's Lynn for a weekend away in London, said: 'I just saw this shadow which seemed to blot out the sun. Suddenly I realised it was the underside of a car. I pulled over into a lay-by and somehow ran across four lanes of traffic to get to the driver, who was lying in the road.
'I don't know how I didn't get knocked over but something in my brain just kicked in.
'Next minute I was kneeling beside him. I didn't think he would be breathing – I didn't think anyone could survive that.
'I asked him his name and address and date of birth. He kept going in and out and I thought he was going to go, but I just kept talking to him, telling him to stay awake and he kept coming back round.
'I kept telling him everything was going to be OK, even though I really didn't know how it could be.
'Suddenly there were paramedics and ambulance crews everywhere and they were telling me I was doing a great job and was a hero, and that he wouldn't be alive if it wasn't for me.'
MORE: Daniella didn't have time to stop and think before she rushed to help Leigh Windsor
Daniella suffers from asthma and anxiety and was recently diagnosed with a tumour in her leg. She usually relies on crutches to get around.
She said: 'I usually find it difficult to get around, but in that moment I didn't notice. Something just happened and I knew I had to get there.'
Mr Windsor remains in intensive care at the Lister Hospital in Stevenage.
His family have said he is in the very best hands and that they want to pass on their thanks to Daniella for her actions, which they say saved his life.
Witnesses who saw the crash should call Herts police on 101.
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