Ciara Neno said that she was driving with her husband when a "black fog" descended suddenly on the motorway causing an Iceland truck in front of them to "literally disappear".
Mrs Neno described how she managed to brake and avoid the truck before hearing sickening "thumps" behind them as other motorists were not so lucky.
She said: "It was quite frankly the scariest night of my life and we are extremely lucky to be alive.
"A black fog came down very very fast and the Iceland truck in front of us literally disappeared.
"We managed to brake and not hit the truck but the carnage had started and all we heard was 'thump thump thump' and we were waiting to be hit and end up under the lorry.
"I got onto the emergency services and my husband started dragging people from smoking cars.
"The noise and the smell was horrendous and there was several explosions as the fires took hold.
"I cannot stress how blessed we feel to be able to walk away from this, too many others were not so lucky."
Describing conditions on the section of the motorway at the time of the accident, Ciara from Weston-Super-Mare told Sky News: "There was a very thick fog bank which lifted as quickly as it came.
"There were no fireworks at the time - just music from the rugby club.
"It was horrifying to hear that speculation was rife that the fog only came after the accident and that driver distraction caused it.
"As we were near the very start of the crash I can categorically state that this was not the case."