Seven people died and 51 injured during a series of crashes involving 34 vehicles on southbound carriageway of the M5 near Taunton, Somerset, in 2011.
Motorists have told of entering a wall of thick fog -described by some as being like a "white curtain", "emulsion" or "custard being poured from a jug" -and were unable to prevent multiple collisions.
The collision happened at 8.20pm on November 4, just five minutes after a £3,000 fireworks display finished 200ft away at the Taunton rugby club ground.
Grandparents Anthony Adams, 73, and his wife Pamela, 70, from South Wales; Michael Barton, 67, and his daughter Maggie, 30, from Berkshire; battle re-enactor Malcolm Beacham, 46, from Somerset; and lorry drivers Terry Brice, 55, from Gloucestershire, and Kye Thomas, 38, from Cornwall, all died.
Last year firework contractor Geoffrey Counsell, 51, who had been operating the display at the rugby club, was cleared at Bristol Crown Court of breaching health and safety laws on the night of the accident.
West Somerset Coroner Michael Rose told the inquest: "I dismiss the possible cause being the smoke from the firework display itself."
Mr Rose said: "There were in the Taunton area pockets of very dense fog - in some cases of such density that only occur possibly once or maybe twice in a decade."
He added "Part of this area of fog may have extended to the perimeters of Taunton RFC and, in all probability, to a height of 25ft or more to reach the motorway."