Firework smoke was not to blame for one of Britain's worst motorway pile-ups in which seven people died, a coroner has ruled.
But West Somerset coroner Michael Rose said he could not rule out the possibility that the smoke had mixed with thick fog and maybe further reduced motorists' visibility.
Mr Rose said he was considering using his powers to make recommendations to ensure crashes like this did not happen again.
Seven people died and 51 were injured during a series of collisions between 34 vehicles on the south-bound carriageway of the M5 near Taunton, Somerset, on the evening of November 4 2011.
The pile-up happened at 8.20pm - just five minutes after a £3,000 fireworks display operated by Geoffrey Counsell had finished only 200ft away at Taunton rugby club.
Motorists described 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.
Other drivers have described smelling smoke or gunpowder on the motorway.
Grandparents Anthony Adams, 73, and his wife Pamela, 70, from Newport, South Wales; Michael Barton, 67, and his daughter Maggie, 30, from Windsor, Berkshire; battle re-enactor Malcolm Beacham, 46, from Woolavington, Somerset; and lorry drivers Terry Brice, 55, from South Gloucestershire, and Kye Thomas, 38, from Cornwall, all died.
Last year Mr Counsell, 51, was cleared at Bristol Crown Court of breaching health and safety laws on the night of the accident.
During the eight-day hearing, Mr Counsell said he did not consider smoke to be a danger to motorists and insisted it did not drift on to the M5 from his display.
The lorry driver said the build-up of smoke during the display did not give him any concern and there was "no reason whatsoever" to consider smoke as a potential hazard.
But some of the people at the display described seeing a "wall" of smoke drift across the pitch towards spectators.
Others said they thought the display was too large and in the wrong place because of its proximity to the motorway.
Mr Counsell, an experienced operator, agreed with the question that if there was a fault that night, it was with the rules, regulations and guidance.
Mr Rose concluded that all the victims had all died as a result of the road accident caused when vehicles entered an "area of greatly reduced visibility".
"This incident has been described as one of the worst motorway accidents that have occurred in Great Britain and in the town of Taunton the third most serious in the last 100 years," Mr Rose said.
"Was the area of reduced visibility caused by smoke from the firework display, fog or wholly or in part by fog containing a concentration of particles from the firework display?
"I dismiss the possible cause being the smoke from the firework display itself.
"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.
"Varying plumes of smoke from the firework display - particularly that caught below the inversion area - may have mixed with the fog and therefore I cannot rule out the possibility that they drifted under the inversion to the motorway and may have added to the intensity of the obscuration."
Today's hearing had been delayed by two hours because of a car fire on the M5 near Taunton.
Mr Rose said he had considered all the published literature for firework display organisers and "none contained a warning against using fireworks in foggy conditions".
He said he was considering making a report to prevent future deaths but first wanted to hear from the Department of Business, Innovation and Skills, the Health and Safety Executive, the Highways Agency, the British Pyrotechnists Association and Taunton Deane Borough Council.
The coroner said he would consider whether firework operators need to be trained in the problems of running displays during fog, whether there should be a minimum distance from major roads for holding public displays, whether public displays should be licensed and whether fog detection devices should be installed on the M5 in the Taunton area.
Speaking after the hearing, Tonia White, the daughter of Mr and Mrs Adams, said she was pleased with Mr Rose's conclusions.
"I think the coroner's recommendations are a big step forward," she said.
"That's we were looking for - moving on and making sure this doesn't happen to anybody else.
"We want to see the licensing of firework displays, the literature changed to include the visibility problems that have arisen.
"I think the coroner has gone as far as he could go and I think we have had our questions answered.
"As the coroner said he cannot rule out the fact the smoke didn't play a part and that's quite important to us.
"The coroner has summed it all up for us in a lot of ways."
Her sister Elaine Adams added: "I think the outcome today has been very, very positive and from my point of view I would like to see more legislation around fireworks, more so around operators and the way they are allowed to go and set up businesses.
"They certainly need more training and they need people who are accountable. I wouldn't say I am 100% confident it will happen but I certainly hope that is the way it will go."