A massive motorway pile-up caused by smog from a nearby fireworks display in Somerset may have been avoided if the organiser properly fulfilled his health and safety duties, a jury has heard.
Seven people died and 51 were injured after 34 cars collided when a section of the M5 was suddenly "engulfed in thick smog" in November 2011.
Bristol Crown Court heard that the smog "built up and built up" during and after a fireworks display at Taunton Rugby Club and drifted across the northbound carriageway.
Motorists had "no chance" of escaping the series of crashes, which began just six minutes after Geoffrey Counsell's 15-minute display - involving 1,500 shots.
Counsell, 51, of Ashill in Somerset, is charged with breaching health and safety regulations by failing to ensure the safety of others, which he denies.
Prosecuting, Peter Blair QC told the jury that Counsell boasted of a "stringent safety regime" but could not fulfil his responsibilities operating as a "one-man band" during the display.
"We say people were exposed to an appreciable risk to their personal safety that night as a consequence of the way Mr Counsell prepared for and then went about his business," Mr Blair said.
"We say that ensuring the safety of people using that motorway needed particular care if you were going to undertake a big firework display involving the use of a substantial volume of these hazardous explosives, which generated a lot of smoke.
"Smoke can have an appreciable effect on reducing visibility - that should be obvious. It didn't take long from the build-up of smoke to the dramatic loss of visibility.
"We say, if he had adequately undertaken his health and safety properly, some or all of the consequences which unfolded on the 4th of November may have been avoided."
Mr Blair said the prosecution and defence would disagree whether there was a "substantial risk" to safety from the smoke generated by the fireworks display.
"We don't have to prove he personally foresaw the extent of those consequences on the motorway but he had a duty of responsibility and he breached it," Mr Blair added.
The jury was told that Counsell, operating under the name Firestorm Pyrotechnics, was contracted to carry out his "showstopper" £3,000 show, called the Jupiter Display.
In a leaflet promoting his business, Counsell described the 15-minute display as the "biggest and most lavish" piece of "unforgettable" pyrotechnics he offered.
Counsell, who claimed he had 20 years of experience, said "highly trained" experts would carry out his shows "while working to the highest possible safety standards".
Mr Blair said the claims were "an exaggeration" and "sales talk", with Counsell describing himself as "managing director" when he was in fact a "one-man band".
Counsell was awarded certificates in pyrotechnics in 1999 and 2008, after studying courses featuring modules including health and safety, the court heard.
Extracts from one course, run by the British Pyrotechnists Association, describing the importance of proper risk assessments were read to the jury.
"In the particular circumstances of this firework display on the 4th November 2011, we say Mr Counsell failed to prepare adequately for it," Mr Blair said.
"The prosecution don't accept that he did all that was reasonably practical."
Mr Blair said Counsell attended Taunton Rugby Club three weeks before the show to discuss matters with employee Colin Bentley.
"He handed over some paperwork, which unfortunately Mr Bentley didn't keep," Mr Blair said.
"That was, as Mr Bentley understood, a document that Mr Counsell had prepared assessing the risks for this planned fireworks display.
"Mr Bentley's evidence was that he wasn't handed sketches of the site."
After the tragedy, police officers visited Counsell and discovered two sketches of the site at his home - though he later claimed these were drawn up following the display.
Mr Blair said Counsell told officers he only had one copy of his original risk assessment, which was handed to Mr Bentley.
One sketch estimated the distance between the firing zone to the motorway - later calculated at around 220 yards (200m) - to be 547 yards (500m).
A second did not feature the motorway or a nearby railway line at all, Mr Blair said.
"They have fundamental misconceptions of the site, as to how far away the motorway was, as to how the fall-out might affect the motorway," Mr Blair said.
The paperwork found at Counsell's home contained a list of tasks, all of which had Mr Counsell's name typed beside them, the prosecutor added.
"We say Mr Counsell compromised his ability to ensure the safety of others by conducting a display of this magnitude without adequate assistance," he said.
"This was the largest type of display that he advertised. A one-man band doing everything himself; no-one watching, monitoring, spotting the problems to assist his single pair of eyes. He was about to ignite 40kg (88lb) of explosive devices.
"He had one pair of legs, one pair of arms and one pair of eyes."
Mr Blair said his lack of assistance had serious consequences as the thick smoke began to build up during the display.
"He didn't react as he should have, by stopping what he was doing, by reviewing the potential consequences, given that the traffic was trundling past."
Representing Counsell, Adrian Darbishire QC, told the jury his client was well respected in the fireworks field.
"He was good," Mr Darbishire said. "People trusted him. There were no problems with his displays.
"There's nothing to say about Geoffrey Counsell, he was just an ordinary man who had a regular job and an interest that he loved.
"He was, I am sure, no different from those people involved in the crash. They were ordinary people, each of whom had their own enthusiasms and loves."
Mr Darbishire said the trial could not examine how the seven people came by their deaths, only whether Counsell breached the Health and Safety at Work Act.
Anthony and Pamela Adams, Maggie and Michael Barton, Malcolm Beacham, Terry Brice and Kye Thomas died and 51 people were injured, including some seriously, in the pile-up.
"This is not an inquest, this is not a public inquiry, this is a trial," Mr Darbishire said.
"The prosecution's case is that the crash was caused by an area of reduced visibility.
"The Crown suggest that the reduced visibility was caused by fireworks at the display.
"The defence say that the evidence you will see just doesn't establish that possibility with any real certainty."
Mr Darbishire said witnesses would say they saw smoke from the display actually drifting away from the motorway.
"You will see many witnesses are absolutely clear that what they saw and indeed in some cases what they drove through was not smoke, nor smog but fog."