One lorry driver was twice the drink drive limit and another was on a hands-free call with the vehicle on cruise control when a crash on the M1 killed eight people, a court heard.
Polish national Ryszard Masierak, 32, allegedly stopped for 12 minutes in the slow lane in the early hours of August 26 2017, despite there being miles of hard shoulder available.
His lorry caused an obstruction on the motorway forcing others to slow before moving around him, the court was told.
As minibus driver Cyriac Joseph waited with his hazard lights on for a chance to go around the heavy goods vehicle, a second lorry driven by Briton David Wagstaff, 54, smashed into the back of the van, forcing it into and under Masierak's lorry, jurors heard.
Opening the case, prosecutor Oliver Saxby told Reading Crown Court it was "an entirely avoidable collision, with the most catastrophic and tragic of consequences".
Addressing jurors he said it was "caused by the dangerous driving of these two defendants".
"In Mr Masierak's case, under the influence of alcohol, parking up in the slow lane of the M1, an act as flagrant as it was dangerous.
"What on earth did he think he was doing?
"And in Mr Wagstaff's case, on a hands-free call, his lorry on cruise control and he on auto-pilot, completely failing to notice what was ahead of him in full view for some time, inattention on a gross scale."
Mr Joseph and seven of his Indian passengers, en route to London from Nottingham to catch a coach to Disneyland, were killed in the collision on the Saturday of the Bank Holiday weekend.
The other fatalities, five men and two women, were Panneerselvam Annamalai, Rishi Ranjeev Kumar, Vivek Baskaran, Lavanyalakshmi Seetharaman, Karthikeyan Pugalur Ramasubramanian, Subramaniyan Arachelvan and Tamilmani Arachelvan.
Four other mini-bus passengers were injured in the collision on the southbound M1 near Milton Keynes.
The court heard Wagstaff, from Stoke, was driving at 56mph, and did not brake or decelerate before ploughing into Mr Joseph's van just after 3am.
Masierak and Wagstaff both deny eight counts each of causing death by dangerous driving, and four counts each of causing serious injury by dangerous driving.
Masierak faces a further eight charges of causing death by careless driving, while over the prescribed alcohol limit.
It is alleged that he had 55 micrograms of alcohol per 100 millilitres of breath.
Some family and friends of those killed and injured in the crash sat in court for the first day of the trial.
Masierak, of Evesham, Worcestershire, who appeared in court wearing a grey sweatshirt, was assisted in following proceedings by a police translator.