Michael South was fatally injured in the collision after seizing control of the ambulance on the A64 near York.
An official investigation found a series of failings led to a decision to transport him in an ambulance without a specialist crew.
READ: Provisional trial date set for Sheffield woman, 83, accused of murdering husbandIts report said he had suffered from a severe depressive disorder for more than two decades including several relapses in the months leading to the tragedy.
He was treated at St James’s Hospital in Leeds for self-inflicted stab wounds sustained three days before his death on April 10, 2015.
The father-of-one, 40, of York, was transferred to his home city for treatment for his illness but as the ambulance neared Askham Bryan he tried to grab the steering wheel, forcing the driver to pull over.
Fearing for their safety, both ambulance staff fled the vehicle but the keys were left in the ignition.
Mr South drove off, driving erratically for 15 miles along the A64 before swerving into a bus near Flaxton. He died at the scene. Six people were injured.
The report ordered by NHS England said his illness had effectively been untreated prior to his transfer and as a result his “safety and the safety of those with whom he had contact was not ensured”.
READ: Doncaster motorcyclist seriously injured in collision diesThe independent investigation team said information about the risk he posed to himself and others “was not properly conveyed, recognised or managed”.
Staff did not see key details in his medical notes including a previous bid to grab the wheel of a car when he was ill.
It said treatment of mainly superficial physical injuries was given “significant priority” over his mental health needs.
He was twice assessed by trainee psychiatrists but his remaining care was left to nurses on a busy ward with no specialist training in mental illness, illustrating “a divide between mental and physical health care which is a problem throughout the NHS and which requires action at a national level”.
An inquest recorded an open verdict and, in submissions to the investigation, NHS managers said they did not accept his death could have been foreseen.
But the inquiry team said there was evidence “for the very real potential for an impulsive, potentially fatal action” to be predictable.
The tragedy could have been prevented if he had travelled in a secure ambulance with trained mental health staff.
In response to six recommendations, NHS leaders said “significant changes” had been made.
A range of psychiatric staff were available 24/7 to deal with hospital patients in Leeds, important clinical information was better shared and new training was given to hospital staff dealing with patients with mental health problems.
READ: Furious mum finds angry note saying daughter ‘does not look disabled’ on her car at MeadowhallNew procedures were in place to ensure patients were safely conveyed to mental health units.
Yvette Oade, chief medical officer at Leeds Teaching Hospitals NHS Trust, said it had “listened, learnt and taken action since these sad events occurred”.
“We continue to find ways to further improve our care for patients with both mental health and physical health needs,” she said.
Cathy Woffendin, of mental health provider Leeds and York Partnership NHS trust, said they were “deeply sorry” and many lessons had been learned.
She added: “It was such a rare incident and there was no warning as to the events that would eventually unfold that day, which meant a life was tragically lost and continues to have a significant impact on family members and the staff involved.”