The owners of a coach which overturned on a slip road to services on the M1 are "obsessed" with safety, its chief executive has said.
The coach hit the kerb, a tree and lamppost before overturning
Richard Bowker of National Express said drivers have to meet "tough standards" and pass random drink-drive tests.
Mr Bowker also said the firm was fully co-operating with police investigating the crash at Newport Pagnell, Bucks.
The driver of the coach, a 34-year-old man from West Bromwich, has been arrested on suspicion of drink-driving.
Thirty people were injured, including six who were seriously hurt, in the crash on Monday afternoon, as the coach travelling from Birmingham to Stansted airport entered the services on the southbound carriageway of the motorway.
Speaking on BBC Radio 4's Today programme, Mr Bowker said: "To be even considered to drive for National Express you have to pass very rigorous tests and the recruitment test, particularly around drugs and alcohol, is very tough.
"Once you are driving for National Express then the standards to which you have to perform are higher in terms of drink than the legal requirement.
"We randomly test and that means that it is extremely likely that you will be caught at some point."
The single-decker coach clipped a kerb, hit a lamppost and a tree and toppled over on to the driver's side in the accident.
Police were called by service station managers and four ambulances went to the scene.
The six with serious injuries were taken to hospitals in Milton Keynes, Northampton and Oxford.
Emergency services said 11 people, described as "walking wounded", suffered minor injuries and 17 were discharged at the scene.
The driver, who had to be cut free from the wreckage, was breath-tested at the scene and is now under arrest in hospital on suspicion of drink driving.
He is being treated for an injured arm and cracked ribs.
Three of his passengers are still being treated at Northampton hospital. They are a woman, 45, from Church Stretton near Shrewsbury, Shropshire; a man, 60, from Goussanoville in France and a man, 28, from Coventry.
A woman, 57, from Victoria, Australia, and a man, 20, from Gdansk, Poland, are being treated at Milton Keynes General Hospital.
A further two passengers - a woman, 39, from Worcester and a man, 61, from Kenilworth, Warwickshire, are being treated at the JRII in Oxford.
Mr Bowker continued: "This is now a police investigation and obviously we need to learn the detail of this ourselves as quickly as possible.
"It is so incredibly rare for us to have an incident like this... travelling by coach is far, far safer than travelling by car - it was last week, it will be this week and it will be in the future."
The coach had left Birmingham at 1400 BST.