Just over 100 vital minutes elapsed between the moment that the Princess's body, unrestrained by a seat belt, was tossed crazily inside Dodi Fayed's Mercedes and the moment that her prone form was wheeled into Pitie-Salpetriere hospital three miles away.
The delay could have meant the difference between her death and some kind of continuing, though probably grievously damaged, life, according to two US journalists, who claim to have provided the first comprehensive account of the crash that killed her.
A central claim of Death of a Princess: An Investigation, by Tom Sancton and Scott Macleod of Time magazine is that the apparent cause of the Princess's death, a torn pulmonary vein, might have been successfully repaired had she been got to hospital earlier. They quote a US heart surgeon, John Ochsner, retired head of surgery at Ochsner clinic in New Orleans, who said that the pulmonary vein cannot have been seriously torn or she would have died immediately at the scene from loss of blood. The fact that she survived long enough to reach hospital suggests that the tear was minor. "With that lesion if you can get them in a hospital and on a heart-lung machine early enough, you can save them. But time is of the essence," Dr Ochsner said.
The book alleges that there was a delay of an hour and three-quarters before the Princess reached hospital, due to the French practice of providing as much expert care at the roadside as possible. It is unclear, however, how much of that time was spent releasing her from the Mercedes in which she was trapped by one leg. Firemen eventually cut her free.
The French emergency service, Samu, takes anaesthetists and experts in resuscitation to the scene of an accident, sometimes by helicopter, and is highly regarded in Britain. Efforts would have been made to reduce the shock to the Princess's body, caused by loss of blood and fluids, and immobilise her for the journey to hospital in case of spinal injury. She was apparently given a blood transfusion at the roadside.
Dr Frederic Mailliez, the emergency doctor who was first on the scene, said: "The philosophy here is to try to stabilise the patient as much as you can because travelling with this kind of status can be very dangerous for a patient. So we try to restore a little bit of blood pressure and some other things before we start to drive."
In essence, this is little different from the approach that would be taken in England. Attitudes to emergency care have changed in the last decade with a growing recognition that providing immediate help at the roadside can increase the chances of survival for seriously injured patients over the old policy of "scoop and run" - picking accident victims off the roadside and rushing them to hospital. That has meant that most emergency ambulances in Britain now carry a paramedic specially trained in resuscitation. But efforts at resuscitating or stabilising a patient have to be strictly limited. The crucial proviso is that patients must still be got to hospital inside the so-called "golden hour".
The "golden hour" is the time within which medical or surgical intervention by a specialist trauma team has the greatest chance of saving life. If more than 60 minutes has elapsed by the time the patient reaches the operating table, the chances of survival fall sharply.
Dr Andrew Mason, spokesman for the British Association of Accident and Emergency Medicine and a consultant at Medway Hospital, Kent, said he had confidence in the French system of emergency care but agreed it would be unusual for doctors to treat a patient at the scene of an accident for as long as an hour and three-quarters. "If it is true, it does seem a bit strange. We tell our ambulance crews to spend 20 minutes at the outside stabilising or resuscitating patients at the scene. If they are not making an impression they should scoop and run."
Reports that the ambulance carrying the Princess was ordered to drive slowly to avoid worsening her condition and that braking and accelerating could be "bad for blood pressure" were also puzzling, Dr Mason said. If a spinal injury was suspected - a possibility in the Princess's case - the ambulance driver might be asked to proceed slowly, for fear of aggravating it. But where there is significant blood loss, speed is of the essence.
Dr Mason said: "If someone is conscious and dying of loss of blood the best you can do is resuscitate them at the scene and transfer them to a specialist trauma centre within the golden hour. The more you can lessen the surgical shock, caused by the loss of blood and fluids, the better. But you have only got a limited period - 60 minutes - to reverse or stabilise the patient's condition. If you miss that you have lost the patient."
However, Dr Mason said the allegations in the book awaited confirmation and the exact details of what happened were unclear. "My information is that Samu is extremely good. If they were there, Princess Diana was getting the best treatment she could have got."
Even if the Princess had got to hospital sooner the chances of saving her would have been slim, given the extent of her injuries. If the tear in her pulmonary vein had been minor it might have been possible to repair it. But it is highly unlikely that was the only place from which she was losing blood.
Dr Tim Coats, a senior lecturer in accident and emergency care at the Royal London Hospital, Whitechapel, said: "She would have been bleeding from many places internally and finding where she was bleeding from is a problem in itself. In this kind of severe incident, it is very difficult to spot because there are so many things happening." He added: "If someone's heart is not beating the primary concern of the doctor is to get it beating again, not surgery. If you can't get the pulse back after five minutes the person is dead."
In London and some European and American cities, helicopter ambulances can speed the delivery of seriously injured road accident victims to hospital. There is little evidence that they improve survival, however. Studies in London have shown that a road ambulance with siren and blue light can reach the scene of most accidents as quickly as the helicopter. The cost of maintaining the London helicopter ambulance, with its own heliport on the roof of the Royal London Hospital at Whitechapel, is equivalent to that of 13 road ambulances.
At 1am on a summer night in central Paris, a helicopter is unlikely to have improved Princess Diana's chances of survival over a conventional ambulance. Given the ferocity of the crash and the extent of her injuries, only a miracle could have saved her. But there is here a collective longing to re-write history and there are plenty more theories that could be built around what might have been.