Driving while sad is far more likely or result in a crash than being tired or using a mobile phone a new study has shown.
Many motorways now display signs which warn motorists to pull over if they are feeling tired, but new research suggests that a healthy emotional state is far more important for safe driving.
Feeling sad, angry or agitated behind the wheel raises the risk of an accident by nearly ten times. In contrast, fatigue makes a crash three times more likely while talking on a mobile only doubles the risk.
In fact talking on a mobile phone was found to be no more distracting for motorists than fiddling with the radio or air conditioning, American researchers from Virginia Tech found.
And although parents may feel that having a child in the back seat is distracting, interacting with a youngster while driving actually lowers the chance of a smash. Likewise applying make-up or eating food was found not to cause significant problems.
"We have known for years that driver-related factors exist in a high percentage of crashes, but this is the first time we have been able to definitively determine the extent to which such factors do contribute to crashes," said Dr Tom Dingus, lead author of the study and director of the Virginia Tech Transportation Institute.
"Driving while observably angry, sad, crying or emotionally agitated increases the risk of a crash by 9.8 times compared with model driving.
"Other factors posed much lower risks than previously thought, or even have protective effect, such as interacting with children in the rear sat."
To find out what distracts drivers the most, researchers used in-car video cameras to monitor the motoring habits of more than 3,500 people across the US and compared it with crash data. They found that more than half of motorists engage in some kind of risky behaviour every time they take to the road. During the study period there were more than 900 accidents where either people were injured or property was damaged.
Reaching for an item, was found to raise the risk of a crash by nine times, although picking up a mobile was only six times. Dialling a number raised the chance of an accident by more than 12 times.
In contrast, chatting to a passenger barely raised the risk at all, and dancing to music in the car seat was found to have no impact.
Unsurprisingly, the biggest risk factor for a crash was consuming drink or drugs behind the wheel, which raised the chance of having an accident more than 35 times.
However texting or browsing online was found to be a big problem, raising the risk of a smash by six times. Researchers said it was likely to be the 'single factor' which had caused an increase in crashes in the US in recent years.
"An increased need or want to remain connected and productive via cell phones has the potential to escalate distraction-related crashes in the future," added Dr Dingus.
They believed that more than 30 per cent of crashes could be avoided if all distractions were removed from cars. A recent poll by the AA found that 38 per cent of motorists admit to being distracted while driving while 12 per cent believe it is find to use a mobile for short calls.
Edmund King OBE, AA president said: "It is tragic that lives are being lost through needless distractions from a passenger or hand held mobile telephone to text, call and now even video on the move. In a recent poll members tell us they see other drivers using hand held mobile phones on nearly every journey.
"It seems our psyche is such that, when driving, many find it hard to single task. Being self-disciplined by quickly shutting out distractions is critical to staying safe on the roads."
The RAC said that government statistics which show that just four per cent of road deaths were caused by distractions, were 'dramatically downplaying' the effects of distractions.
RAC spokesman Simon Williams said: "The fact this was a long-term study using video to observe drivers' behaviour at the wheel makes it particularly significant and its findings therefore need to be heeded as they show just how prevalent distractions in accidents are.
"Not surprisingly the research shows just how dangerous the use of hand-held phones when driving can be, but it also highlights the dangers of other forms of distraction such as interacting with passengers, reaching for an object and the impact of drowsiness and fatigue.
"All it takes is a moment's loss of concentration from reaching for a mobile phone, fiddling with a sat-nav or adjusting an in-vehicle device to lose control and find yourself involved in a life-changing accident."
Gary Rae, campaigns director for Brake, the road safety charity, added: "This research confirms our own evidence and experience, that it's distraction from any source in a vehicle that poses the real danger to drivers.
"Virginia Tech's 'distraction list' shows that dialling, reaching and texting on mobile phones score highly. That's why we advise drivers to switch of their devices and put them out of reach."
The research was published in the Proceedings of the National Academy of Science.
The Telegraph, London