Drivers increase their crash risk nearly tenfold when they get behind the wheel while angry, sad, crying or emotionally agitated, a new study has found.

Researchers found that drivers more than double their crash risk when they choose to engage in distracting activities that require them to take their eyes off the road, such as using a handheld cell phone, reading or writing, or using touchscreen menus on a vehicle instrument pane.

Drivers engage in some type of distracting activity more than 50 per cent of the time they are driving, they said.

For the study, researchers from the Virginia Tech University in the US considered 905 higher severity crashes involving injury or property damage in the data set and found that, overall, driver-related factors that include fatigue, error, impairment, and distraction were present in nearly 90 per cent of the crashes.

The study represents the largest naturalistic crash database available to date, with more than 1,600 verified crash events ranging in severity from low, such as tire and curb strikes, to severe, including police-reportable crashes.

"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 - using high-severity, crash-only events that total more than 900 - the extent to which such factors do contribute to crashes," said Tom Dingus from Virginia Tech.

Travelling well above the speed limit creates about 13 times the risk, and driver performance errors such as sudden or improper braking or being unfamiliar with a vehicle or roadway have an impact on individual risk.

Researchers found several factors previously thought to increase driver risk, including applying makeup or following a vehicle too closely, actually had a lower prevalence in the naturalistic driving study, meaning they were minimally present or were not present at all in the crashes analysed.

Factors such as interacting with a child in the rear seat of a vehicle were found to have a protective effect, or had a risk lower than the base risk value.

All factors analysed were compared to episodes of model driving, or episodes in which the drivers were verified to be alert, attentive, and sober, marking the first known time such a comparative analysis has been made, researchers said.

"Our analysis shows that, if we take no steps in the near future to limit the number of distracting activities in a vehicle, those who represent the next generation of drivers will only continue to be at greater risk of a crash," said Dingis.

The findings were published in the journal Proceedings of the National Academy of Sciences.