Almost a quarter have felt drowsy or have nodded off, while 18 per cent said stress "affected" their driving.
The survey by the Royal Society for the Prevention of Accidents (Rospa) also found that 17 per cent of those who drive for a living are concerned about the safety implications of the hours they spend on the road.
Despite the revelations, motoring organisations gave the findings only a qualified welcome.
"We should get this into perspective. Company car drivers are no less safe than other motorists, they just drive longer," said an RAC spokesman.
However, many experts support the study. "People forget how stressful driving can be. After all if you work in a factory for long hours and become unsafe, your employer is required to take you off the job. But get into a car and nobody bothers," said Jim Horne, who runs the sleep laboratory at Loughborough University.
Rospa believes that people driving as part of their job are likely to be linked with more than a quarter of Britain's 3,600 annual road fatalities.
"Accident rates among fleet vehicle users have been estimated to be 30- 40 per cent higher than among private car users, but driver training can reduce accident rates by almost twice that amount," said Rospa occupational safety adviser Roger Bibbings.
The findings come as Rospa publishes a Managing Occupational Road Risk guidance booklet.
Dicing with death
There is, roughly, one death per 15 million car trips in the UK. Here are seven activities that also lead, on average, to one death, in order of danger:
Two million cigarettes smoked per death from lung cancer
Three million acts of unprotected sex per death from Aids
Four million bicycle rides per cyclist killed
10 million flights on a jet aircraft per passenger fatality
25 million beef steaks eaten per death from CJD
30 million walks in the rain per death by lightning
75 million trains boarded per train-crash deathReuse content