According to French researchers even when drivers recognise that they are tired and sleepy many carry on driving and put themselves at risk of being involved in a serious accident.
Studies have shown that sleepiness in drivers is an important factor contributing to the burden of traffic related injury and death.
While estimates of the proportion of car crashes attributable to sleepiness range from 3% to 33%, little is known about the extent to which drivers are able to assess that they are sleepy while driving.
In a large study of French workers researchers examined the association between self reported driving while sleepy and the risk of serious road traffic accidents (RTAs) in 13,299 drivers.
They collected data on sleepiness and other driving behaviours in 2001, and serious RTAs in 2001-3.
Socioeconomic status was recorded, and a range of other factors that could affect the results were taken into account.
They found that self-reported driving while sleepy in the previous 12 months was a "powerful predictor" of serious traffic accidents over the next 3 years.
This say the researchers suggests that drivers' awareness of their sleepiness while driving is not enough to stop them from having road traffic accidents.
Dr. Emmanuel Lagarde, from Universite Victor Segalen, Bordeaux and colleagues based their findings on responses to questions put to some 13,300 male and female experienced drivers who were between the ages of 35 and 50 years.
They say in response to the question "in the past 12 months, have you ever driven while sleepy?" - 63 percent of respondents answered never, 36 percent a few times, 0.8 percent about once per month, 0.3 percent about once a week, and 0.2 percent more than once a week.
The team say the risk of serious traffic accidents rose proportionally with the frequency of self-reported driving while sleepy.
It seems that drivers who reported driving while drowsy "a few times" in the previous 12 months were 50 percent more likely to have been involved in a serious traffic accident than drivers who reported not driving while sleepy over the same period.
Drivers who admitted to driving while sleepy "once a month or more often" in the past year were nearly three times more likely to have been involved in a serious accident.
Lagarde and colleagues say their results support those of previous studies done in New Zealand, France, and the United States, and have "important consequences for public safety because of the high proportion of drivers concerned".
The researchers say that the study demonstrates that drivers know when they are tired, but may not act accordingly, and either underestimate the impact of sleepiness on their driving performance or overestimate their capacity to fight sleepiness.
They suggest that messages on prevention should therefore focus on convincing sleepy drivers to stop driving and sleep before resuming their journey.
Another study has found a link between unsafe driving behaviour and four wheel drive vehicles.
British researchers say that drivers of four wheel drive vehicles are more likely to flout laws regarding mobile phones and seat belts than drivers of other cars.
The researchers from Imperial College, London, say this constitutes a major public health concern and greater efforts are needed to educate the public and enforce laws.
For the study the researchers recorded observed behaviour at three different sites in West London. They observed private passenger vehicles from Monday to Friday for one hour in the morning (9-10 am), afternoon (1-2 pm), and early evening (4-5 pm).
In all a total of 38,182 normal cars and 2,944 four wheel drive vehicles were included in the analysis.
They found that almost one in six drivers (15.3%) was not wearing a seat belt and one in 40 (2.5%) was using a hand held mobile phone while he or she passed the observer.
It seems the drivers of four wheel drive vehicles were the worst offenders and were almost four times more likely than drivers of other cars to be seen using hand held mobile phones and were also more likely not to comply with the law on seat belts.
The researchers say the findings support the theory of risk compensation, which predicts that drivers of four wheel drive vehicles feel safer and therefore take more risks when driving.
They warn that while four wheel drive vehicles are possibly safer in a crash, their owners may be placing themselves and other road users at increased risk of injury with such behaviour.