Fact sheet

Fatigue in traffic: causes and effects


The role of fatigue must not be underestimated when studying the causes of crashes. Crashes in which driver fatigue plays a role are not only a matter of having spent too long behind the wheel; fatigue can also be caused by too little sleep, stress, or the time of the day. According to a conservative estimate based on studies abroad, driver fatigue is involved in 10-15% of all severe crashes.

Although technical aids to prevent fatigue crashes are being developed, they are not yet ready for use. In the road haulage industry, the only means that are presently available to combat crashes involving fatigue are safety culture and consistently applying (and enforcing) the driving and resting hours. Information campaigns should make non-professional car drivers aware of the risks of fatigue.

Background and content

Everyone feels tired once in a while. Yet, what exactly do we mean when we say ‘tired’? Fatigue has various connotations; there is no unambiguous definition of fatigue. The term is attributed with various meanings: physical (e.g. as a result of heavy and protracted physical labour) as well as neurobiological (biologically determined sleep-awake rhythms). In addition, it also has a mental/psychological meaning: not having the energy to do things, and a subjectively experienced reluctance to continue with a task.

This fact sheet discusses the causes of fatigue, the prevalence of fatigue, the effects of fatigue on driver behaviour and on the number of crashes, as well as the possible measures for professional drivers on the one hand, and non-professional drivers on the other.


What causes fatigue?

Fatigue has many causes. It used to be directly and almost exclusively associated with the amount of time that one has been carrying out a particular task, for example the time one spends behind the wheel without interruption (time-on-task). This is still seen as one of the most important causes. However, research has found other factors that are equally important (Van Schagen, 2003).

  1. In the first place, fatigue is a lack of sleep. This can be chronic or acute.
    A chronic lack of sleep is the result of not having enough sleep during a long period. The average person needs 8 hours of sleep in every 24-hour cycle. The quality of the sleep is also of great importance, besides the quantity. If sleep is interrupted regularly, this leads to a chronic lack of sleep, just as too little sleep does. The quality of sleep is influenced by, among other things, sleep disorders like sleep apnoea (a temporary breathing stoppage while sleeping) or narcolepsy (the tendency to suddenly fall asleep). But it can also be a side effect of chronic diseases and/or medication or the result of external factors such as a noisy or unpleasant sleeping environment.
    An acute lack of sleep is also the result of too little sleep, but is less structural than chronic lack of sleep. An acute lack of sleep can occur after just one bad or short night. If there has been too little sleep during a 24-hour period, we refer to it as a partial acute lack of sleep. There is a complete acute lack of sleep if there has been no sleep at all in a period of 24 hours.
  2. Fatigue or sleepiness can also occur without lack of sleep. This type of fatigue is usually linked to the daily sleep cycle or the biorhythm. This means that at certain times in the 24-hour cycle the human body has a greater need for sleep than at other times. This happens most and lasts longest early in the morning (approximately between midnight and 6 a.m.) and, to a lesser extent, about 12 hours later (approximately between 2 p.m. and 4 p.m.). At these moments, there is a natural tendency to sleep, and if this cannot be given in to, a sleepy feeling occurs.
  3. The following factors have indirect influence: age, physical condition, the use of alcohol, external factors such as temperature, noise, vibrations, and also the routine of a task. For example, driving alone on a boring road for a long time may not itself cause fatigue or sleepiness, but it can cause the consequences to manifest sooner.
How often does fatigue occur?

As fatigue has many causes, it seems valid to conclude that everybody is very tired once in a while. Questionnaire and interview surveys (for an overview, see Van Schagen, 2003) show that about 30% of the population are very tired regularly. The estimation is that about 10% of the population suffer from a serious type of sleeplessness. About 3% of the population suffer from a sleep disorder, of which sleep apnoea is the most common.

Many studies have shown that fatigue while driving a car is also a very frequent phenomenon. Approximately 25% of professional as well as non-professional drivers say that they have at least once fallen asleep behind the wheel; about 50% say they have sometimes been very tired while driving or that they have nearly fallen asleep. In a Dutch questionnaire (Goldenbeld et al., 2011) a quarter of all drivers indicated having been so tired at least once in the past year that it was hard to keep the eyes open, and 4% indicated to have fallen asleep behind the wheel in the past year. These percentages are higher for lorry drivers; 62% and 20%, respectively.

What are the consequences in general?

Fatigue leads to a reduction in alertness, longer reaction times, memory problems, poorer psychometric coordination, and less efficient information processing. Fatigue also has an effect on the frame of mind. The motivation to carry out a task diminishes, the communication and interaction with the surroundings deteriorates, and one gets irritated quicker and reacts more aggressively towards people and things. In other words, fatigue leads to diminished action capability and action preparedness (Meijman 1991; in Brookhuis 2001).

All the above-mentioned functions are important for carrying out the driving task accurately and safely. It can, therefore, be expected that fatigue also leads to a worsening of carrying out the driving task. Various studies have indeed shown this to be the case (see Van Schagen, 2003; Horrey et al. 2011). Such research generally involves a driving simulator in which subjects either drive a long distance or are not allowed to sleep for a long time. The results of this type of study are reasonably uniform. In the first place, it was found that tired people experience more difficulty in keeping their lane, more often cross or nearly cross the side marking, make greater steering adjustments and do so more abruptly. The drivers also react less accurately to deceleration by the driver in front.

What is the role of fatigue in road crashes?

Fatigue also plays a role in the occurrence of road crashes. However, it is difficult to say to what extent this is the case. As opposed to, for example, the use of alcohol, drugs and/or medicines, it is by no means simple to determine the extent of fatigue in an objective way and it must be estimated in a much less direct way. The police crash reports indicate that 1-4% of all registered crashes are sleep-related. In the Netherlands, the police give sleep/fatigue as the primary cause in fewer than 0.5% of all registered serious crashes. However, it is highly likely that these police reports greatly underestimate the problem. In most countries, police are not (yet) very alert to fatigue being the crash cause. In addition, most normal healthy drivers are not likely to readily admit that they were very tired or had fallen asleep at the time of the crash. Furthermore, the crash itself will have caused most of the symptoms of fatigue to disappear.

Questionnaire studies and in-depth crash analyses have led to completely different conclusions about the role of fatigue in road crashes. The results of individual studies vary greatly, but on the basis of research using questionnaires it can be concluded that fatigue played a role in 5-10% of all crashes. In-depth analysis, however, indicated that this was the case in 10-25% of all crashes. The higher percentages have particularly been found in studies that examined lorry crashes and/or fatal crashes (Van Schagen, 2003; SafetyNet, 2009). All these estimates are from studies in other countries than the Netherlands. In the Netherlands, two in-depth studies examining run-off-road crashes in the Province of Zeeland (Davidse et al., 2011) and a part of the Province of South-Holland (Davidse, 2011) found that fatigue played a role in 17-19% and 7-19% of these crashes, respectively.   If we extrapolate the above estimates for the Netherlands, and start of from a conservative estimate of 10-15%, then around 50 to 75 fatal crashes per year are (partly) related to fatigue or falling asleep behind the wheel (crash data 2011). These are crashes registered by the police which according to this registration do not involve alcohol.

Furthermore, Van Schagen (2003) also discussed a number of studies which have examined whether fatigue also leads to a higher risk of being involved in a crash. These show that people with a sleep disorder and people with an acute lack of sleep have a considerably higher (3-8 times higher) risk of being involved in an injury crash.

Which road users have the highest risk?

Most of the fatigue crashes occur on trunk roads and motorways, during the late evening and early morning under good conditions, when the driver has spent a considerable length of time on the road. The result is that one either goes off the road or has a frontal collision with an oncoming vehicle. The consequences are usually very severe: one brakes too late so that the collision speed is high.

Some road user categories are relatively often involved in fatigue-related crashes (SafetyNet, 2009). These are:

  • people younger than 25 years old;
  • people suffering from sleep disorders;
  • people who drive at night;
  • long distance drivers;
  • professional drivers;
  • people who work in shifts.
What do drivers themselves think of fatigue driving and how do they prevent it?

Goldenbeld et al. (2011) showed that drivers consider fatigue driving as dangerous and that they recognize their own fatigue in themselves. According to drivers, yawning, not being able to keep one’s eyes open and loss of concentration are the most important signs of fatigue. However, far from all drivers act on this. Twenty per cent indicated that they have begun driving or continued driving while they considered themselves too tired for this. For lorry drivers, this was 37%.

The moment fatigue sets in, a driver may decide to stop driving and take a nap or to ask a passenger to take over the driving task. According to drivers themselves, these two strategies are the most effective, yet they are not the most used strategies. Drivers mainly choose to allow fresh air into the car (by opening the window or switching on the air conditioning), to talk to a passenger, to briefly stop driving for some food or exercise, or to turn up the volume Lorry drivers indicate more frequently  stopping to drive to rest or sleep when they are fatigued. Of course, this is compulsory for them according to regulations on driving and resting hours.

What are the possible measures?

Until now, other than, for example, for alcohol, it has been impossible to fix a legal limit of the extent of fatigue that permits further traffic participation that can be enforced by the police. This makes it difficult to implement laws and regulations and their enforcement. This is certainly the case for the ordinary, non-professional driver. There are slightly more possibilities for professional drivers because they can be dealt with through laws and regulations on driving and resting hours. Nevertheless, various measures can be considered (see for example Van Schagen, 2003; Methorst, 2006; SafetyNet, 2009; Jettinghoff et al., 2010).

Haulage companies

  • First of all, they have the possibility to use rules to set maximum driving and minimum resting hours that take the human need for rest and sufficient night time sleep adequately into account.
  • Then, of course, the haulage companies have to plan the work in such a way that chauffeurs can really adhere to the rules. Compliance control of these rules remains important. In addition, haulage companies should also take their responsibility of informing their drivers about the causes and results of fatigue.
  • With regard to the occurrence of fatigue, the influence of personal living circumstances (life style) of individual drivers and their own responsibility should not be forgotten.
  • Also, screening drivers for sleep disorders, especially sleep apnoea, may be considered a task of the haulage companies.
  • The so-called Fatigue Management programmes, especially in Australia and the United States, are based on these starting points, but also the ideas about a safety culture for haulage companies fit into this. This concept is gradually been given more attention in the Netherlands, and was used in the Fatigue scan for haulage companies published in 2011.

Non-professional drivers

At this moment, the possibilities of measures aimed at drivers in general concentrate mainly on giving information about causes, effects, fatigue symptoms, and advice about possibilities of limiting (if only temporarily) the effects of fatigue. This can at least increase the awareness of the role of fatigue in road safety, which is a precondition for changing behaviour. In itself, however, it is too small a step. An example is the national Dutch information campaign 'Don't become a sleepdriver' which was used in the period 2008-2011.

Infrastructural measures

With regard to infrastructure, rumble strips in the longitudinal direction are already being used to warn drivers auditively and kinetically if their vehicle is about to go off the road. Experiments are being carried out with Lane Departure Warning Systems, a technologically more advanced system with the same purpose. In addition, roadside barriers and obstacle-free zones beside the road are also very important in limiting the consequences of the typical fatigue crash, viz. going off the road (see also the archived SWOV Fact sheet Run-off-road crashes).

New developments: automatic fatigue detection systems

Work is also being done on the development of intelligent detection and warning systems. Such a system automatically detects whether a driver is tired. The idea is that the system makes the driver aware of the tiredness, possibly advises him what to do, and undertakes action to keep the driver awake, or even to make further driving impossible. These are promising developments, but they are not expected to lead to large-scale use in the short term. A number of technical problems (how can fatigue best be detected) and matters of principle (when and how should such a system intervene) need to be solved first. It is important to follow the developments and, if possible, facilitate them. We should already now be thinking about the way in which such systems can be implemented in due time.


Based on the data in the literature it can be concluded that fatigue has many causes and that almost everybody is bound to drive occasionally while very tired. Another conclusion is that fatigue has a negative effect on the driving skill and road safety. Therefore, fatigue in traffic is not just a safety problem for 'large' countries where people have to drive great distances and not just a problem for long-distance professional drivers. Presently, there are few possibilities of preventing fatigue-related crashes. Both non-professional drivers as well as professional drivers and their employers should be aware of the causes of fatigue and its road safety effects. Haulage companies can make this part of a Fatigue Management programme. Automatic in-vehicle detection and warning systems may be possibilities for the future.

Publications and sources

Davidse, R.J. (red.) (2011). Bermongevallen: karakteristieken, ongevalsscenario's en mogelijke interventies; Resultaten van een dieptestudie naar bermongevallen op 60-, 70-, 80- en 100km/uur-wegen. R-2011-24. SWOV, Leidschendam.

Davidse, R.J., Doumen, M.J.A., Duijvenvoorde, K. van & Louwerse, W.J.R. (2011). Bermongevallen in Zeeland: karakteristieken en oplossingsrichtingen; Resultaten van een dieptestudie. R-2011-20. SWOV, Leidschendam.

Brookhuis, K.A. (2001). Vermoeidheid in het Europese goederen en personenvervoer: initiatieven in Europa. Experimentele & Arbeids Psychologie, Rijksuniversiteit Groningen, Groningen.

Goldenbeld, C., Davidse, R.J., Mesken, J. & Hoekstra, A.T.G. (2011). Vermoeidheid in het verkeer: prevalentie en statusonderkenning bij automobilisten en vrachtautochauffeurs; Een vragenlijststudie onder Nederlandse rijbewijsbezitters. R-2011-4. SWOV, Leidschendam,

Horrey,W.J., Courtney, T.K., Folkard, S., Noy, Y.I., et al. (red.) (2011). Advancing fatigue and safety research. Accident Analysis and Prevention, Special Issue, vol. 43, nr. 2, p. 495-594.

Jettinghoff, K., Starren, A., Houtman, I. & Henstra, D. (2005). I love uitgerust achter het stuur!? Vermoeidheid in het verkeer: maatregelen in het buitenland en hun toepasbaarheid in Nederland. Twijnstra Gudde, Amersfoort. 

Methorst, R. (2006). Naar een nieuwe aanpak van vermoeidheid in het verkeer. Directoraat-Generaal Rijkswaterstaat, Adviesdienst Verkeer en Vervoer AVV, Rotterdam. 

SafetyNet (2009). Fatigue. Geraadpleegd 6 juli 2012 op: http://ec.europa.eu/transport/road_safety/specialist/erso/safety-issues/index_en.htm

Schagen, I.N.L.G. van (2003). Vermoeidheid achter het stuur; Een inventarisatie van oorzaken, gevolgen en maatregelen. R-2003-16. SWOV, Leidschendam.

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01 Aug 2012