Cognitive components of simulated driving performance: Sleep loss effects and predictors

Driving is a complex task, which can be broken down into specific cognitive processes. In order to determine which components contribute to drowsy driving impairments, the current study examined simulated driving and neurocognitive performance after one night of sleep deprivation. Nineteen professional drivers (age 45.3 ± 9.1) underwent two experimental sessions in randomised order: one after normal sleep and one after 27 h total sleep deprivation. A simulated driving task (AusEd), the psychomotor vigilance test (PVT), and neurocognitive tasks selected from the Cognitive Drug Research computerised neurocognitive assessment battery (simple and choice RT, Stroop Task, Digit Symbol Substitution Task, and Digit Vigilance Task) were administered at 10:00 h in both sessions. Mixed-effects ANOVAs were performed to examine the effect of sleep deprivation versus normal sleep on performance measures. To determine if any neurocognitive tests predicted driving performance (lane position variability, speed variability, braking RT), neurocognitive measures that were significantly affected by sleep deprivation were then added as a covariate to the ANOVAs for driving performance. Simulated driving performance and neurocognitive measures of vigilance and reaction time were impaired after sleep deprivation (p < 0.05), whereas tasks examining processing speed and executive functioning were not significantly affected by sleep loss. PVT performance significantly predicted specific aspects of simulated driving performance. Thus, psychomotor vigilance impairment may be a key cognitive component of driving impairment when sleep deprived. The generalisability of this finding to real-world driving remains to be investigated.     

A meta-analysis of the effects of cell phones on driver performance

The empirical basis for legislation to limit cell phones while driving is addressed. A comprehensive meta-analysis of the effects of cell phones on driving performance was performed. A total of 33 studies collected through 2007 that met inclusion criteria yielded 94 effect size estimates, with a total sample size of approximately 2000 participants. The dependent variables of reaction time, lateral vehicle control, headway and speed and the moderating variables of research setting (i.e., laboratory, simulator, on-road), conversation target (passenger, cell phone) and conversation type (cognitive task, naturalistic) were coded. Reaction time (RT) to events and stimuli while talking produced the largest performance decrements. Handheld and hands-free phones produced similar RT decrements. Overall, a mean increase in RT of .25s was found to all types of phone-related tasks. Observed performance decrements probably underestimate the true behavior of drivers with mobile phones in their own vehicles. In addition, drivers using either phone type do not appreciably compensate by giving greater headway or reducing speed. Tests for moderator effects on RT and speed found no statistically significant effect size differences across laboratory, driving simulation and on-road research settings. The implications of the results for legislation and future research are considered.     

How do prolonged wakefulness and alcohol compare in the decrements they produce on a simulated driving task?

The effects of alcohol ingestion were compared with those of prolonged wakefulness on a simulated driving task. Eighteen healthy, male subjects aged between 19 and 35 years drove for 30 min on a simulated driving task at blood alcohol concentrations of 0.00, 0.05 and 0.08%. Subjective sleepiness was assessed before and after the driving task. Driving performance was measured in terms of the mean and standard deviation (S.D.) of lane position (tracking); the mean and S.D. of speed deviation (the difference between the actual speed and the posted speed limit); and the number of off-road occurrences. Ratings of sleepiness increased with increasing blood alcohol concentration, and were higher following the driving task. With increasing blood alcohol concentration, tracking variability, speed variability, and off-road events increased, while speed deviation decreased, the result of subjects driving faster. The results were compared with a previous study examining simulated driving performance during one night of prolonged wakefulness [Arnedt, J.T., MacLean A.W., 1996. Effects of sleep loss on urban and motorway driving stimulation performance. Presented at the Drive Alert... Arrive Alive International Forum, Washington DC], using an approach adopted by Dawson and Reid [Dawson, D., Reid, K., 1997. Fatigue, alcohol and performance impairment. Nature 388, 23]. For mean tracking, tracking variability, and speed variability 18.5 and 21 h of wakefulness produced changes of the same magnitude as 0.05 and 0.08% blood alcohol concentration, respectively. Alcohol consumption produced changes in speed deviation and off-road occurrences of greater magnitude than the corresponding levels of prolonged wakefulness. While limited to situations in which there is no other traffic present, the findings suggest that impairments in simulated driving are evident even at relatively modest blood alcohol levels, and that wakefulness prolonged by as little as 3 h can produce decrements in the ability to maintain speed and road position as serious as those found at the legal limits of alcohol consumption.     

Monotony of road environment and driver fatigue: a simulator study

Studies have shown that drowsiness and hypovigilance frequently occur during highway driving and that they may have serious implications in terms of accident causation. This paper focuses on the task induced factors that are involved in the development of these phenomena. A driving simulator study was conducted in order to evaluate the impact of the monotony of roadside visual stimulation using a steering wheel movement (SWM) analysis procedure. Fifty-six male subjects each drove during two different 40-min periods. In one case, roadside visual stimuli were essentially repetitive and monotonous, while in the other one, the environment contained disparate visual elements aiming to disrupt monotony without changing road geometry. Subject's driving performance was compared across these conditions in order to determine whether disruptions of monotony can have a positive effect and help alleviate driver fatigue. Results reveal an early time-on-task effect on driving performance for both driving periods and more frequent large SWM when driving in the more monotonous road environment, which implies greater fatigue and vigilance decrements. Implications in terms of environmental countermeasures for driver fatigue are discussed.     

The link between fatigue and safety

The objective of this review was to examine the evidence for the link between fatigue and safety, especially in transport and occupational settings. For the purposes of this review fatigue was defined as ‘a biological drive for recuperative rest’. The review examined the relationship between three major causes of fatigue – sleep homeostasis factors, circadian influences and nature of task effects – and safety outcomes, first looking at accidents and injury and then at adverse effects on performance. The review demonstrated clear evidence for sleep homeostatic effects producing impaired performance and accidents. Nature of task effects, especially tasks requiring sustained attention and monotony, also produced significant performance decrements, but the effects on accidents and/or injury were unresolved because of a lack of studies. The evidence did not support a direct link between circadian-related fatigue influences and performance or safety outcomes and further research is needed to clarify the link. Undoubtedly, circadian variation plays some role in safety outcomes, but the evidence suggests that these effects reflect a combination of time of day and sleep-related factors. Similarly, although some measures of performance show a direct circadian component, others would appear to only do so in combination with sleep-related factors. The review highlighted gaps in the literature and opportunities for further research.     

Simultaneous interaction with in-vehicle systems while turning left: comparison among three groups of drivers

The growing introduction of new technologies inside vehicles represents a set of extra information sources. Its use can interfere with the driving activity and induce performance decrements. An on-road experiment was conducted to investigate how drivers reacted to the simultaneous interaction with in-vehicle systems. Forty-eight subjects (elderly, reference and novice) participated in the tests. They were asked to interact with one (road guidance system) or two (road guidance system and mobile phone) on-board devices. Their performance was evaluated for the turn left manoeuvres and main effects of system conditions were revealed. The simultaneous interaction with both in-vehicle systems induced participants to some performance decrements on the driving task: higher number of late activations of the turning indicator; lower mean speeds before the intersection; lower values of instant speed at the beginning of the turn manoeuvre; higher steering wheel variability; activations of the braking pedal with less pressure, later, and performing a higher number of strokes; and also less glances towards important areas of the environment. No significant difference was observed for the guidance task. The comparison between driver groups' performance did not reveal enough significant differences in order to draw a general conclusion.     

Event-related potentials and secondary task performance during simulated driving

Inattention and distraction account for a substantial number of traffic accidents. Therefore, we examined the impact of secondary task performance (an auditory oddball task) on a primary driving task (lane keeping). Twenty healthy participants performed two 20-min tests in the Divided Attention Steering Simulator (DASS). The visual secondary task of the DASS was replaced by an auditory oddball task to allow recording of brain activity. The driving task and the secondary (distracting) oddball task were presented in isolation and simultaneously, to assess their mutual interference. In addition to performance measures (lane keeping in the primary driving task and reaction speed in the secondary oddball task), brain activity, i.e. event-related potentials (ERPs), was recorded. Performance parameters on the driving test and the secondary oddball task did not differ between performance in isolation and simultaneous performance. However, when both tasks were performed simultaneously, reaction time variability increased in the secondary oddball task. Analysis of brain activity indicated that ERP amplitude (P3a amplitude) related to the secondary task, was significantly reduced when the task was performed simultaneously with the driving test. This study shows that when performing a simple secondary task during driving, performance of the driving task and this secondary task are both unaffected. However, analysis of brain activity shows reduced cortical processing of irrelevant, potentially distracting stimuli from the secondary task during driving.     

Are OSAS patients impaired in their driving ability on a circuit with medium traffic density?

OBJECTIVES: Many studies have demonstrated that patients with Obstructive Sleep Apnea Syndrome (OSAS), a very common sleep-related breathing disorder, are usually impaired in their driving ability because of decreased sleep quality. However, most of the simulation procedures in laboratories are designed to create monotonic conditions with low traffic density, if any, thereby leading to a dramatic decrease in performance in OSAS patients because of the lack of stimulation. The aim of this study was therefore to evaluate driving abilities in OSAS patients involved in a driving simulation task with medium traffic density, in order to replicate as far as possible real world conditions. The behavioral and physiological attributes likely to predict driving performance in these patients were also investigated. METHODS: After a normal night of sleep, 12 OSAS patients and 8 healthy controls performed 6 driving sessions during a 24-h period of sustained wakefulness. Driving performances (speed, lateral position, distances...) were measured and correlated to sleep parameters and to a waking EEG recorded during the task. RESULTS: Compared to controls, patients showed difficulties in speed adjustment. However, they maintained longer inter-vehicle distances, including during overtaking. Their waking EEG, while driving, showed increased spectral power in theta (3.9-7.8Hz) but also in beta (12.7-29.2Hz) activity, alpha power (7.9-12.6Hz) being increased in both groups due to sustained wakefulness. Poor sleep indices were correlated to increased theta and beta activities, as well as to more cautious behavior. DISCUSSION: In medium traffic density conditions, driving performance in OSAS patients remained at near normal levels, but with more cautious behavior than controls. This could be the result of a bigger effort to stay awake, as suggested by an increased beta activity in these patients.     

Fatigue, sleep restriction and driving performance

We ran a randomized cross-over design study under sleep-deprived and non-sleep-deprived driving conditions to test the effects of sleep restriction on real driving performance. The study was performed in a sleep laboratory and on an open French highway. Twenty-two healthy male subjects (age = 21.5 +/- 2 years; distance driven per year = 12,225 +/- 4739 km (7641 +/- 2962 miles) [mean +/- S.D.]) drove 1000 km (625 miles) over 10 h during five 105 min sessions on an open highway. Self-rated fatigue and sleepiness before each session, number of inappropriate line crossings from video recordings and simple reaction time (RT) were measured. Total crossings increased after sleep restriction (535 crossings in the sleep-restricted condition versus 66 after non-restricted sleep (incidence rate ratio (IRR): 8.1; 95% confidence interval (95% CI): 3.2-20.5; p < 0.001)), from the first driving session. The interaction between the two factors (conditionxtime of day) was also significant (F(5, 105) = 3.229; p < 0.05). Increasing sleepiness score was associated with increasing crossings during the next driving session in the sleep-restricted (IRR: 1.9; 95% CI: 1.4-2.4) but not in the non-restricted condition (IRR: 1.0; 95% CI: 0.8-1.3). Increasing self-perceived fatigue was not associated with increasing crossings in either condition (IRR: 0.95; 95% CI: 0.93-0.98 and IRR: 1.0; 95% CI: 0.98-1.02). Rested subjects drove 1000 km with four shorts breaks with only a minor performance decrease. Sleep restriction induced important performance degradation even though time awake (8h) and session driving times (105 min) were relatively short. Major inter-individual differences were observed under sleep restriction. Performance degradation was associated with sleepiness and not fatigue. Sleepiness combined with fatigue significantly affected RT. Road safety campaigns should encourage drivers to avoid driving after sleep restriction, even on relatively short trips especially if they feel sleepy.     

Effects of chronotype and time of day on the vigilance decrement during simulated driving

The current study tested for the first time the effect of individual differences in circadian rhythmicity (chronotype) on both driving performance and its evolution along time on task. Morning-type and evening-type female participants were tested in morning (8 am) and evening (8 pm) sessions, in which we controlled for prior sleep duration and prior wake. Measures of body temperature, subjective activation and affect, reaction times (RT) in the Psychomotor Vigilance Task (PVT), behavioral performance (error position) and EEG alpha power during simulated driving were collected. The main result showed strong linear increments of mean and standard deviation of error position along time on task (vigilance decrement) when evening-type participants drove at their non-optimal time of day, that is, during the morning session. In contrast, driving performance in the morning-type group remained stable over time on task and was not affected by time of day. This finding can be due to differences in personality traits (e.g., conscientiousness, sensation seeking) and task appraisal associated to extreme chronotypes. The consideration of chronotype in vigilance and driving tasks can enhance safety and human performance by promoting work schedules and countermeasures to prevent failures in the accomplishment of tasks under non-optimal circadian conditions.     

Daytime driving simulation performance and sleepiness in obstructive sleep apnoea patients

BACKGROUND: Sleepiness related car accidents are common in obstructive sleep apnoea syndrome (OSAS) patients. The objective measurements of sleepiness used in clinical setting quantify the tendency to fall asleep in quiet situations that are completely different from driving. METHODS: We studied 30 OSAS patients with subjective (subjective sleepiness scales) and objective (multiple sleep latency test, MSLT) sleepiness measurements, associated with driving simulation test (DST), previously validated in young healthy subjects. The results of subjective and objective sleepiness tests were compared with simulated driving performance in order to evaluate the suitability of our DST for measuring alertness. RESULTS: Subjective and objective sleepiness measurements were significantly correlated with driving performance on the simulator. The most significant correlates of sleepiness were the measures of the primary vehicle control task on the simulator: lane position variability and crash data. The comparison of DST and MSLT results suggested our driving simulated approach could be used to evaluate daytime sleepiness in the clinical setting of OSAS patients. CONCLUSIONS: Our DST is a suitable objective tool to detect sleepiness in OSAS patients, and could be useful in the clinical setting of sleep medicine and research.     

Driver state examination—Treading new paths

A large proportion of crashes in road driving can be attributed to driver fatigue. Several types of fatigue are discussed, comprising sleep-related fatigue, active task-related fatigue (as a consequence of workload in demanding driving situations) as well as passive task-related fatigue (as related to monotonous driving situations). The present study investigated actual states of fatigue in a monotonous driving situation, using EEG measures and a long-lasting driving simulation experiment, in which drivers had to keep the vehicle on track by compensating crosswind of different strength. Performance data and electrophysiological correlates of mental fatigue (EEG Alpha and Theta power, Inter Trial Coherence (ITC), and auditory event-related potentials to short sound stimuli) were analyzed. Driving errors and driving lane variability increased with time on task and with increasing crosswind. The posterior Alpha and Theta power also increased with time on task, but decreased with stronger crosswind. The P3a to sound stimuli decreased with time on task when the crosswind was weak, but remained stable when the crosswind was strong. The analysis of ITC revealed less frontal Alpha and Theta band synchronization with time on task, but no effect of crosswind. The results suggest that Alpha power in monotonous driving situations reflects boredom or attentional withdrawal due to monotony rather than the decline of processing abilities as a consequence of high mental effort. A more valid indicator of declining mental resources with increasing time on task seems to be provided by brain oscillatory synchronization measures and event-related activity.     

Reliability of simulator driving tool for evaluation of sleepiness,fatigue and driving performance

Study objective

To compare the impact of extended wakefulness (i.e., sleepiness) and prolonged driving (i.e., fatigue) at the wheel in simulated versus real-life driving conditions.

Design

Participants drove on an INRETS-MSIS SIM2 simulator in a research laboratory or an open French highway during 3 nocturnal driving sessions. A dose–response design of duration of nocturnal driving was used: a 2 h short driving session (3–5 AM), a 4 h intermediate driving session (1–5 AM) and an 8 h long driving session (9 PM–5 AM).

Participants

Two groups of healthy male drivers (20 for simulated driving and 14 drivers for real driving; mean age ± SD = 22.3 ± 1.6 years), free of sleep disorders.

Measurements

Number of inappropriate line crossings, self-rated fatigue and sleepiness were recorded in the last hour of driving sessions to control the effects of prior waking time and time of day.

Results

Compared to the daytime reference session, both simulated and real driving performance were affected by a short nocturnal driving session (P < .05 and P < .001, respectively). Extension of nocturnal driving duration affected simulated performance nonlinearly and more severely than that of real driving (P < .001).Compared to the daytime reference session, short nocturnal simulated and real driving sessions increased self-perceived fatigue and sleepiness. Real and simulated driving conditions had an identical impact on fatigue and sleepiness during extended periods of nocturnal driving.

Conclusions

In healthy subjects, the INRETS-MSIS SIM2 simulator appropriately measures driving impairment in terms of inappropriate line crossings related to extended wakefulness but has limitations to measure the impact of extended driving on drivers’ performance.     

The deterioration of driving performance over time in drivers with untreated sleep apnea

Sleep apnea increases risk of driving crashes when left untreated. This study examined the driving performance decrements of untreated, undiagnosed sleep apnea drivers compared with healthy controls in a monotonous highway driving simulator task. It was hypothesized that the sleep apnea group would perform worse during a driving simulator test compared with the control group. A significant group by time interaction occurred indicating that sleep apnea participants’ performance degraded more quickly over the course of the drive. In contrast with previous studies, this sleep apnea group did not include sleep disorder center patients, but rather community volunteers whose screening indicated a significant apnea/hypopnea index of 15 or greater. There may be inherent differences between patients and nonpatients with sleep apnea, as patients may have a more significant impact on their quality of life, causing them to seek treatment. Still, the results are clear that although the sleep apnea group drove similarly to the control group at the start of the drive, they are sensitive to time on task effects. These results support the need to diagnose and treat sleep apnea.     

Road accidents caused by sleepy drivers: Update of a Norwegian survey

The current study tests, updates and expands a model of factors associated with sleepy driving, originally based on a 1997 survey of accident-involved Norwegian drivers (Sagberg, F., 1999. Road accidents caused by drivers falling asleep. Accident Analysis & Prevention 31, 639–649). The aim is to establish a robust model to inform measures to tackle sleepy driving. The original questions on (i) tiredness-related accidents and (ii) incidents of sleep behind the wheel in the last 12 months were again posed in 2003 and 2008, in independent surveys of Norwegian drivers involved in accidents reported to a large insurance company. According to those drivers at-fault for the accident, tiredness or sleepiness behind the wheel contributed to between 1.9 and 3.9 per cent of all types of accident reported to the insurance company across these years. Accident-involved drivers not at fault for the accident reported a reduction in the incidence of sleep behind the wheel for the preceding year, decreasing from 8.3 per cent in 1997 to 2.9 per cent in 2008. The reasons for this are not clear. According to logistic regression analysis of survey responses, the following factors were robustly associated with road accidents involving sleepy driving: driving off the road; good road conditions; longer distance driven since the start of the trip; and fewer years with a driving licence. The following factors are consistently associated with reports of sleep behind the wheel, whether or not it leads to an accident: being male; driving further per year; being younger; and having sleep-related health problems. Taken together these findings suggest that young, inexperienced male drivers who drive long distances may be a suitable target for road safety campaigns aimed at tackling sleepy driving.     

Evaluation of experience-based fatigue countermeasures

Background and objective

Strategies adopted by drivers in order to cope with fatigue and falling asleep at the wheel include a variety of activities that may invigorate the body and/or the mind. The objectives of the current study were to examine the effectiveness of an energy drink and a non-traditional manual-dexterity/mastication activity as fatigue countermeasures.

Method

Twenty subjects participated in this driving simulator study. Each driving session lasted 2 h and each driver drove under three conditions: after consumption of an energy drink, while engaged in a self-paced manual-dexterity/mastication secondary task (shelling and eating sunflower seeds), and in a control condition with neither. Fatigue effects were assessed on three dimensions: subjective evaluations (using the Swedish Occupational Fatigue Inventory), a physiological indicator (heart rate variability), and driving performance measures (speed, steering, and lane deviations).

Results

The subjective and physiological measures showed a significant effect of both treatments in counteracting the effects of fatigue when compared to the control condition. The results of the driving performance measures indicated that the energy drink was effective in counteracting fatigue, while the secondary task was as effective as the energy drink in counteracting fatigue on measures that did not rely on hand movements.

Conclusions

Drinking an energy drink prior to the driving task has a significant, positive effect in counteracting fatigue, though it may have long-term negative rebound effects. The manual-dexterity/mastication secondary task can temporarily counteract the subjective and physiological effects of fatigue while driving, but can interfere with vehicle handling.     

Driver distraction: the effects of concurrent in-vehicle tasks, road environment complexity and age on driving performance

This paper presents the findings of a simulator study that examined the effects of distraction upon driving performance for drivers in three age groups. There were two in-vehicle distracter tasks: operating the vehicle entertainment system and conducting a simulated hands-free mobile phone conversation. The effect of visual clutter was examined by requiring participants to drive in simple and complex road environments. Overall measures of driving performance were collected, together with responses to roadway hazards and subjective measures of driver perceived workload. The two in-vehicle distraction tasks degraded overall driving performance, degraded responses to hazards and increased subjective workload. The performance decrements that occurred as a result of in-vehicle distraction were observed in both the simple and complex highway environments and for drivers in different age groups. One key difference was that older drivers traveled at lower mean speeds in the complex highway environment compared with younger drivers. The conclusions of the research are that both in-vehicle tasks impaired several aspects of driving performance, with the entertainment system distracter having the greatest negative impact on performance, and that these findings were relatively stable across different driver age groups and different environmental complexities.     

Efficient driver drowsiness detection at moderate levels of drowsiness

Previous research on driver drowsiness detection has focused primarily on lane deviation metrics and high levels of fatigue. The present research sought to develop a method for detecting driver drowsiness at more moderate levels of fatigue, well before accident risk is imminent. Eighty-seven different driver drowsiness detection metrics proposed in the literature were evaluated in two simulated shift work studies with high-fidelity simulator driving in a controlled laboratory environment. Twenty-nine participants were subjected to a night shift condition, which resulted in moderate levels of fatigue; 12 participants were in a day shift condition, which served as control. Ten simulated work days in the study design each included four 30-min driving sessions, during which participants drove a standardized scenario of rural highways. Ten straight and uneventful road segments in each driving session were designated to extract the 87 different driving metrics being evaluated. The dimensionality of the overall data set across all participants, all driving sessions and all road segments was reduced with principal component analysis, which revealed that there were two dominant dimensions: measures of steering wheel variability and measures of lateral lane position variability. The latter correlated most with an independent measure of fatigue, namely performance on a psychomotor vigilance test administered prior to each drive. We replicated our findings across eight curved road segments used for validation in each driving session. Furthermore, we showed that lateral lane position variability could be derived from measured changes in steering wheel angle through a transfer function, reflecting how steering wheel movements change vehicle heading in accordance with the forces acting on the vehicle and the road. This is important given that traditional video-based lane tracking technology is prone to data loss when lane markers are missing, when weather conditions are bad, or in darkness. Our research findings indicated that steering wheel variability provides a basis for developing a cost-effective and easy-to-install alternative technology for in-vehicle driver drowsiness detection at moderate levels of fatigue.     

Developing measures of fatigue using an alcohol comparison to validate the effects of fatigue on performance

The effects of 28 h of sleep deprivation were compared with varying doses of alcohol up to 0.1% blood alcohol concentration (BAC) in the same subjects. The study was conducted in the laboratory. Twenty long-haul truck drivers and 19 people not employed as professional drivers acted as subjects. Tests were selected that were likely to be affected by fatigue, including simple reaction time, unstable tracking, dual task, Mackworth clock vigilance test, symbol digit coding, visual search, sequential spatial memory and logical reasoning. While performance effects were seen due to alcohol for all tests, sleep deprivation affected performance on most tests, but had no effect on performance on the visual search and logical reasoning tests. Some tests showed evidence of a circadian rhythm effect on performance, in particular, simple reaction time, dual task, Mackworth clock vigilance, and symbol digit coding, but only for response speed and not response accuracy. Drivers were slower but more accurate than controls on the symbol digit test, suggesting that they took a more conservative approach to performance of this test. This study demonstrated which tests are most sensitive to sleep deprivation and fatigue. The study therefore has established a set of tests that can be used in evaluations of fatigue and fatigue countermeasures.     

Road accidents caused by drivers falling asleep

About 29600 Norwegian accident-involved drivers received a questionnaire about the last accident reported to their insurance company. About 9200 drivers (31%) returned the questionnaire. The questionnaire contained questions about sleep or fatigue as contributing factors to the accident. In addition, the drivers reported whether or not they had fallen asleep some time whilst driving. and what the consequences had been. Sleep or drowsiness was a contributing factor in 3.9% of all accidents, as reported by drivers who were at fault for the accident. This factor was strongly over-represented in night-time accidents (18.6%), in running-off-the-road accidents (8.3%), accidents after driving more than 150 km on one trip (8.1%), and personal injury accidents (7.3%). A logistic regression analysis showed that the following additional factors made significant and independent contributions to increasing the odds of sleep involvement in an accident: dry road, high speed limit, driving one's own car, not driving the car daily, high education, and few years of driving experience. More male than female drivers were involved in sleep-related accidents, but this seems largely to be explained by males driving relatively more than females on roads with high speed limits. A total of 10% of male drivers and 4% of females reported to have fallen asleep while driving during the last 12 months. A total of 4% of these events resulted in an accident. The most frequent consequence of falling asleep--amounting to more than 40% of the reported incidents--was crossing of the right edge-line before awaking, whereas crossing of the centreline was reported by 16%. Drivers' lack of awareness of important precursors of falling asleep--like highway hypnosis, driving without awareness, and similar phenomena--as well as a reluctance to discontinue driving despite feeling tired are pointed out as likely contributors to sleep-related accidents. More knowledge about the drivers' experiences immediately preceding such accidents may give a better background for implementing effective driver warning systems and other countermeasures.