Sleepy at the wheel: Knowledge, symptoms and behaviour among car drivers

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1 Transportation Research Part F 10 (2007) Sleepy at the wheel: Knowledge, symptoms and behaviour among car drivers S. Nordbakke *, F. Sagberg Institute of Transport Economics, P.O. Box 6110, Etterstad N-0602, Oslo, Norway Received 6 May 2005; received in revised form 9 February 2006; accepted 6 March 2006 Abstract Driver sleepiness has been shown to be one of the most important risk factors in road crashes. The aim of the present study was to increase the understanding of drivers actions when feeling sleepy. A national Internet panel survey was conducted among private drivers in the autumn of Reported symptoms of sleepiness differed between drivers who had fallen asleep and those who had not, but had been afraid to do so. The results indicate that drivers in general have a good knowledge of the various factors influencing the risk of falling asleep while driving. Furthermore, most of them are well aware of the most effective measures to prevent falling asleep at the wheel, such as stopping the car and take a nap. In spite of all their knowledge, most of the drivers continue driving when recognising sleepiness while driving. A short trip, appointments, and the wish to arrive at a reasonable hour are the most frequently reported reasons for continuing driving while fatigued or sleepy. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Sleepiness; Behaviour; Road safety 1. Introduction It has been documented repeatedly that a considerable proportion of car drivers have fallen asleep while driving (Gårder & Alexander, 1995; McCartt, Ribner, Pack, & Hammer, 1996; Nordbakke, 2004; Sagberg, 1999) and that this kind of driver impairment makes up one of the major causes of serious car crashes (Arnold et al., 1997; Fell, 1994; Horne & Reyner, 1995; Maycock, 1997; Reyner & Horne, 2002; Sagberg, 1999; Summala & Mikkola, 1994; UK Department of Transport, 2002). In order to implement adequate measures for fatigue management among drivers it is important to know why so many drivers go on driving despite being sleepy. As shown by Reyner and Horne (1998), almost all incidents of falling asleep at the wheel are preceded by subjective symptoms of sleepiness. In other words, there are subjective warning signals that should ideally be sufficient for the driver to take adequate countermeasures. Therefore, assuming that the driver is aware of her/his sleepiness, continued driving can be supposed to result from either inadequate knowledge about the * Corresponding author. Tel.: ; fax: address: sno@toi.no (S. Nordbakke) /$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi: /j.trf

2 2 S. Nordbakke, F. Sagberg / Transportation Research Part F 10 (2007) 1 10 risk of falling asleep while driving, or a failure to act according to the knowledge. There may, however, be individual differences between the drivers regarding their sensitivity to various symptoms of sleepiness. One question addressed in this study is whether some symptoms are more prevalent in some groups than in others, e.g., those who have fallen asleep at the wheel and those who have only been afraid to do so. Furthermore, the present study attempts to map out various aspects of driver s knowledge about risks related to driving while sleepy, their beliefs about the efficiency of various countermeasures against sleepiness both before and while driving, and what they actually do about the problem when and if it occurs. An important objective is to assess the consistency between knowledge and beliefs on one hand, and behaviour on the other. In the case of discrepancies between knowledge and behaviour an interesting issue is to assess to what extent various motivational factors result in behaviour that is not in accordance with basic beliefs and knowledge. The study is concerned with the issue of how drivers manage their sleepiness once it has occurred, and to a less extent with the factors that determine driver sleepiness in the first place. The latter issue has been comprehensively covered in previous research, focusing on the importance of amount and quality of sleep (Carter, Ulfberg, Nyström, & Edling, 2003; Connor et al., 2002; Stutts, Wilkins, Osberg, & Vaughn, 2003), time at the wheel (Folkard, 1997; Mackie & Miller, 1978), the circadian rhythm of sleepiness and its related performance variation (Åkerstedt & Folkard, 1993, 1997; Hamelin, 1987; Horne & Reyner, 1995; Kecklund & Åkerstedt, 1995; Mackie & Miller, 1978), sleep-related disorders (Connor, Whitlock, Norton, & Jackson, 2001), as well as on driver background factors (Gårder & Alexander, 1995; Sagberg, 1999; Sagberg & Bjørnskau, 2004). 2. Method Data were collected by means of a questionnaire administered on the Internet by the survey company Tns Gallup. The respondents were sampled from a panel of pre-recruited persons, having volunteered for Internet surveys by Tns Gallup. The panel is composed so as to be approximately representative of the population in the age group years. For this study, 2783 persons drawn randomly from the panel received an with an invitation to participate in the survey, provided they held a driver s licence. Responses were received from 1513 drivers; 45% were women and 55% were men. The mean age of the respondents was 39.6 years. There are only minor differences in background characteristics between the net sample and the grand sample. There was an under-representation of women by 2% points and of the youngest age group by 4% points. This is most likely explained by fewer licence holders in those groups. As the share of licence holders in the population is as high as about 85%, only a small part of the non-responding can be explained by not holding a licence. A more important reason for non-responding may be the fact that there was a 5-day deadline for responding. The response rate of this survey does not differ from other surveys in this panel with the same allotted time to respond. Information about the topic of the questionnaire may have selectively triggered those who had experienced driver fatigue to respond, thus resulting in a selection bias in the sample. As the main purpose of this study was to investigate the symptoms, knowledge, and behaviour among just that group of drivers, this bias is no limitation for the interpretation of results. In the presentation of the results, probability values less than 0.05 are taken as the criterion of statistical significance unless otherwise specified. 3. Results 3.1. Awareness of symptoms The drivers who had fallen asleep or had been afraid to do so were asked whether they had experienced certain specified symptoms of sleepiness before this happened, referring to the latest incident. The most frequently reported symptom is difficulty to keep the eyes open, reported by 55% of the drivers (Table 1). Yawning, more frequent eye blinks, difficulties to concentrate on the driving, body movement and changing posture, and increased reaction time, are also relatively frequent, being reported by more than one third of the drivers. Most symptoms are reported more frequently by younger than by older drivers. This may indicate a memory effect, as the last incident of falling asleep or being afraid to fall asleep may be longer ago for older than for younger drivers.

3 S. Nordbakke, F. Sagberg / Transportation Research Part F 10 (2007) Table 1 Drivers reporting sleepiness symptoms the last time they fell asleep or were afraid to do so, by age, gender and sleepiness experience (percent) All Age drivers years years years years Statistical significance Gender Statistical significance Sleepiness experience Female Male Fallen asleep Been afraid to fall asleep Statistical significance N Difficult to keep n.s p = p <.001 eyes open Yawning p = n.s p <.001 More frequent p = n.s n.s. eyeblinks Difficult to p < n.s n.s. concentrate on driving Changing position, p = n.s n.s. moving Slower reaction p = n.s n.s. to traffic events Increased variation n.s p < p =.080 in speed Dreamlike state n.s n.s p <.001 of consciousness Misjudgement of road n.s p = n.s. or traffic Freezing p < p < p =.030 However, for changing position, moving the age difference goes in the opposite direction, which could indicate that this is a more common symptom among older drivers. There are only minor differences between men and women in how they recognise their sleepiness. However, men experience more difficulties in keeping their eyes open than women. Women on the other hand, report to a larger degree than men freezing and variation in speed as symptoms of their sleepiness. For some symptoms the prevalence differed significantly between the drivers who had fallen asleep and those who had only been afraid to do so. Two of the symptoms ( difficult to keep the eyes open and dreamlike state of consciousness ) were reported significantly more frequently by those who had fallen asleep. On the other hand, three other symptoms ( yawning, increased speed variation, and freezing ) were reported less frequently by those drivers. A reasonable assumption is that those who have actually fallen asleep are most likely to report the symptoms occurring immediately before falling asleep, whereas those who have been afraid to fall asleep tend to report earlier symptoms. In other words, the two groups may report warning signs occurring in different phases in the process of developing fatigue. A possible related explanation (that needs verification by additional evidence) could be that difficulties to keep eyes open and entering a dreamlike state of consciousness make up the most important warnings that drivers should beware. On the other hand, yawning and increased variation in speed are possibly more properly acknowledge as sleepiness indicators that drivers take seriously, since relatively more drivers experiencing these symptoms have managed to stay awake Awareness of symptoms: multivariate analyses The differences in reported symptoms between drivers who had actually fallen asleep and those who had only been afraid to do so, could possibly be explained by different background characteristics of those two driver groups. For example, there was a higher percentage of male drivers among those who had fallen asleep

4 4 S. Nordbakke, F. Sagberg / Transportation Research Part F 10 (2007) 1 10 Table 2 Odds ratios (OR) for reporting various symptoms of sleepiness during driving, by gender, age and driver sleep history Symptom Driver characteristics Having fallen asleep during driving (vs. been afraid to fall asleep) Female driver Age years (vs years) Age years (vs years) Difficult to keep eyes open 3.00 *** Yawning 0.59 *** 1.95 *** More frequent eyeblinks 1.99 *** 1.36 * Difficult to concentrate on driving 1.80 ** 1.30 * Changing position, moving 0.59 ** Slower reaction to traffic events (0.79) 1.58 ** Increased speed variation 0.74 * 1.50 ** Dreamlike state of consciousness 2.57 *** 1.65 * Misjudgement of road or traffic 0.68 * Freezing (1.18) 2.35 *** 4.40 *** An OR above 1.0 indicates that drivers in that category report the given symptom more frequently than the drivers in the comparison category, and an OR below 1.0 indicates reporting less frequently. Only ORs that are statistically significant are included (the two ORs in parenthesis are close to being significant: p < 0.07). not significant. * p < ** p < *** p < than among those who had only been afraid to fall asleep (71% vs. 42%). To assess the possible influence of gender and other driver background characteristics, a multiple regression analysis was carried out for each of the symptoms. The dependent variable was presence vs. absence of the symptoms, and the independent variables were driver sleepiness experience (fallen asleep vs. been afraid to fall asleep), gender, age (four age groups), and frequency of driving (daily vs. less frequent). Table 2 shows the significant odds ratios associated with the background factors, for each symptom. The results of the multivariate analyses in general confirm the bivariate associations presented in Table 1 regarding differences in reported symptoms between drivers who have fallen asleep and those who had only been afraid of doing so. Thus, the differences cannot be explained by differences in background factors between the two groups Drivers beliefs about sleepiness countermeasures The countermeasure believed to be most effective is to replace the driver (when possible). As shown in Fig. 1, more than 90% think this is very effective. However, this is a countermeasure that is possible only Overtaking Speeding Talk in cellphone Eat sweets/drink soft drinks/sodas Talk to oneself/sing Put on loud music Avoid heavy meals during the drive Prevent passengers from falling asleep Drink coffee Drink water Stop for a bite Ask passengers to talk to you Open the window Stop and take a nap Stop and get out of the car Swap drivers Percent Fig. 1. Drivers judgment of sleepiness countermeasures. Percentage of drivers considering a measure to be very effective.

5 S. Nordbakke, F. Sagberg / Transportation Research Part F 10 (2007) when there is a second driver available. Looking at countermeasures that are applicable to situations when there is only one driver, it is clear that taking a break is considered the most effective. There were three different types of breaks that were asked about, and stopping and getting out of the car was the one considered most effective (80%), with stopping to take a nap on second place (70%). Stopping to eat was considered efficient by far fewer drivers (30%). The other countermeasures believed by more than 30% of the drivers to be very effective included opening the window, and asking passengers to talk to the driver Drivers knowledge and beliefs about crashes and risk factors related to falling asleep at the wheel The respondents were asked to state their degree of agreement to a list of statements regarding sleep at the wheel and sleep-related crashes (Table 3). It appears that a large majority (almost three out of four drivers) acknowledge that falling asleep at the wheel can happen to anyone, and is not limited to special subgroups. A similar proportion agrees that 24 h without sleep is comparable to a BAC level of 0.1% regarding effects on performance. Rather few believe that males and young people are more prone to fall asleep compared to other groups. Concerning the statement that sleep-related crashes are more serious, the drivers are almost equally divided between agree and disagree. It is notable that almost two-thirds of the drivers believe that rumble strips will wake up a sleeping driver. Table 3 Drivers degree of agreement to statements about risk of falling asleep and sleep-related crashes Statement Agree Neither agree nor disagree Falling asleep at the wheel can occur to anybody Only persons with sleep disorders fall asleep while driving Only physically unfit persons fall asleep while driving Males are more prone to fall asleep while driving than females Young drivers are more prone to fall asleep while driving than older drivers Sleep-related crashes are more serious than other road crashes Most sleep-related crashes happen during daytime Rumble strips will awake drivers who have fallen asleep A 24-h sleep deprivation results in a deterioration of driving performance comparable to that of a 0.1% BAC The number of valid answers varies between 1502 and Disagree Heavy meal before/during a drive 14 Time of the day Tiresome/stressfull day at work Little sleep the night before Alcohol the night before Time at the wheel Use of medications Time since last sleep period Little sleep several days before Percent Fig. 2. Drivers beliefs regarding risk factors for falling asleep at the wheel. Percentage of drivers believing a factor influences the risk to a large extent. (The number of valid answers varies between 1484 and 1499.)

6 6 S. Nordbakke, F. Sagberg / Transportation Research Part F 10 (2007) 1 10 It seems that the problem of drivers who fall asleep during daytime is somewhat underestimated, since only 11% agree to the statement most sleep-related crashes occur during daytime. Earlier research (e.g., Sagberg, 1999) has shown that when all kinds of crashes are considered, the absolute number of sleep-related crashes is higher during daytime, although the crash rate is higher during the night. This is explained by the difference in traffic volume between day and night. Concerning individual risk factors, about 80% believe that little sleep over several days affects the likelihood of falling asleep at the wheel to a large extent (Fig. 2). Slightly fewer (70%) believe that the same is true for elapsed time since last sleep period, and still fewer (58%) for time at the wheel (continuous driving for a long period). It is also notable that only 20% believe that the time of day for starting a trip influences the risk of falling asleep to a large extent What drivers do to prevent falling asleep: pre-trip countermeasures Fig. 3 shows that the precaution reported by the largest proportion of drivers (71%) to be taken to a large extent is to avoid or be moderate with alcohol on the evening before the trip. The remaining countermeasures are reported by less than half of the drivers to be taken to a large extent. All countermeasures were reported more frequently by older than by younger drivers What drivers do to prevent falling asleep: in-transit countermeasures The precautions taken most frequently during the trip in order to keep awake, as shown in Table 4, are to open the window and to stop and get out of the car for a while; both of these are reported by about half of the drivers on the last occasion when they fell asleep or were afraid to do so. Next on the list is listening to music (reported by 36%) and talking to oneself or singing (24%). A comparison of the reported frequencies to the drivers beliefs regarding the efficiency of the various measures (which were shown in Fig. 1 and also indicated as efficiency ratings in Table 4) shows that the measures believed to be most efficient are not necessarily the ones reported most frequently. Even though taking a nap is considered as the second most efficient measure (next after replacing the driver), few drivers (10%) did this when they got sleepy. More often the drivers engaged themselves in activities that they consider being less effective, like different activities in the car (opening up the window, putting on music, singing/talking to themselves, etc.) without stopping. The only action the drivers took that corresponds with their opinion of what is effective when feeling sleepy, is to stop and get out of the car. This measure is regarded as equally effective as taking a nap. Avoid heavy meal right before the drive Plan starting time/sleeping habits Sufficient sleep over a long period Plan stop during the drive Sufficient sleep the night before Avoid/be moderate with alcohol Percent Fig. 3. Pre-trip precautions to avoid falling asleep. Percentage of drivers taking a precaution to a large extent. (The number of valid answers varies between 1498 and 1506.)

7 S. Nordbakke, F. Sagberg / Transportation Research Part F 10 (2007) Table 4 Countermeasures taken by drivers when they got sleepy the last time they fell asleep or were afraid to do so What did you do to avoid falling asleep when you started feeling tired? Total (n = 1280) Efficiency rating a Opened the window 52 ** Stopped and got out of the car 50 *** Put on music 36 * Talked to myself or sang 24 * Ate sweets/drank sodas 17 * Drank coffee 15 ** Asked passenger(s) to talk to me 13 ** Drank water 11 ** Stopped and had a nap 10 *** Stopped to eat 8 ** Drove faster or overtook 2 * Talked in mobile phone 2 * Percent, ranked by reported frequency. *<20%, **20 35%, ***>35%. The ratings are based on the data presented in Fig. 1. Table 5 Drivers taking various countermeasures in transit, by age groups (percent) All drivers years years years years p Opened the window n.s. Stopped and got out of the car Put on loud music Talked to myself or sang Ate sweets/drank sodas Drank coffee Asked passenger(s) to talk to me Drank water Stopped and had a nap Stopped to eat n.s. Drove faster or overtook n.s. Talked in mobile phone N The final column shows the p-values for significant chi square tests. What kind of countermeasure a driver takes seems to be somewhat dependent on age. The most striking difference between age groups is that older drivers take more real actions than the younger drivers. Various countermeasures in transit by age is shown in Table 5. The likelihood for a driver to stop either to just get out of the car for a while or to have a nap increases with age. On the other hand, the effort to prevent sleep by activities within the car while continuing driving decreases with age, such as putting on loud music, talking to oneself/singing, eating sweets/drinking sodas. Drinking coffee is more common among those above 26 years, and it is especially common in the age group between 36 and 45 years. The youngest drivers, those below 25 years, drink on their part more water than the older drivers Actions taken after waking up 3.5 percent of the drivers who have fallen asleep at the wheel some time or other, report that the sleep episode resulted in an accident. The drivers who had not experienced an accident as a result of the drivers sleep, were asked to answer the following question: What did you do when you woke up again last time/that time you fell asleep? The frequency distribution of the response alternatives is shown in Table 6.

8 8 S. Nordbakke, F. Sagberg / Transportation Research Part F 10 (2007) 1 10 Table 6 What drivers did after they woke up again last time/that time they fell asleep Actions taken when they woke up % Continued to drive without having a break 26 Took a short break 37 Parked the car and took a nap 23 Stopped to drink and eat 14 Swap driver 15 Stopped and continued the next day 2 Other 4 Several alternatives possible; N = 653; percent. The road has rumblestripes Work as a driver A wellknown road It will be allright when I start driving Wish to get back from work quickly Wish to get back at a reasonable hour A short trip to the destination Must keep an appointment A short trip Percent Fig. 4. Conditions influencing drivers to continue driving despite being sleepy. Percent of drivers reporting each condition (n = 1098; only drivers who sometimes drive when feeling tired are included). As many as 26% continued to drive without having a break first. However, the most common measure after having fallen asleep is to have a short break (37%). Quite a few also report that they had a nap (23%). It is interesting that as many as 15% report that they swapped drivers after waking up again, which means that a great deal actually had the opportunity to swap drivers before they fell asleep Self-reported sleepy driving: reasons for driving when sleepy Seventy three percent of the drivers confirm that it happens that they continue driving even when they feel too tired. Fig. 4 shows the percentage of drivers giving various reasons for driving despite being sleepy. The most frequently reported reasons seem to be related to the length of driving (first and foremost they drive if it is a short trip, but also if it is a short remaining distance to the destination). Reasons related to time pressure also seems to be of importance, such as the wish to keep an appointment, the wish to get back home (from e.g., a weekend trip, visit to friends/family) at a reasonable hour or the wish to get back from work quickly. 4. Discussion The drivers in this study have good knowledge about the risk of falling asleep while driving, as well as its associated hazards. It seems to be widely agreed that falling asleep while driving can happen to anyone. Even though the drivers may acknowledge falling asleep as an important cause of road accidents, the severity of sleep-related accidents seems to be somewhat underestimated. This may possibly prevent the drivers from taking sleepiness and fatigue seriously enough. Most drivers are also aware that sufficient previous sleep is an important precondition for not getting sleepy while driving. Despite this knowledge, however, a large proportion of the drivers do not act in accordance with this, and therefore do not get sufficient sleep before a long drive. The same applies for what they do when

9 S. Nordbakke, F. Sagberg / Transportation Research Part F 10 (2007) experiencing sleepiness while driving; even though they consider stopping and taking a nap to be a very efficient countermeasure to prevent falling asleep, only few drivers actually do so when feeling tired and fatigued. More often drivers engage in activities (when feeling tired while driving) that they consider less effective, like different activities in the car (opening up the window, putting on music, singing/talking to themselves, etc.) without stopping. The only measure taken that corresponds with the drivers opinion of what is effective is to stop and get out of the car. This measure is regarded by the drivers as equally effective as taking a nap. When under time pressure, this might represent a quicker alternative to taking a nap. The preference for this countermeasure might perhaps also be related to the experienced degree of sleepiness. This measure, however, seems to have only a temporary palliative effect on sleepiness. This study verifies earlier research showing that most drivers experience various symptoms of sleepiness before they fall asleep during driving. The problem seems to be that those symptoms are not taken seriously enough. This may partly be due to an underestimation of the relationship between the various physiological and behavioural signals on one hand, and the risk of falling asleep on the other hand. In addition, some drivers may overestimate their ability to fight the symptoms of sleepiness. Further studies of driver behaviour in the various phases of sleepiness will hopefully shed more light on this issue. It is notable that the reported sleepiness symptoms tend to differ somewhat between drivers who had actually fallen asleep and those who only have been afraid to do so. For example, difficulties to keep the eyes open is reported more frequently among those who have fallen asleep, whereas yawning was reported more often among those who were afraid to fall asleep. This may imply that yawning is an earlier sign of sleepiness than increased blinking duration. This seems to be consistent with recent results from simulator studies of driver behaviour associated with the various stages of fatigue (Sagberg, Jackson, Kürger, & Muzet, 2004). The study shows that reporting of symptoms is more frequent among young than old drivers. If this can be related to a memory effect, or that younger people are more aware of sleepiness symptoms in general, requires further research. However, the study shows quite clearly that women to a larger degree than men report freezing and increased variation in speed when feeling sleepy. In spite of the drivers knowledge of the risk and of the significance of sleep, most drivers continue to drive when recognizing sleepiness while driving. Almost three out of four among these drivers report that they sometimes continue driving even when feeling too tired to drive. The major arguments are related to the distance of driving primarily that it is a short drive, but also that it is a short distance left to the destination. Social factors seem to contribute as well; the drivers often mention appointments and the wish to come home at a reasonable hour as the most important reasons for continuing driving while feeling fatigue or sleepy. A possible limitation concerning generalizing from the present findings could be sampling bias due to the use of an internet-based sample as well as self-selection of drivers especially interested in the topic of driver fatigue. On the other hand, it was the aim of the study to investigate just the group of drivers that had experienced fatigue, and we have no particular reasons to believe that the relationships between symptoms, knowledge, beliefs and the use of countermeasures in the present sample differs substantially from the expected relationships in the general driver population. In conclusion, there seem to be some notable discrepancies between knowledge and action when it comes to measures to prevent sleepiness and falling asleep, both before and during driving. However, older drivers tend to take more necessary actions before a trip and are also more prone to stop and take a nap during a trip. This might indicate that they have learned by experience. As driver sleep does not always have a happy outcome, younger drivers should learn about important countermeasures without having to experience such hazardous incidents. That few drivers take the necessary actions when feeling sleepy and tired at the wheel might indicate that driver fatigue and sleepiness are not taken seriously enough and that the drivers overestimate their own capabilities, but it might also be a matter of lack of knowledge on when to act on their sleepiness behind the wheel. In combination with social and motivational factors this results in a high prevalence of sleepy driving. It is a big challenge to convince drivers of the importance of paying sufficient attention to the early warning signs, and to make them stop driving in time. Information to drivers should focus on the fact that the transition from fully awake to being asleep is very gradual, and often poorly perceptible, on the importance of taking the early signs (like e.g., yawning, more frequent eye blinks) very seriously, and on the fact that a driver fighting against sleep is likely to lose in the long run.

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