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Sleep-related distraction: Why it’s important to investigate and document driver fatigue in road traffic collisions

Hours Posted on Hours Updated on

Christina M. Rudin-Brown, Ph.D., CCPE
Human Factors North, Inc., Canada

Bio

Dr. Missy (Christina) Rudin-Brown, CCPE, is a human factors specialist with Toronto-based Human Factors North, Inc. (HFN). She has over 25 years’ experience in road and transportation safety research, investigation and policy analysis, having worked for over 10 years as a senior human factors investigator and manager with the Transportation Safety Board of Canada (TSB) and, before that, for 4 years as a senior research fellow in human factors at the Monash University Accident Research Centre (MUARC) in Melbourne (Australia). From 1999 to 2009, she conducted human factors research with Transport Canada’s Road Safety Directorate, supporting the development of federal motor vehicle standards and regulations. Dr. Rudin-Brown has been a human factors expert witness in over 30 road safety and motor vehicle forensic collision cases since joining HFN in 2012. She has also published over 100 peer-reviewed publications in the transportation human factors area, including two co-edited books.

Abstract

Sleep-related fatigue affects a person’s susceptibility to distraction. This has obvious consequences in terms of road safety and traffic collisions. A fatigue investigation methodology developed and used previously in other modes of transport was applied as part of a forensic human factors assessment of a motor vehicle collision where driver distraction played a role. The investigation found that sleep-related fatigue and driver distraction were contributing factors. While many collisions are investigated primarily for the determination of fault, criminal activity, or negligence, the results from this investigation demonstrate the importance of identifying, investigating, and documenting the risk factors and consequences associated with fatigue and its management in the road traffic context. Adopting a systems approach to forensic human factors investigation that includes the investigation of sleep-related fatigue can help to capture those – otherwise missed – opportunities to improve our roads and make driving safer.

Background

While a vehicle operator falling asleep at the controls is the most obvious symptom of fatigue in transportation, less extreme fatigue levels are reliably associated with performance impairments in, for example, attention, information processing, memory, and reaction time. An even less well-known effect of sleep-related fatigue is its ability to increase one’s susceptibility to distraction. This has obvious consequences for road safety, considering what is known about the effects of distracted driving on collision risk; according to 2019 to 2023 data in Transport Canada's National Collision Database, distracted driving contributes to an estimated 20% of fatal motor vehicle collisions.1

Compared to other industries, where work hours are generally predictable, fatigue in transportation is made more likely by challenges to the human body's circadian rhythm caused by shiftwork. Commercial drivers and shift workers often work long and irregular schedules — sometimes in challenging conditions — that are not always conducive to getting sufficient restorative sleep. Although shift scheduling practices and hours of service regulations are designed to improve safety and efficiency of operations and to minimize the risk of employee fatigue, there remains a need to better understand and identify fatigue risks in the traffic safety context. Investigating transportation collisions is one way to do this.

Investigating for fatigue using a systems approach can identify the factors that contribute to fatigue, determine when fatigue does (and does not) contribute to a collision, and document the findings. Investigating leads to safety improvements in the future by providing the opportunity for drivers and road safety interest-holders to learn from the experience of others.

This article presents some of the literature on sleep-related fatigue and its little-known relationship with distraction. A fatigue investigation methodology, developed and used previously in other modes of transport, was applied as part of a forensic human factors assessment of a motor vehicle collision where driver distraction played a role.2 The importance of investigating and documenting the risk factors and consequences associated with fatigue and its management in the road traffic context is highlighted.

The collision

A highway collision took place on a Saturday night in August at approximately 2:30 a.m. Weather was clear at the time and road conditions were dry. The driver of a sedan was travelling south on a divided highway in the middle of three southbound travel lanes. The speed limit was 100 km/h. As the driver approached the location on the highway where a friend’s vehicle was stopped with a flat tire, they pulled over as far as possible onto the shoulder and left lane and stopped, activating the vehicle’s flashing hazard lights and brake lights. A portion of the stopped vehicle remained in the travel lane.

At around the same time, a young man was driving a company vehicle – a pick-up truck – also southbound, on the same highway, travelling in the left-most travel lane. He was on his way home after working a 16-hour shift as a construction apprentice. As his vehicle approached the area where the sedan was stopped, he was looking inside his vehicle, down and towards the right, adjusting the volume on his personal device. He reported not looking forward for “about 2 seconds”. His truck was travelling between 106 and 111 km/h and he did not see the sedan in his travel lane. The pick-up struck the rear of the sedan at 106 km/h. Both vehicles came to rest in the left lane over 60 metres south of the point of impact. The sedan driver was seriously injured. The pick-up truck driver was charged by police for speeding.

Sleep-related fatigue 

Sleep is a fundamental biological need. Normal, healthy adults need between 7 and 9 hours of sleep every 24 hours to feel well-rested and to be able to maintain vigilance throughout the day; anything less can result in fatigue.3 Because it is biological in nature, sleep-related fatigue cannot be prevented by, for example, characteristics of personality, intelligence, education, training, skill, or conscious motivation. While experiencing fatigue is a normal physiological and behavioural state, for drivers, experiencing fatigue while operating a vehicle can be catastrophic.

Less extreme levels of fatigue are reliably and significantly associated with performance impairments in cognitive functioning, including slowed reaction time,4 amongst others. At the extreme fatigue end, a person has difficulty maintaining wakefulness and, without being kept active and aroused, readily falls asleep. 

Performance decrements associated with fatigue are significant risk factors and predictors of occupational accidents and injuries, including motor vehicle accidents.5 An analysis of driving data collected under naturalistic conditions that used measurements of drivers’ eye closure to identify drowsy driving found that drowsiness was involved in 8.8%–9.5% of all crashes examined and 10.6%–10.8% of crashes that resulted in significant property damage, airbag deployment, or injury.6 Crashes that occurred in darkness were more than three times as likely to involve drowsiness as those that occurred during daylight.6 

Shift scheduling practices can create conditions that increase the risk of employee fatigue. For example, shifts that are too long limit an individual’s opportunities for sleep because they do not allow sufficient time to commute, or for personal activities like eating and hygiene / grooming.

Being awake for longer is also associated with greater pressure for sleep and, therefore, greater fatigue, especially at night.7 Research on the effects of fatigue vs. alcohol on cognitive psychomotor performance found that 17 hours of wakefulness produced impairments in psychomotor functioning on a computer test of hand-eye coordination and reaction time that were equivalent to a blood alcohol concentration (BAC) of 0.05%.8 This is the proscribed level of alcohol intoxication for drivers in many countries, and is subject to administrative penalties such as immediate roadside license suspensions and vehicle impoundment in Canada. Twenty-four hours of sustained wakefulness resulted in cognitive psychomotor performance at a level equivalent to the performance deficit observed at a BAC of roughly 0.10%. (A BAC of 0.08% is the criminal driving limit in all Canadian provinces and territories.)

Shift timing during the 24-hour circadian cycle also contributes to fatigue. In terms of sleep, day hours are not biologically equivalent to night hours. Many years of evolution have anchored human biology with sleep occurring during night hours and wakefulness within day hours. This means that the biological drive for sleep during the night hours is much stronger than during the day hours.9 It also means that fatigue can result from fewer hours of continuous wakefulness if these hours occur at night rather than during the day, even for regular night workers. Night shifts (those that include at least 3 hours of work between the hours of 11:00 p.m. and 06:00 a.m.) are the most significant single factor predicting severe sleepiness, leading to 6 to 14 times higher risk for “severe sleepiness” than day shifts.10 Therefore, safety is more likely to be compromised during night shifts than day shifts, particularly where night work is coupled with extended hours or overtime.11

Driver distraction

Drivers spend about 80%-90% of their time driving looking ahead at the roadway and roadside, using eye fixations in the forward field.12 While they can switch their attention rapidly from one information source to another, humans can attend well to only one information source at a time.13 Therefore, a driver’s attention can be compromised when they are distracted, which is defined as “a diversion of attention away from activities critical for safe driving towards a competing activity”.14 

Visual distraction occurs when a driver looks away from the road scene and instead focuses visual attention on another target, such as a handheld device.15 Cognitive distraction occurs when a driver’s attention is withdrawn from the processing of information necessary for the safe operation of a motor vehicle and applied to a non-driving related activity.16 Manual distraction occurs when a driver takes one or both hands off the steering wheel to manipulate a control, device, or other non-driving-related item. All types of distraction, which can co-occur, slow driver reaction time and increase the likelihood that a driver will miss critical visual stimuli in the visual field and roadway ahead. The visual and manual distraction associated with handheld use of cellphones or other devices is why it is illegal in most jurisdictions to use handheld devices while driving.

It is recognized that driver eye glances away from the forward visual scene are significantly associated with crashes and near-crash events. Research conducted under “naturalistic” conditions, where everyday drivers operate their own vehicles while a range of video and vehicle performance data is recorded and later analyzed, has consistently found that driver eye glances away from the forward visual scene are significantly associated with crashes and near-crash events.17-18 More specifically, in-vehicle tasks that require glances totaling more than 2 seconds increase near-crash / crash risk by at least two times.17 Activities that require a driver to handle and look at a device result in the driver looking away from the road, and lead to impaired vehicle control and increased missed events.18 One of the largest naturalistic driving studies, the Second Strategic Highway Research Program 2 (SHRP2), found that 68% of the crashes that occurred involved some form of observable distraction.19

Gaze eccentricity

Location of a control or device within a vehicle will affect how much, and for how long, a driver looks at it. Because they require central (foveal) vision, driver glances towards in-vehicle locations that are farther away from the forward line of sight are associated with poorer detection of target objects (missed targets) in front of the vehicle and slower reaction times to identify forward-located targets.20

Driver survey studies21-23 have found that, among young drivers, music-related activities (e.g., playing and changing music on a smartphone) are the most common mobile phone activities undertaken when driving. Music search and selection tasks using kinetic interfaces like the ones used in smartphones increase the amount of time that drivers spend with their eyes off the roadway.24

In 2013, the United States introduced driver distraction guidelines25 for North American automobile manufacturers, to limit and reduce the potential for distraction from in-vehicle devices and controls. These guidelines recommended that glances to a device should require a maximum downward viewing angle of 30 degrees from the average driver’s seated eye position to the geometric centre of the display. In 2019, Transport Canada also published guidelines to limit distraction from visual displays in vehicles, recommending that visual displays should not interfere with the driver's view of the road or any of the existing controls and displays and should be positioned in the forward view as close as possible to road centre, in line with the driver's forward view without obstruction.28 

Sleep-related distraction

An interesting and less well-known aspect of fatigue is its relationship with distraction. Regardless of its causes, sleep disruption leads to fatigue and increases one’s propensity to be distracted. A laboratory study looking at the relationship between fatigue and distraction demonstrated that, when the sleep of normal, healthy university students on the night before testing was restricted to only five hours, their performance on a proven, monotonous reaction time test – the psychomotor vigilance test (PVT) – deteriorated.26 In that study, 16 healthy young adults (8 men, 8 women) attended a laboratory in the afternoon on two occasions: once after their sleep was restricted to five hours, and another after they had a normal night’s sleep. Participants underwent 30-minute PVT sessions in a sound-dampened cubicle both with, and without, an attractive distraction task – a popular television program playing on a nearby television. The television was located in the participant’s visual periphery, 90° away from the PVT monitor. Although they were instructed to ignore the television (whether it was turned on or off) and attend fully to the PVT, when they were sleep-deprived, participants made significantly more (almost six times as many) head turns toward the television and demonstrated significantly more (about 5 times as many) PVT “lapses” (reaction times of longer than 500 milliseconds) than when they were fully rested. The distractive effect of fatigue was so strong that, even when participants were sleep-deprived but the television was turned off during the “no distraction” condition, both an increase in head turns and in lapses occurred. The study authors interpreted this finding as showing that, “even in nondistractive environments, sleepy people will seek distraction, possibly in an attempt to overcome sleepiness or boredom”.

A more recent study explored how lack of sleep can affect one’s “distractedness” and found that people whose sleep was restricted were less able to recover from distractions than were people who were well-rested.27 In that study, over 200 people were assessed in a sleep lab. Between 10:00 p.m. and midnight, they worked individually on a procedural task that required several steps to complete. While they were working, participants were periodically interrupted. These repeated distractions meant that they had to reengage each time with where they were in the sequence of task steps. Afterwards, half of the participants spent the night at home and slept normally; the others stayed at the sleep lab and did not sleep at all. The participants were assessed on the same procedural task the next morning. Fifteen percent of the sleep-deprived participants were unable to complete the task compared to only one percent of the rested group. Those who were sleep-deprived but were able to complete the task made significantly more errors than those who were well-rested. The authors concluded that sleep-deprived individuals should not perform procedural tasks that are associated with “interruptions and costly errors” or, if they must, they should perform such tasks only for short periods.

Fatigue - Analysis

Due to the relationship between distraction and fatigue, and because the pick-up truck driver had been working many shifts before and was in the process of travelling home after a work shift at the time of the crash, a fatigue analysis for the pick-up driver in the collision described above was carried out. The analysis examined six known risk factors for sleep-related fatigue: 1) acute sleep disruptions; 2) chronic sleep disruptions; 3) continual wakefulness; 4) circadian rhythm effects; 5) sleep disorders; and 6) medical conditions. Hours of work and rest were plotted, as was commuting time, with 30 minutes of “personal time” applied at the start and end of each workday. Personal cellphone records were used to estimate sleep and wake times. On the pick-up driver’s days off, and where cell phone records were not clear, a sleep time of 8 hours was assumed since he normally had restricted opportunities for sleep due to many back-to-back work shifts and because he had reported that “it was tiring working a lot of shifts”.

An acute sleep disruption is understood to be a reduction in the quantity or quality of sleep occurring within the prior three days. On the night before the collision, the pick-up truck driver had a maximum opportunity for sleep of seven hours. The night before that, he had an opportunity to get only 5 hours of sleep. The night before that, he had a sleep opportunity of only 6 hours. This is less than the recommended 7 to 9 hours of sleep per 24 hours for adults.3

On both days leading to the crash, the pick-up driver had worked 16-hour shifts that began at 10:30 a.m. and ended at approximately 02:30 a.m. Assuming a wake-up time of 09:30 a.m. on the day of the collision would allow him half an hour to shower and eat, and half an hour to commute the approximately 23-minute drive to his place of work that day, the pick-up driver would have been awake for at least approximately 17 hours at the time of the crash (at 02:40 a.m.). This duration is associated with decrements in cognitive psychomotor functioning equivalent to those seen with a BAC of 0.05%.8 The timing of driving during the nighttime circadian trough (between approximately 10:30 p.m. and 04:30 a.m.) also increased the influence of the extended period of wakefulness on fatigue.

Chronic sleep disruption occurs where sleep quantity or quality disruptions are sustained for periods longer than three consecutive days. In the 20 days prior to the collision, the pick-up driver had a period of only two days (and two nights), when he did not work. His average total nightly sleep duration was only 6 hours, suggesting a chronic sleep disruption that would have placed him at an elevated risk of performance impairments associated with fatigue during that time.

The pick-up driver’s work timesheets showed that he had worked many shifts on 18 of the 20 days before the collision, with only 2 days off and 3 days (of 17) where he worked 10 hours or less. On the other 14 days he worked 16-hour shifts. The sleep-wake history revealed that, at the time of the crash, and even if he had taken every possible opportunity to sleep in the preceding 5-day work period, the truck driver was at an increased risk for fatigue due to three risk factors: acute and chronic sleep disruption, and continual wakefulness. 

Distraction - Analysis

Based on where in the vehicle the pick-up driver had been looking towards his handheld device, the downward viewing angle (from horizontal) to the device would likely have been greater than the recommended maximum downward angle of 30 degrees, requiring the driver’s view (gaze eccentricity) to be away from the forward roadway and would be expected to result in poorer detection of, and longer reaction time to, hazards in the roadway ahead. This was confirmed by the driver’s report that he had been looking towards his device “for about two seconds” and did not detect the stopped vehicle in the roadway ahead until the airbags in his vehicle deployed.

As a requirement for use of a company vehicle, the pick-up driver had signed a vehicle use agreement with his employer. The agreement stipulated that handheld cellphone use while the vehicle was in motion was prohibited.

Company safety program for fatigue, distraction

An employer has a responsibility to ensure a safe work environment for employees. For employers that provide employees with company vehicles, that includes road and vehicle safety elements, both to protect the employees and the travelling public. Fatigue and distraction are known road safety risks, and employers are responsible for managing and preventing the risks through their shift scheduling practices, vehicle oversight, employee safety and education programs, and incident reporting and investigation.

A company’s role in managing fatigue in its employees involves educating staff at all levels on the causes of and mitigations for fatigue, defining appropriate policies and procedures with respect to fatigue management, ensuring a working environment that minimizes fatigue as much as practicable, and striving for continual improvement in reducing fatigue by incident reporting and investigation. An employee’s contribution to preventing fatigue includes applying knowledge of the effects of fatigue to take all reasonable steps to report for work well-rested, making effective use of fatigue countermeasures, recognizing the signs of fatigue in themselves and in co-workers, and taking action to ensure that fatigue arising from activities inside or outside of work does not lead to performance issues when working.

At the time of the crash, the pick-up driver’s employer had a company safety plan, which included a 1-page ‘vehicle safety’ section. The company also had a generic road safety program for employees who commuted to and from work or who drove for work. The program was intended to prevent or mitigate the potential effects of various road safety risks, and included training (in-class, presentation, video), a manual, and a vehicle use / device use agreement to limit driver distraction.

Discussion

The pick-up truck driver was experiencing sleep-related fatigue and was distracted in the seconds leading up to the collision, which, when considered in the context of the high speed at which the vehicle was travelling and the location away from the forward roadway towards where the pick-up driver was looking, prevented him from detecting and responding to the stopped sedan in time to avoid the impact. The fatigue analysis showed that, because he had been working a very challenging work schedule in the weeks preceding the collision that did not allow adequate time to obtain sufficient restorative sleep, and because he had been awake and working for at least about 17 hours at the time of the accident and during early morning hours, the pick-up driver was at high risk of performance impairments from fatigue, including slowed reaction time and an increased risk of becoming distracted.

Regardless of its causes, sleep disruption leads to fatigue and increases one’s propensity to be distracted. The distractive effect of fatigue is so strong that, even when there are no distracting secondary tasks readily available, sleepy people tend to seek distraction, possibly in an attempt to overcome sleepiness or boredom. 

Although work and rest time of shift workers (like the pick-up driver) are not subject to professional drivers’ hours of service regulations, there are legal considerations regarding an employer’s responsibility when assigning work and a company vehicle. Informing drivers about company driving policies around device use and how to manage fatigue is important. With regards to managing the potential safety consequences of fatigue, although the pick-up driver’s employer was aware of the number of shifts that he had been working in the weeks prior and of the number of hours that he worked on the day of the crash, they did not flag the potential for fatigue, nor did they investigate the collision to determine if fatigue played a role.

Conclusion

Fatigue and distraction are known road safety risks, and the investigation of this collision concluded that both factors contributed in this case. While many road collisions are not investigated or are investigated primarily for the determination of fault, criminal activity, or negligence, there remains significant untapped benefit that could be gained if the investigation of road crashes were to be expanded in terms of breadth and depth. The importance of identifying, investigating, and documenting the risk factors and consequences associated with fatigue and its management in the road traffic context is clear. Adopting a systems approach to forensic human factors investigation that includes the investigation of sleep-related fatigue will help to capture those – otherwise missed – opportunities to improve our roads and make driving safer.

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