Driver fatigue – an accident waiting to happen
This topic is sponsored by NRMA ACT Road Safety Trust.
Although we often associate driver fatigue with long-haul truck drivers, it can affect all of us.
Strange things happen in the dead of night. A car leaves a dark and lonely highway, apparently at full speed, and slams into an unforgiving tree. Nobody sees it happen and the driver is dead. On another country road, a car drifts from its lane for no obvious reason and smashes into an oncoming truck, killing all the car's occupants.
More than likely, these crashes were caused by fatigue: drivers either falling asleep at the wheel or so exhausted they made serious and fatal driving errors.
Fatigue is thought to be one of the biggest killers on Australian roads, rivalling the effects of speed and alcohol. But the full extent of its role is not really known unlike alcohol and drugs, fatigue can't be tested for in post-mortems. This is the reason for the big difference between the lowest and highest estimates of the role of fatigue in the Australian road toll.
One study based on coronial and police reports found that fatigue played a part in only 5 per cent of fatal crashes in 1988. A more recent survey (for 1994) raised this figure to about 18 per cent. It included not only those crashes in which police identified fatigue as a cause, but also cases where the crash description suggested 'loss of concentration' had been a contributing factor. A third review found that around 30 per cent of rural crashes in Western Australia could be attributed to fatigue. And a fourth study, by the Australian Transport Safety Bureau, reckoned that fatigue was a factor in over 16 per cent of the total crashes on Australian roads in 1998.
In the Australian Transport Safety Bureau study, fatigue-related crashes were defined by first excluding all crashes involving alcohol, unlicensed drivers or pedestrians and those occurring in areas where the speed limit was less than 80 kilometres per hour, and then counting all remaining head-on crashes and any single-vehicle crashes between midnight and 6 am, and between 2 pm and 4 pm (the two periods of the day when the effects of fatigue are most evident). This is a pragmatic definition; it has the advantage of being repeatable in other studies, but it risks missing some crashes in which fatigue was a factor and of counting others where it wasn't.
Why does fatigue cause accidents?
The effects of fatigue on driver performance have been documented in numerous studies in which subjects were required to perform driving tasks after long hours of wakefulness. Fatigue manifests itself in:
- slower reaction times: fatigue increases the time taken to react in an emergency;
- reduced vigilance: subjects perform worse on attention-based tasks when sleep-deprived. For example, a fatigued driver will be slower to notice oncoming hazards, such as roadworks or a railway crossing; and
- information processing: fatigue reduces both the ability to process information and the accuracy of short-term memory. Thus, a fatigued driver may not remember the previous few minutes of driving and will be slower in evaluating oncoming hazards.
The Centre for Sleep Research at Flinders University in South Australia has likened fatigue-induced impairments to those caused by alcohol: a person kept awake for 17 hours will perform at a standard comparable to that of someone with a blood-alcohol concentration (BAC) of 0.05 per cent (the legal limit in Australia). After 24 hours without sleep, a person will have capabilities similar to someone with a BAC of 0.10 per cent.
But probably the greatest hazard posed by fatigue is the risk of sleep itself. A fatigued driver who remains awake will probably be able to take some (perhaps belated) action to avert a crash, but one who has fallen asleep must rely solely on luck for survival.
Researchers have long noted that fatigue-related accidents tend to occur in two distinct periods of the day between midnight and 6 am, and between about 2 pm and 4 pm. These periods coincide with typical low-points in our daily pattern of alertness, or circadian rhythm (the word 'circadian' is derived from two Latin words: circa, meaning 'about', and dies, meaning 'day').
Most organisms follow a daily routine (circadian rhythm). Songbirds, for example, mark sunrise and sunset with their vocal-chords. Many Australian marsupials sleep during the day and go about their business in the relative cool of the night. Are these routines based purely on external factors? For example, do nocturnal animals simply get up when they notice that the sun has set, or is their behaviour also governed by some internal timing mechanism?
Scientists have shown that most organisms have internal 'clocks'. If the sun failed to rise one day, songbirds would still sing their usual tune. Plants whose leaves track the sun will continue to do so if kept in a perpetually dark room as noted in 1729 by the French scientist d'Ortous de Mairan.
Scientists have gathered molecular evidence for an internal clock in humans (Box 1: Circadian rhythms at the molecular level), but circumstantial evidence is provided by the modern phenomenon of jet-lag. Travellers who have moved between time zones say, from Australia to the United Kingdom (a difference of about 10 hours) typically find it difficult to sleep, even when tired. It might be 10 pm in London and theoretically bedtime, but according to the body-clock it's 8 am and time to get up.
In humans, the circadian rhythm is controlled by a small region of the brain called the suprachiasmatic nucleus (SCN). The SCN is located in the hypothalamus, which regulates many functions of the autonomic nervous system.
One of the main ways in which the SCN transmits its time-related information is by stimulating the production of melatonin, a hormone manufactured in the pineal gland at the base of the brain. Melatonin levels typically increase in the body after sunset and reach their peak between 12 midnight and 6 am. This corresponds with the body's lowest levels of alertness and body temperature and its lowest capacity for the processing of incoming information. A second, smaller trough in these functions occurs in the afternoon, commonly between about 2 and 4 pm.
These two dips in the circadian rhythm are dangerous for drivers. Fatigue-related crashes are thought to be about twice as high at 2 pm as they are at 10 am, and nearly six times as high at 2 am.
What sleep does
Scientists have identified five stages of sleep:
- stage 1 (light sleep),
- stages 2-4 (deep or delta-wave sleep),
- stage 5 (rapid-eye-movement (REM) sleep), which is the stage in which our most vivid dreams occur.
Each sleep cycle comprising the five stages takes about 90 minutes. Thus, someone sleeping for 8 hours will sleep through about 5½ cycles.
The different stages consume different amounts of the sleep quota: stage 1, for example, usually makes up less than 10 per cent of a full night's sleep, while the REM stage might span about 25 per cent (although percentages vary by age group).
Surprisingly little is known about the physiological role of sleep and the ways in which it restores the brain to its full functions. But the effects of fatigue on the brain can be measured. Studies have shown that after 24 hours of sustained wakefulness the brain's metabolic activity can decrease by up to 6 per cent in total and by up to 11 per cent in specific areas of the brain particularly those that play a role in judgement, attention and visual functions.
Measures to prevent fatigue-related crashes
Changes to road design, such as those listed below, could help prevent fatigue-related crashes:
- sealing road shoulders so that drivers can maintain better control if they drift off the road;
- providing 'audio-tactile' edge linings so that drivers can hear and feel when their tyres cross the line;
- ensuring that there is an adequate number of rest areas so that long-distance drivers are able to take frequent breaks;
- building divided highways to minimise the risk of head-on collisions; and
- removing roadside hazards such as poles and trees to prevent collisions.
Public education campaigns to warn of the dangers of driving while fatigued also play an important role in reducing fatigue-related crashes.
It is clear that the best way to manage fatigue is simply to get enough sleep. Medical researchers suggest that 8 hours a night is about the right amount for most people, although some, particularly those with sleep disorders (Box 2: Sleep disorders), might find this difficult to achieve.
Other fatigue management techniques might help. For example, recent research at Flinders University has shown that subjects waking from a 10-minute nap demonstrate an immediate significant increase in alertness and mental performance that lasts for at least an hour afterwards. In contrast, a 30-minute nap fails to produce a similar immediate increase (although it does induce an increase about 30 minutes after the end of the nap). One useful practice for fatigued drivers, then, is to pull over and take a short 'power' nap.
Various road-safety publications outline other fatigue-management techniques. For example, VicRoads advocates that drivers should:
- understand the signs of fatigue (eg, constant yawning, blurred vision, slowed reactions, heavy or sore eyes, poor concentration, impatience, not remembering the last few kilometres of the trip, etc). Technologies are being developed that might assist this;
- avoid driving during 'normal' sleep times (between midnight and 6 am for most people);
- stop if feeling sleepy and take a nap;
- obtain sufficient high-quality sleep between periods of driving; and
- avoid alcohol.
Maintaining a sensible sleep regime is the key. Driving and drowsiness are not good bedfellows; in the dead of night, it's better to wrap yourself around a pillow than around a highway ghost-gum.
Posted December 2002.