Driver Fatigue: Concepts, measurement and crash countermeasures

Federal Office of Road Safety - Contract Report 72
(Performing organisation Human Factors Group, Department of Psychology, Monash University)

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Abstract:

This report addresses the topic of driver fatigue, an issue which is receiving increasing attention in the road safety field. A range of subject areas is reviewed in detail, including concepts and theories directly related to fatigue, the measurement of fatigue, factors contributing to the onset and development of fatigue, the degree to which fatigue is associated with road crashes, countermeasures having potential for offsetting the degrading effects of fatigue on safety, and an identification of research issues having promise for reducing the role of fatigue in crashes.

Executive Summary

Concepts and theories of fatigue

There is a lack of an agreed definition of fatigue, even as to whether the term refers to a phenomenon or a theoretical entity (intervening variable or hypothetical construct). Fatigue has subjective, objective (performance) and physiological components which may occur in the short-term or as a chronic state.

Many theories of fatigue have been proposed, varying in their precision and the type of concepts they employ. Neural models may be more suited to the explanation of muscular fatigue than to driver fatigue. Arousal theories can explain why fatigue develops in the low demand situation of highway driving, they link the concepts of attention and fatigue and they allow for psychological and physiological measures of fatigue. One disadvantage of these theories is that the physiological measures sometimes give inconsistent results. Other theories attempt to link the concepts of fatigue and stress.

Janssen's (1979, in Sanders, 1986) hierarchical model of driving holds promise for predicting specific performance effects of driver fatigue. The model of fatigue as an increased probability of falling asleep at the wheel proposed by Naatanen and Summala (1978) has direct implications for fatigue measurement and countermeasures.

Measurement of fatigue

Clear criteria are available and should be used to decide whether fatigue is measured on-road or in a simulator (Willumeit, Kramer and Neubert, 1981). Regardless of which experimental setting is chosen, several continuous measures should be collected.

It is inadvisable to use a performance measure on a new task which follows the supposedly fatiguing task. Subjects seem able to motivate themselves to mask the effects of fatigue, resulting in an inability to gauge the true magnitude of impairment.

Steering movements or lateral placement has been shown to be a valid measure of fatigue both on-road and in simulators. In contrast, speed maintenance or variation, brake reaction time and accelerator behaviour cannot be recommended as fatigue measures.

Rating scales seem to be the best measures of the subjective component of fatigue because they allow assessment of both frequency and severity of symptoms.

Heart rate (and related measures) cannot be considered a valid index of driver fatigue. Recording EEG enables one to tell if the driver has fallen off to sleep or is on the verge of doing so, and has been shown to validly measure fatigue. Eye movements change to a less efficient scanning pattern with fatigue and give a good index of the decrease in driving skill with fatigue.

The subsidiary auditory reaction time task has been shown to be a valid on-line measure of driver fatigue. However, it is unclear to the current authors whether the task may not in itself affect levels of driver fatigue.

Tests of persistence, the Stroop effect and Critical Flicker Fusion have been shown to give an index of fatigue largely unaffected by drivers' attempts to compensate for fatigue.

Analyses of bodily fluids are not recommended as fatigue measures.

Another indicator of the presence or severity of fatigue is the time taken to recover from it. This measure allows the separation of acute and chronic fatigue and it allows the fatiguing effects of different tasks to be compared.

Fatigue and road crashes

The inadequacy of epidemiological research means that a precise estimate of the contribution of fatigue to road crashes in Australia cannot be made. The research that has been conducted, however, suggests that the involvement of fatigue is probably lowest in all (property-damage only, casualty and fatality) crashes, higher in casualty crashes and highest in fatal crashes. The true incidence probably ranges between 5 and 10% in all crashes, about 20 to 30% in casualty crashes and about 25 to 35% in fatal crashes. The contribution of fatigue may even reach 40 to 50% in particular types of crashes, for example, fatal single-vehicle semi-trailer crashes. It is likely that alcohol is also involved in many fatigue-related crashes.

Countermeasures and further research

Two types of educational countermeasures are proposed: teaching drivers to recognise the signs of fatigue and teaching attention maintenance strategies to actively "fight" fatigue.

Better design of log books, including time stamping of fuel receipts and bills of lading would make detection of violation of regulations limiting the length and distribution of hours of work easier. Field fatigue tests and incidental driving performance tests are two enforcement strategies which should be explored.

Countermeasures which involve changes to road design may be expensive and take a long time to implement. However, these may be useful if confined to particular areas which have a record of a large number of fatigue-related crashes.

Subsidiary reaction time devices have been shown to be effective as fatigue monitors. At present there is no empirical evidence to support or deny the claim that they may encourage drivers to drive for longer periods.

Laboratory studies show that stimulants may have short term benefits in counteracting fatigue but they may increase driver risk taking and have adverse effects in the long term.

Auditory input in the form of a car radio may be helpful in counteracting fatigue.

There are good prospects for progress in defining and alleviating the role of fatigue in road crashes. Such progress would involve further research in widely varying aspects of fatigue in driving: concepts and theories of fatigue, the nature of the fatigue-crash relationship and the development of countermeasures. Research in concepts and theories of fatigue and the fatigue-crash relationship are longer-term notions, not so directly related to the implementation of countermeasures but necessary to understand more about the relationships between fatigue, hours of service and driving performance.