OR 23: Fatigue-related crashes: An analysis of fatigue-related crashes on Australian roads using an operational definition of fatigue


This report examines a proposed operational definition of fatigue, and the occurrence and characteristics of fatigue-related road crashes within Australia as identified by the operational definition.

Fatigue represents a significant social and economic cost to the community in relation to road crashes, especially fatal road crashes. Fatigue-related crashes are often more severe than other crashes as drivers reaction times are often delayed or drivers have not employed any crash avoidance manoeuvres. However, the identification of fatigue related crashes is hindered by the absence of a universally accepted definition of fatigue. Furthermore, it is difficult to quantify the level of driver fatigue due to the difficulties in objectively measuring the degree of fatigue following a crash.

The Australian Transport Safety Bureau (ATSB) has proposed an operational definition of a fatigue-related road crash that would provide a common, objectively based methodology. This definition should be useful in monitoring fatigue-related crashes and gauging trends over time or between regions. The definition is based on a set of well researched selection criteria and uses crash characteristics routinely collected by different traffic authorities.

The criteria for the operational definition implemented in this report included single vehicle crashes that occurred during critical times (midnight-6am and 2pm4pm), and head-on collisions where neither vehicle was overtaking at the time of the crash. Excluded were crashes that occurred on roads with speed limits under 80 km/h, or involved pedestrians or unlicensed drivers or drivers with high levels of alcohol (blood alcohol concentration over 0.05g/100ml).

Using this criteria, this study found that 16.6 per cent of fatal crashes in 1998 involved driver fatigue. When comparing among the States and Territories, the Northern Territory had the highest rate of fatigue-related crashes per 100 million vehicle kilometres travelled (0.66). However, within individual States and Territories, New South Wales had the highest percentage of fatal crashes involving driver fatigue (22.0 per cent). The study also found that between 1990 and 1998 the proportion of fatal crashes involving driver fatigue increased from 14.9 per cent in 1990 to 18.0 per cent in 1994, after which there was a decline to 16.6 per cent in 1998. This trend was also observed when the number of fatigue-related crashes was worked as a proportion of all fatal crashes 80km/h or over. This was done to take into account the fact that the number of fatal crashes occurring in speed zones of 80km/h or over increased throughout the 1990s and that roads have also been re-zoned over this time period.

The operational definition identified a relationship between the time and type of fatigue related crashes. More single vehicle crashes occurred in the early morning (midnight- 6am) than the afternoon (2pm-4pm). However, the incidence of head-on crashes was highest between midday and 6pm and lowest between midnight and 6am, this relationship may be related to traffic densities. That is, higher traffic densities during the day would increase the likelihood of fatigue-related crashes involving multiple vehicles in head-on collisions and, conversely, lower traffic densities during the early morning would increase the likelihood of fatigue-related crashes involving single vehicles.

Some of the findings of this study were similar to other studies in that the operational definition identified a higher number of male fatigued drivers/riders than female, and more fatigued drivers/riders under 29 years of age compared with older age groups. The operational definition and other studies also found that most early morning fatigued drivers/riders were less than 29 years of age, and fatigued drivers/riders over 50 years of age were involved in more afternoon crashes than in early morning crashes.

There also appeared to be a relationship between the age of the fatigued driver/rider and the type of fatigue-related crash (single vehicle or head-on). Single vehicle crashes involved a higher proportion of fatigued drivers/riders under 29 years of age compared with head-on crashes. However, fatigued drivers/riders over 50 years of age were involved in more head-on crashes. This relationship may be linked to the time of crash. That is, single vehicle crashes are more likely to occur in the early morning and early morning crashes are more likely to involve fatigued drivers/riders under 29 years of age. Therefore, single vehicle crashes involve more fatigued drivers/riders under 29 years of age. A similar argument could explain the relationship between older fatigued drivers/riders and head-on crashes.

Using the operational definition, 29.9 per cent of fatal articulated truck crashes in 1998 involved driver fatigue, which was almost twice the proportion of all fatal crashes involving fatigue (16.6 per cent). However when speed limits were controlled for, by only including those crashes occurring at crash sites with speed limits of 80km/h or over, the difference between articulated truck crashes and all crashes was smaller. That is, in 1998, 34.5 per cent of fatal articulated truck crashes in speed zones of 80km/h or over involved fatigue, whilst 24.9 per cent of all fatal crashes involved fatigue.

The operational definition also found that the proportion of fatigue-related articulated truck crashes between 1990 and 1998 increased from 31.0 per cent in 1990 to 38.6 per cent in 1994, and this was followed by a decrease to 29.9 per cent 1998. Similar trends were also observed when speed zones were controlled, with an initial increase in the proportion of fatigue-related crashes between 1990 and 1994, followed by a decrease till 1998.

Although fatigue is more highly represented in articulated truck crashes, this does not necessarily imply that the truck driver was the fatigued driver in a crash involving more than one vehicle. The fatigued driver in a head-on crash was identified by observing which vehicle had driven onto the wrong side of the road. Therefore, in head-on fatigue related crashes involving an articulated truck, truck drivers were estimated to be the fatigued driver in only 16.8 per cent of crashes, whilst passenger car drivers were fatigued in 66.0 per cent of crashes.

The identification of fatigue-related crashes by the operational definition was compared with fatigue-related crashes identified by coroners/police. While researchers generally acknowledge that coroners/police underestimate the incidence of fatigue, it was the only measure available for comparison in this report. The operational definition compared relatively well; however, two limitations and possible modifications for the operational definition were highlighted. Firstly, nearly two-thirds of crashes identified as fatigue related by coroners/police, but not by the operational definition, were excluded because they were single vehicle crashes that did not occur during the critical time periods. Secondly, just over a third of crashes identified as fatigue-related by the operational definition, but not by the coroners/police, had been attributed to speed, drugs, or drugs and alcohol by coroners/police. This may suggest that the operational definition should be modified to exclude speed and drug related crashes, and extend the critical time periods for single vehicle crashes. However, excluding drug and speed related crashes may reduce the objectivity of the operational definition and the ability to consistently implement the definition across various traffic authorities. For instance the identification of speed involvement can vary between different traffic authorities, and not all drivers involved in fatal crashes are tested for drugs. Furthermore, extending the critical time periods may lead to an increase in the number of crashes falsely identified as fatigue-related. Clearly, more analysis is needed before the definition is modified.

In conclusion, while the operational definition may include some crashes that are not fatigue-related and exclude others that are, it nevertheless provides a practical and useful index of the relative incidence of fatigue-related crashes.

Download Complete Document: Fatigue_related_sum [PDFPDF: 181 KB]

Type: Research and Analysis Report
Sub Type: Other Report
Author(s): Kim Dobbie
ISBN: 1 877071 05 6
Publication Date: 15/05/02