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Are sleep-related road traffic accidents preventable?

1.3 Driver sleepiness

1.3.3 Are sleep-related road traffic accidents preventable?

There is now a wealth of research demonstrating that healthy people do not fall asleep whilst driving without experiencing a period of increasing sleepiness beforehand (Reyner and Horne 1998b, Lisper et al. 1986, Otmani et al. 2005, Horne et al. 2004) . However, the problem comes when drivers fail to take signs of sleepiness as signs of increasing risk of falling asleep (Kaplan et al. 2007). The following section discusses if people are aware of sleepiness and therefore able to stop driving before an incident occurs, the implicating factors resulting in srRTIs.

1.3.3.1 Insight into sleepiness

It has long been recognised that people have a legal responsibility to be fit to drive; but opinion on falling asleep at the wheel has changed over the years. McCutcheon (1998) reports a case in Scottish law of H.M. Advocate v. Ritchie (a driver who fell asleep) in 1926, which stated “that a person is obliged to take account of the risk of falling asleep when driving”. The same paper reports this decision being rejected in 1963, and then approved in 1991. From the legal perspective it makes a lot of difference whether a driver is able to fall asleep without any warning or if drivers consciously decide to continue driving while knowing that they are sleepy. Examples of different court outcomes are reported in the preface of this thesis.

There is evidence that people are aware of sleepiness prior to having a srRTI as If people who have fallen asleep while driving are asked what happened prior to the incident they are able to identify and recognise known warning behaviours. These behaviours include: struggling to keep eyes open, yawning, difficulty connecting to driving, slower response to traffic change and an increased variation in speed (Lisper et al. 1986, Nordbakke et al. 2007).

Further evidence of ability to recognise sleepiness is apparent in the correlation between subjective sleepiness scores and driving simulator performance. Here, when sleep restricted participants rate themselves feeling sleepy at times when srRTIs occurred (Horne et al. 2004, Biggs et al. 2007). In a study of young healthy drivers it was found that participants had an average of 45 minutes from when they felt sleepy until a driving incident occurred (Reyner and Horne 1998b). This time interval would be sufficient to implement a countermeasure before a RTI occurs. However, it was found that some drivers although knowing they were sleepy did not feel they were likely to fall asleep and therefore may not use a countermeasure. The correlation between subjective sleepiness ratings and driving performance has also been noted in healthy participants during 24h sleep deprivation (Baranski 2007).

Additionally subjective (KSS) scores have also been shown to correlate to EEG measures of sleepiness (Horne et al. 2004, Kaida et al. 2006), demonstrating that participants have awareness of their underlying sleepiness. As it has been shown experimentally that subjective sleepiness as rated on the KSS is linked driving incidents it can also be expected to do so on real roads. When reporting KSS 9 participants report they are fighting sleep; if someone is fighting sleep it is likely they will be doing something to “fight” it using a countermeasure such as open a window. If a person has taken any action in this manner it can be said that they knew they were sleepy (Horne et al. 2004).

All studies of awareness of sleepiness reported above have been conducted under laboratory conditions, where participants are repeatedly asked to report a subjective sleepiness rating. It should be acknowledged that when on real roads, without this prompt drivers may not regularly assess how sleepy they are so could miss early signs of sleepiness and continue to drive.

The research reported here has been completed using healthy participants; similar research has not been conducted with older drivers or OSA participants, consequently there is no evidence of awareness of sleepiness in these groups.

1.3.3.2 Factors affecting sleep related incidents

Below is a brief outline of the factors which can contribute to srRTIs this is to demonstrate that despite one underlying cause (the driver being to sleepy to drive) there are many contributing factors resulting in increased likelihood of a srRTI occurring.

Prior sleep

Having inadequate sleep prior to driving has been shown to be a major cause of sleep- related accidents (Fell et al. 1997, Connor et al. 2002).

Time of day

As detailed in section 1.2.2 a greater number of sleep-related accidents happen in the early hours of the morning and in the early afternoon when sleep pressure is at its highest (Horne et al. 1995, Pack et al. 1995). The effect of time of day is exacerbated in shift workers driving home in the early hours of the morning (Horne et al. 1999, Akerstedt et al. 2005).

Type of road

Analysis of RTI data has shown sleep-related accidents to be more common on major roads, such as motorways (Horne et al. 1995). It is likely that the monotonous conditions of this type of road facilitate sleepiness (Horne et al. 1999). It is thought that a road with little variation leads to lower arousal and therefore increased sleepiness (Thiffault et al. 2003). In line with this, traffic density also has an effect, as sleep-related accidents on motorways are more likely when there is low traffic density (Flatley et al. 2004), in situations with greater traffic density drivers may be able to maintain alertness.

General sleepiness levels

Using the Epworth sleepiness scale as a measure of general sleepiness, it has been shown that people who are generally sleepier are more likely to fall asleep while driving (Maycock 1996).

It may be hypothesised that the longer a person drives the more likely they are to fall asleep. However, evidence is conflicting, as using a simulator it has been shown that in healthy participants there is no significant effect of time on task to driving incidents, but that driving performance is worsened by prior sleep restriction (P. Philip et al. 2005). In another study, driving impairment over time was reported and the authors suggested that 80 min is the safe limit for monotonous driving (Ting et al. 2008).

Individual difference

Ability to maintain performance when sleep deprived varies greatly between individuals (Van Dongen et al. 2004). Whilst driving a significant difference in ability to cope with sleep deprivation has been found between individuals (Philip et al. 2006).

Driving experience

It is possible that those drivers who often drive at night or when sleep deprived will be at higher risk of falling asleep at the wheel due to greater exposure. However, there is some evidence to show that it is possible for people to adapt to this situation, such as professional drivers adapting to night time driving (Dalziel et al. 1997). Similarly it has been suggested that inexperienced drivers (holding a licence for less than three years) have greater impairment at a driving task following sleep deprivation than experienced drivers (Lenne et al. 1998).

Sleep disorders

Accident surveys have been used to show obstructive sleep apnoea sufferers to be more likely to have srRTIs than healthy people.