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Driving is a complex activity that requires a variety of physical, mental and cognitive functions and their interaction and coordination (Eby and Molnar, 2012; Karthaus and Falkenstein, 2016). As people age, a range of age-related functional impairments could negatively affect their safe driving ability, including decline in the sensory, cognitive and psychomotor functions (Attebo et al., 1996; Ball et al., 1998; Haegerstrom-Portnoy et al., 1999; Karthaus and Falkenstein, 2016).

2.2.3.1 Age-related impartments in sensory function

Age-related sensory function impairments generally refer to visual and hearing problems. Among these, visual impairments are most closely linked to driving safety, as visual information accounts for a high proportion of the information required by the driver during driving (Attebo et al., 1996; Karthaus and Falkenstein, 2016). A variety of visual abilities could decline due to the ageing process. Haegerstrom-Portnoy et al. (1999) indicated that the ageing process decreases static and dynamic visual acuity and contrast sensitivity. Also, a study by Schieber (1994) showed that the time that older participants took to recover from the deleterious effects of glare is about three times longer compared to younger counterparts. In addition, the useful field of view (UFOV) is positively associated with driving performance (Ball et al., 2005), and it is negatively affected by the ageing process (Sekuler et al., 2000). Older drivers who have seriously impaired UFOV are more likely to have been involved in car collisions (Huisingh et al., 2015; Karthaus and Falkenstein, 2016). In addition, motion sensitivity also increases with age, for example, Trick and Silverman (1991) found that participants aged over 70 years old exhibited levels of motion sensitivity about twice those of participants aged 30 years old and under. These impaired visual abilities can negatively affect drivers’ safe driving ability (Eby and Molnar, 2012). For example, they could increase the risk of crashes among older drivers when driving at night and in environments with low light levels; they could also result in difficulties for older drivers in clearly reading warning, direction, information, road work signs, as well as traffic lights and road markings and lines (Eby and Molnar, 2012). Apart from the visual decline due to normal ageing processes, eye diseases among older drivers also seriously affect their vision and thus reduce safe driving performance. For example, eye cataracts are a common condition that influence a higher proportion of people aged 65 and over, significantly increasing their probability of being involved in a car crash (Owsley et al., 1999; Karthaus and Falkenstein, 2016). The hearing

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function also seriously declines due to the ageing process, which could result in missing important auditory information such as warning signals, potentially endangering older drivers (Karthaus and Falkenstein, 2016).

2.2.3.2 Age-related impairments in cognitive functions

Apart from sensory abilities, a range of cognitive abilities and the seamless interaction of these abilities are also crucially important for driving safety, including attention, perceptual motor skills, memory, and decision-making (Anderson et al., 2005). The attention abilities that are relevant to driving include selective attention, divided attention and sustained

attention (McDowd and Birren, 1990). Selective attention refers to the ability of the driver to effectively select suitable and useful stimuli and to disregard distracting and useless stimuli during driving (McDowd and Birren, 1990). Pollatsek et al. (2012) conducted experiments both on a driving simulator and in real road environments to examine the influence of age on selective attention, and they revealed that older participants exhibited reduced selective

attention compared to younger participants. Divided attention refers to the ability when people have to concentrate on multiple sources of information or perform several tasks at a same time (McDowd and Birren, 1990; Brouwer et al., 1991). Brouwer et al. (1991) administered a driving simulator investigation to examine the effect of age on divided attention and found that compared to younger participants, older participants had a significantly declining ability of divided attention. Sustained attention refers to the ability to concentrate on an activity for a long duration, which is closely related to the vigilance and alertness of driving (McDowd and Birren, 1990). It was found to decline with age (Davies and Davies, 1975; Mani et al., 2005).

Apart from attention abilities, the speed of processing various information also plays an essential part in driving, especially when driving in difficult and complex situations, such as heavy traffic and adverse weather situations, and it has been found the speed of information processing slows down with age (Panek et al., 1977; Pichora-Fuller, 2003). After the

information gained while driving has been successfully processed, executive functions enable drivers to effectively plan an effective strategy and execute corresponding responses, and the executive functions can also deteriorate during ageing (Daigneault et al., 2002; Insel et al., 2006).

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2.2.3.3 Age-related impairments of psychomotor abilities

Psychomotor abilities refer to a person’s capabilities to control and coordinate his or her body (Kelso, 1982; Houx and Jolles, 1993; Shanmugaratnam et al., 2010). Those are strongly related to driving safety are the speed of reaction, mobility and flexibility of limbs and neck, and movement coordination. It has been well recognised that these psychomotor abilities deteriorate due to the normal ageing process and some age-linked illnesses and diseases, such as osteoarthritis, Parkinson’s disease, Huntington’s chorea, deconditioning and

cerebrovascular accidents (Houx and Jolles, 1993; Rodríguez-Aranda et al., 2006; Shanmugaratnam et al., 2010).

One well-established age-linked psychomotor changes concerns increases in older drivers’ reaction time. Ferreira et al. (2013b) argued that the effect of age on reaction time was significant in terms of both psychological assessments and driving performance. Age-linked reduced muscle strength could also make it difficult for older drivers to operate the control of the vehicle accurately and effectively, which would potentially increase the probability of being involved in a vehicle collision (Kallman et al., 1990). In addition, the age-linked impaired limb mobility and flexibility will affect drivers’ ability to effectively operate steering wheel or shift their foot between the accelerator and brake pedals to execute a safe manoeuvre of a vehicle to effective evade a potential collision; and also the declining neck mobility and flexibility would make it difficult for the older adults to effectively check the side mirrors and to make shoulder checks when merging, changing lanes and reversing (Staplin et al., 1999; Marmeleira et al., 2009).

2.2.3.4 Age-related fragility and frailty

Moreover, another significant area of functional decline with increasing age is fragility and frailty. Fragility refers to the possibility of being injured easily or more seriously for a fixed level of severity, and frailty reflects the ability to returning to a healthy condition from an injury (Braver and Trempel, 2004; Kent et al., 2009). Older drivers are more likely to be seriously injured or killed if involved in vehicle collisions, for example, Li et al. (2003) reported that, compared to drivers aged under 60, older drivers aged 70-74 had death rates twice as high and those aged 80 and over five times as high when involved in vehicle collisions of same severity.

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