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Article Excerpt
INTRODUCTION
Information relevant to driving is predominantly visual (e.g., traffic signs, distance to the car ahead, communication with other road participants), and this has led many governments to set firm visual standards for driving licensing. European countries, as well as many states in the United States, require a binocular visual acuity of at least 0.5 (decimal notation, equivalent to 0.3 logMAR) and a horizontal field extent of at least 120[degrees]. If visual criteria are used to determine fitness to drive, sensitivity and specificity of the vision tests should be high. Drivers who meet the vision criteria should demonstrate safe and smooth driving performance, whereas drivers who do not meet the criteria should demonstrate unsafe or otherwise inadequate driving performance. In other words, the number of false alarms (restricting safe drivers from driving) and the number of misses (permitting unsafe drivers to drive) should be as low as possible.
Previous studies have demonstrated that although the relationships between vision requirements and driving safety are significant, they are not conclusive with regard to the identification of individual at-risk drivers (Ball, Owsley, Sloane, Roenker, & Bruni, 1993). Visual acuity is often reported to be a weak predictor of mobility (Black et al., 1997; Kuyk & Elliott, 1999: Long, Rieser, & Hill, 1990; Marron & Bailey, 1982; Turano, Geruschat, Stahl, & Massof, 1999) and driving (Ball & Owsley, 1991; Szlyk, Fishman, Severing, Alexander, & Viana, 1993). (Mobility refers to the ability to walk safely, comfortably, independently, and at a normal or near-normal speed. It is usually assessed by means of an obstacle course in an indoor or outdoor setting. Typical parameters are number of errors, such as contacts with obstacles and loss of balance, and a speed estimator, such as percentage of preferred walking speed and travel speed).
Measures of the visual field (e.g., horizontal extent, mean sensitivity,, percentage remaining visual field) have been reported to be better predictors for mobility (Black et al., 1997; Geruschat, Turano, & Stahl, 1998; Haymes, Guest, Heyes, & Johnston, 1996; Kuyk, Elliott, & Fuhr, 1998; Long et al., 1990; Marron & Bailey, 1982). With regard to driving, it has often been reported that severely restricted visual fields impair driving performance and increase crash risk (Johnson & Keltner, 1983; Szlyk, Alexander, Severing, & Fishman, 1992; Wood & Troutbeck, 1992). However, for less severe visual field defects, the effect on driving performance is much smaller. Wood and Troutbeck (1994), for example, reported only minor changes in driving performance in participants with a simulated binocular visual field extent of 90[degrees] or 105[degrees] and concluded that they compensated for simulated visual field restrictions by driving more slowly rather than by increasing error scores.
The effect of behavioral modifications, such as speed reduction and compensatory viewing strategies, has often been suggested as an intermediate variable between vision and driving safety (Ball et al., 1998; Kuyk et al., 1998; Szlyk et al., 1995; Wood & Troutbeck, 1994). Particularly in the case of visual field defects, compensatory viewing strategies such as an efficient scanning technique or eccentric viewing may be useful in reducing the negative effect of the visual impairment.
In the present study, sensitivity and specificity of the European vision requirements for driving were investigated in a group of drivers with visual field defects caused by ocular pathology, such as age-related macular degeneration, glaucoma, or retinitis pigmentosa. It was investigated whether the predictive power of a model based on the European vision requirements for driving could be improved by taking compensatory viewing efficiency into account. This first model was then compared with a model based on variables that have been reported to be good predictors of driving safety and mobility, such as contrast sensitivity, age, and visual attention.
Ball and Owsley (1991) and Ball et al. (1993) have demonstrated a strong relationship between visual attention and driving. They argued that the low predictive power of visual field measures could be attributable to the difference in complexity between a visual field assessment by means of perimetry and the use of the visual field while driving. They therefore suggested assessing the useful field of view (UFOV). Rather than assessing detection of stimuli in isolation, the UFOV test assesses detection and discrimination of suprathreshold stimuli in a cluttered scene under varying attentional demands. Contrast sensitivity has also been reported to be a better predictor than visual acuity in mobility studies (Black et al., 1997; Cornelissen, Bootsma, & Kooijman, 1995; Geruschat et al., 1998; Haymes et al., 1996; Kuyk & Elliott, 1999; Long et al., 1990; Matron & Bailey, 1982), and therefore its relationship to driving was examined in this study.
The second research question is related to the type of visual field defect. It is often assumed that people with peripheral field loss demonstrate much greater difficulty with mobility than do individuals with central field loss (Marron & Bailey, 1982). Previous studies on driving concur with this hypothesis. Macular degeneration, for example, is usually not associated with increased accident involvement (McCloskey, Koepsell, Wolf, & Buchner, 1994; Szlyk et al., 1993). It should, however, be noted that Szlyk et al. (1995) showed that although participants with age-related macular degeneration did not have increased accident rates, they performed worse on driving simulator indices such as number of accidents, lane boundary crossings, braking response times to stop signs, and driving speed than did a control group of comparable age. In accordance with the hypothesis that people with peripheral field defects are more impaired with regard to driving, Szlyk et al. (1992) reported that a greater proportion of participants with retinitis pigmentosa had accidents as compared with a control group. Owsley, McGwin, and Ball (1998) reported that participants involved in injurious crashes were 3.6 times more likely to report a diagnosis of glaucoma than were controls.
These findings concur with the assumption that ocular pathology causing peripheral field defects impairs driving safety more than does ocular pathology causing central field defects. Attributing the differences in driving safety between different pathology groups solely to the effect of the visual fields, however, is questionable. Owsley, McGwin, et al. (1998), for example, emphasized that glaucoma per se, not visual field sensitivity, was a predictor of crash involvement, suggesting that other characteristics of this pathology elevated crash risk.
When evaluating driving performance, it is important to select valid and reliable criteria. A discussion of various techniques and their methodological weaknesses is provided by Brouwer and colleagues (Brouwer & Ponds, 1994; Brouwer, Withaar, Tant, & van Zomeren, 2002; Withaar, Brouwer, & van Zomeren, 2000). Because of the hierarchical task structure and overlearned nature of driving, a driver has many opportunities to compensate for mild to moderate functional impairments. Assessment of driving performance should take these compensatory abilities into account. In our experience, the Dutch assessment of practical fitness to drive of drivers who do not quite meet the vision requirements for driving complies with this requirement. Drivers are evaluated in their own car and their own neighborhood by an official driving examiner of the Dutch driving license authority, who determines whether they drive smoothly and safely and whether they are able to compensate for the negative effects of their impairment.
METHODS
Participants
The participants were 63 men and 37 women with visual field defects caused by ocular pathology, such as age-related macular degeneration, glaucoma, and retinitis pigmentosa. They were recruited by short reports in newspapers and by pamphlets at ophthalmologists' offices, rehabilitation centers, and patients' associations. All participants were regular drivers, although most of them had been told they...
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