In this study, we demonstrated that several visual, cognitive, and demographic factors were associated with restricted night-driving behavior in elderly drivers. Our data corroborate previous studies using self-reported data, which have found that increasing age, female sex, and poorer cognition were related to restricting night driving.
6,8,11 Among a battery of tests of specific domains of cognitive function, we found that participants with better scores on a specific test of visual search (TMT A) were driving at night.
We also found that better contrast sensitivity and binocular peripheral visual fields were associated with night driving, similar to our previous finding in a different population, where night driving was based on self-report.
7
Reduced contrast sensitivity and not driving at night parallel older drivers' reports of difficulty driving at night and during other conditions when luminance is degraded, such as in poor weather.
4,5 Although not a night-driving study, Woods et al.
21 found that reduced luminance contributed to decreased driver recognition and performance in a small study done on a closed road circuit. It has been shown that in the elderly, loss of contrast discrimination leads to a decline in global motion processing and that performance accuracy and speed are subsequently degraded.
22,23 Such loss could subsequently affect the ability to perceive signs, objects, and other hazards at varying speeds that may make elderly drivers feel less confident, particularly during hours when luminance is reduced.
21 Thus, even without knowledge that contrast sensitivity specifically is lost, the effect on performance may lead older drivers to restrict night driving.
Explanations for declines in contrast sensitivity in the elderly include loss of neuronal efficiency with age and diseases such as age-related lens changes (i.e., cataract formation), retinal pathology, corneal disease, and glaucoma. Among these, cataract is the most common cause of reduced contrast sensitivity; studies have demonstrated an improvement in contrast sensitivity following cataract extraction and intraocular lens implantation.
24 Ophthalmic findings were not recorded in this study, because we were more interested in evaluating visual function changes rather than the ocular pathology. It is possible that participants in this study suffered from ophthalmic disease, which may have explained poor contrast sensitivity, and suggest that reduced night-driving patterns may be a signal for an ophthalmologic evaluation.
The alterations of neurosensory and global motion processing that occur with aging can also explain the association between slower times on tests of visual search (TMT A) and night-driving restriction observed in this study. This finding is consistent with the literature, which shows that cognitive performance is a determinant of driving behaviors, in that skills of psychomotor performance are important for safe and confident driving.
6,11 Feelings of reduced safety and confidence are cited as major reasons for driving cessation in surveys conducted in the elderly.
4,5
Previous studies have been unable to draw a uniform consensus on the role of visual acuity in predicting self-regulated driving behaviors in the elderly. In our earlier studies of self-report, we found that worse visual acuity was related to driving fewer miles but not to the decision to stop driving at night.
3,7 In this study, we also find that visual acuity is not significantly related to an objective determination of night-driving restriction.
Participants with less visual field loss were more likely to be driving at night, adjusting for other factors. We have previously found higher rates of self-reported night driving continuation among participants with less visual field loss.
7 Although persons may be largely unaware of their visual field defect, the reason for cessation of night driving with visual field loss may be due to perceived loss of scotopic vision. Visual field testing, particularly in the periphery, is largely testing scotopic vision. In dark conditions, rod photoreceptors involved in scotopic vision are stimulated. Thus, night drivers with more visual field loss, particularly in the periphery, would predictably suffer greater loss of scotopic vision and find night driving to be a greater challenge than daytime driving. Loss of visual fields may also make it more difficult to perceive road obstacles that appear in a driver's periphery, such as road signs and vehicles in neighboring lanes.
Sex also played a significant role in driving restriction. Although nearly half of the male participants were observed driving at night, only a third of the females were seen doing the same. Similar findings have been reported in prior studies, based on self-report of restriction, including ours.
4,6,8 This disparity may be explained in part through social and cultural phenomena of the elderly participants being tested. Males of this generation tended to start driving earlier than their female counterparts and continued to drive more frequently and for greater distances, largely for vocational purposes.
8 Females in this age group depended less on driving for activities of daily living. Consequently, males may have adopted driving as part of their identity role. Whereas males will continue driving for as long as their health permits, females may give up driving earlier for other reasons.
8 Males also are known to be greater risk takers and may, therefore, be more likely than females to drive in risky driving situations such as at night.
8 We have previously found that females report a poorer sense of direction, which coupled with fear of night driving, may lead to cessation of night driving.
12 Access to an alternate driver was evaluated to determine if this could be an explanation for sex differences in night driving, presuming females may have a larger social network, but no differences were found. It is also possible that females were such infrequent night drivers, that episodes of their night-driving activities were missed during the 5-day DMS study period; if so, there may have been less sex disparity than we report. Even still, females would have been less frequent night drivers than males.
In this study, we observed an association in females between symptoms of depression and night-driving restriction, which was not significant in males. In our analysis, we defined depressive symptoms as a score of >2 on the GDS. Sensitivity analysis using other cutoffs did not change the sex–depression interaction. Interestingly, Brabyn et al.
8 found that males, rather than females, reported restricting their night driving in association with symptoms of depression. The different study findings could be due to the same source bias in the Brabyn study, which used self-report as the source of both depression and night-driving data. Our study used different sources of data; the DMS for real-time night-driving data and self-report data of depressive symptoms, and thus our findings do not have this bias. Additional differences include the populations studied; in the Brabyn study, the self-report of driving at night was much higher in both sexes than we observed in Salisbury. Driving cessation for males in California may be rarer and lead to more symptoms of depression than is true in Salisbury, although it does not explain the difference in females. Our findings suggest that, although females are much less likely in general to drive at night compared with males, depression in females puts them at an even higher risk for not driving at night.
A limitation of this study was the definition of night-driving patterns, which changed appropriately according to season. The length of night-time hours was shortest in the summer (8 hours) and longest in the spring/fall (11 hours) and winter (13 hours); therefore, there were more hours in which subjects could potentially drive at night during the winter compared with other seasons. A participant who routinely stops driving around 8 PM would be driving at night in the winter and not driving at night if observed during the summer. This potential misclassification would likely weaken our associations. Since participants were largely studied in the same season each visit due to the timing of our follow-up visits, we did not have the opportunity to observe change by season in the same person. The participants who changed over time truly changed within the season we observed them. Weather conditions are also known to influence driving patterns and may lead to restriction.
5 Thus some of the night-driving restriction in the winter may be due to weather-related factors rather than just night-time restriction. However, winters are not severe due to the presence of the Chesapeake Bay near Salisbury, and rainy conditions can prevail in all seasons.
Another possible limitation of this study is that we may have missed drivers who infrequently drove at night and were missed during the 5-day DMS testing period. They would be classified in our study as day-driving only. Misclassification is always an issue in studies, and would be true in other studies that rely on self-report of driving at night as well. A strength of this study is that we could base our classification on direct observation and classify those who drove at night as true night drivers.
In summary, this study of elderly drivers found that persons with worse contrast sensitivity and fewer number of points seen in binocular visual fields were less likely to drive at night. We also found that those with worse scores in a test of visual search were less likely to drive at night. These findings likely reflect self-regulation of older drivers, and have a positive implication, because there is reason to believe driving at night would be more hazardous for persons with such limitations. Nevertheless, about a third of drivers who were unable to read greater than 34 letters on the contrast sensitivity chart were still driving at night in this population, suggesting the multiplicity of factors that are involved in decisions to drive at night apart from potential visual limitations. Further investigation into the reasons that such drivers continue to drive at night despite limitations is warranted. Further implications stem from the finding that depression in older females is associated with not driving at night. Because of the cross-sectional nature of the data, we cannot determine if depression preceded or followed restricting night driving, and both are plausible. However, self-restriction of driving habits, justified or not, handicaps independent function. Elderly femles who have given up night driving should be further evaluated for depression to be certain that a treatable condition is not the source of relinquishing elements of independent function.