It has been demonstrated that older drivers are involved in the highest number of traffic accidents per mile driven and are judged to be at fault more often than any other age group.

Standard high-contrast Snellen acuity and other conventional eye tests do not accurately predict difficulties in driving performance experi enced by the aged and those with minor visual impairments. In some cases, even though Snellen acuity might be 20/20, loss of contrast sensitivity under low illumination, reduction in color discrimination, or inability to rapidly recover from glare can present potential driving hazards.

These subtle but important visual deficits occur frequently in the early stages of macular disease (which is now the most common cause of new cases of visual impairment in the elderly), and some also occur with developing cataracts, even before a substantial loss in acuity is found.

Study Description

The present study is an attempt to relate vision functions which are known to decrease with aging to decrements in driving performance. In this study, using a battery of functional vision tests designed specifically to predict the visual performance, we compared vision test scores with the driving records of 97 drivers over 55 years of age. Half of the group had no accidents in the last three years while the other half had three or more accidents. The vision test battery included visual acuity under high contrast and low contrast conditions as well as low contrast-low luminance conditions, peripheral visual fields with and without attentional demand, contrast detection for large targets, sensitivity to disability glare, recovery from exposure to glare, and color vision.

Subjects were recruited from the California Department of Motor Vehicles driving records. Prior to testing, subjects were given questionnaires to evaluate their driving habits and the difficulties they experienced while driving, along with a standard ocular and medical history. When possible, tests were administered binocularly to approximate real-world driving conditions.

If a subject reported being involved in traffic accidents during the past 3 years, one accident history form was filled in for each traffic accident reported. The accident history included questions about the particular lighting and weather conditions during the individual traffic accidents, details about the accident, and what the driver felt was the major cause of the accident.

Results

After accounting for miles driven per year, accident-involved subjects had no differences in accident rate per mile driven regardless of sex or age. They had an average accident rate of 9 accidents per 100,000 miles driven. This is 6 times the average 1.5 accidents per 100,000 miles found for the general population over 55 years of age. They drove the same distance and expressed the same amount of confidence in their driving and visual skills as the accident-free subjects.

Several visual skills which are known to decrease with age were found to be reduced in the accident prone subjects. The skills with the strongest statistical relationships to accident involvement include low contrast-low luminance visual acuity, disability glare, the extent of the standard visual field directly to the right and down and to the right, and the extent of the attentional visual field directly down and down to the right or to the left. The most highly accident prone subjects also showed reductions in contrast sensitivity, and their visual skills showed larger vision decrements than the moderately accident prone drivers.

We defined an accident proneness index taking into account whether the subject was at fault in the accident. There was a statistically significant relationship between overall decreased performance on the vision tests and increased Accident Proneness Index (p=.04).

Even though accident-involved drivers had more difficulty on the vision tests, they were unaware of any problems with their vision. They reported being confident while driving on all types of roads, in unfamiliar places and at night. They also reported the same incidence of being bothered by glare, eye comfort, and changing light levels as non-accident involved drivers and had the same incidence of health conditions.

Conclusions

A minimum screening for older drivers would measure low contrast-low luminance visual acuity, disability glare, and the extent of the horizontal and inferior peripheral visual field. Drivers who are found to be at risk for traffic accidents due to age-related vision deficits should be warned of potential hazards and instructed in ways to avoid dangerous situations.

Any future research attempting to relate vision function loss to accidents will require specific information about the visual environment at the time of the accident. The impact of specific visual losses such as changes in disability glare sensitivity can then be assessed appropriately.

Supported by the California Department of Motor Vehicles and the The Smith-Kettlewell Eye Research Institute.