Section 2: Vision

2.1 Acuity

2.1a. Static Visual Acuity

Errors of refraction are the most common type of visual disorder. An individual with 20/20 vision (or 6/6 metric) is classified as having normal visual acuity. An individual with 20/70 vision or less (even when wearing corrective lenses) is classified as visually impaired. A person is legally blind when their vision is 20/200 or less, even when wearing glasses.

On initial applications for a driver’s license, all United States and Canadian jurisdictions require vision testing (Keeney, 1993). In most states in the United States (42 States or 84 percent), an unrestricted private driver’s license requires visual acuity of 20/40 or better (corrected or uncorrected) in one eye alone. In other countries (e.g., Europe, Asia, Africa, the Middle East, Australia), the most prevalent acuity standard is 20/40 combined, or both eyes tested together (Keeney, 1993). Forty states (Shipp, 1998), the District of Columbia, and three Canadian provinces require vision testing at license renewal (Petrucelli and Malinowski, 1992). Recently, Shipp (1998) assessed the impact of vision-related re-licensing policies on traffic fatalities in 48 contiguous States and the District of Columbia. Data were obtained from the Fatal Accident Reporting System. Results of that investigation revealed lower vehicle occupant fatality rates of older drivers in those states with vision-related re-licensing policies. Thus, as noted by the author, state-level mandatory vision testing for re-licensure may enhance traffic safety and reduce the economic costs of fatal crashes.

Results from a large-scale study (Diller, Cook, Leonard, Dean, Reading, and Vernon, 1998) indicate that individuals who have a history of eye conditions that may affect driving have a higher risk of crashes compared to controls matched on age, gender, and county of residence. In 1979, the Utah Driver License Division implemented a program that restricts drivers with medical conditions according to their functional ability levels. Since the inception of the program, all licensing applicants are required to complete a questionnaire regarding their physical, mental, and emotional health. Individuals who self-report a medical condition are categorized by medical history (e.g., diabetes, neurologic, etc.) and functional ability. Based on results from the questionnaire, an applicant may immediately receive a license or be required to complete a more extensive health history form. On the basis of the screening process, applicants may be denied a license, or receive a fully unrestricted or restricted license. Data obtained from the state licensing agencies between 1992 and 1996 were subsequently linked to the Utah Department of Transportation Crash files. This allowed for the comparison of crash and citation rates of restricted and unrestricted drivers with medical conditions to controls matched on age group, gender, and county of residence. Relevant to this discussion, drivers in the visual impairment category consisted of 13,075 drivers in the unrestricted category and 2,263 drivers in the restricted category. Drivers without restrictions had a significantly greater risk of crashes compared to controls (RR = 2.38, CI = 2.24 - 2.53). The relative risk for drivers with restrictions was 1.31 (CI = 1.10 - 1.56).

2.1b. Dynamic Visual Acuity

The act of driving primarily involves the ability to discriminate an object when there is relative movement between the object and observer. Therefore, tests of dynamic visual acuity rather than static visual acuity would seem to be more relevant for assessments of safe driving performance. In contrast to static visual acuity, dynamic visual acuity is a reliable predictor of crash probability (Fox, 1989; Graca, 1986; Hills and Burg, 1977;Reuben, Silliman, and Traines, 1988). In view of this, it is surprising that tests of dynamic visual acuity are seldom, if ever, included in traditional license renewal assessments. Importantly, declines in dynamic visual acuity and lateral motion detection start at an earlier age and accelerate faster, whereas deterioration in static visual acuity occurs later and progresses more slowly (Shinar and Schieber, 1991).

2.1c. Low Vision and Telescopic Lens

Individuals with low vision have impaired vision that cannot be fully corrected by ordinary prescription lenses, medical treatment, or surgery. Low vision is defined as vision ranging from 20/200 to 20/50, or when the corrected vision becomes a disability to the point at which one cannot function at his or her vocation (Fonda, 1986). Recent estimates suggest that approximately 14 million (one in twenty) Americans have low vision (Kupfer, 1999).

Low vision in older persons is most often the result of pathologies such as cataracts, macular degeneration, glaucoma, and diabetic retinopathy, or from a cerebrovascular accident. Individuals with low vision may experience one or more of the following: overall blurred vision, loss of central vision, and loss of peripheral vision. Telescopic spectacles and other low vision aids are used to assist individuals with low vision.

Despite the importance of research in this area, there has been little research on the use of telescopic lenses and driving performance. The literature that is available generally discusses the benefits and drawbacks of the use of telescopic lenses while driving. However, there is little in the way of data to support the pros or cons of their use. Limitations of telescopic lens systems have been documented by Lippman (1976) and Scott (1976). Those limitations include small central fields, ring scotomas, nearness illusion, movement of the image in the opposite direction of any head movement, reduced resolving power due to vibration, and altered head posturing. However, the papers are more than 25 years old and considerable improvements have been made in the technology of telescopic systems in recent years (Park, Unatin, and Hebert, 1993).

Advances in technology of telescopic systems include rear mounting, miniaturization, micro spiral galilean, vision enhancing systems, and bi-level telemicroscopic and behind the lens systems (Park et al., 1993). Research is needed, however, on the use of the new telescopic systems and driving performance. It is interesting to note that Park et al. (1993) have developed a driving program for visually handicapped telescopic drivers. The program is designed to ensure that every visually impaired telescopic driver meets the legal visual acuity and visual field requirements. In addition, the program is designed to improve the competency of telescopic lens use while driving. Details on the efficacy of the program are, however, lacking at this time.