SD15 - Low Vision Study
Executive Summary: The purpose of this study is to determine whether visual acuity, visual field, or both should be used as criteria for determining whether an individual has adequate vision to operate a motor vehicle safely. Based upon DMV's experience of licensing drivers who use a Bioptic Telescopic Lens (BTL) device, the Medical Advisory Board feels that visual field may be more critical in driving than visual acuity. The basis for this opinion is that most BTL drivers have a visual acuity within the 20/100 to 20/200 range (without the BTL device). The BTL device can only be used to identify a specific object or to read a sign, and cannot function over a wide field of vision. After careful consideration, the Medical Advisory Board requested that a test sample of drivers with low vision (in the 20/70 to 20/200 range) be studied to determine their ability to safely operate a motor vehicle. Such a study needed a resolution from the General Assembly to authorize the testing of the sample. The Medical Advisory Board and DMV developed the Vision Resolution (SJR-3) which allows DMV to conduct and evaluate a skills test of drivers with low vision to determine if current vision standards for motorists in the Commonwealth are too stringent. Based on the results of the study, it may be recommended that the current visual acuity standards be altered. Vision is a complex physical process. Likewise, the terms used to describe vision can be equally complicated. The following descriptions, therefore, are offered to facilitate understanding of the processes detailed in this report. • "20/20" Most people are familiar with this measurement of vision, but do not know what it means. "20/20" is an arbitrary standard of measurement set up several decades ago, in which persons with "normal" vision were asked to identify objects 20 feet away. Their observations then became the benchmark by which the vision of others was measured. The first number represents eyesight of the person being tested, and the second number represents "normal" vision. For example, John's vision is being tested, and he can identify an object when it is 20 feet away. A person with "normal" vision, however, can identify the same object when it is 200 feet away. Therefore, John has "20/200" vision. • Visual Acuity Visual acuity is the component of vision which is measured by the "20/20" comparison described above. Tests for visual acuity (including the ubiquitous eye chart) require a person to use the macula, a tiny portion of the retina which is only millimeters in diameter. The macula of each eye is the "focal point" of a person's vision. Individuals use their maculae to read, or examine details of an object. The rest of the field of vision serves to alert a person to possible items of interest occurring outside the focal point. For example, a person is reading a book when they catch something moving "out of the corner of their eye." Noticing this movement stimulates the person to turn their head and aim the maculae at the source of the movement to obtain further detail. • Visual Field Visual field refers to the entire range of an individual's vision, and is measured in degrees. It has two dimensions; horizontal and vertical. To understand how a person's visual field is measured, imagine sitting in a chair, facing forward, with your eyes focused on a spot in front of you. There is a person standing behind you to your left, about two feet away, holding a pencil vertically adjacent to the back of your head. On command, the person begins to slowly move the pencil forward towards the front of your head. You are staring at the spot in front of you, waiting for the pencil to enter the left edge of your field of vision. When it does, you notify the person with the pencil and they stop. This process is repeated on your right side. Your total horizontal visual field will be determined by how much you can see from the edge of your left field of vision to the edge of your right, without moving your eyes away from the spot on which they are focused. A "standard" (such as 20/20) does not exist to describe the visual field of an person with "normal" vision. Individuals have been examined whose horizontal visual fields exceeded 180 degrees; however, at least 100 degrees of horizontal visual field will be required for participation in the study. Vertical visual field (how much you can see above and below you, without moving your eyes) can be similarly determined. However, as vertical visual field is rarely used in driving, only horizontal visual field will be measured in this study. |
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