Detectable warning surfaces are meant primarily for pedestrians who are unable to see hazards such as intersections with streets. However, many pedestrians with visual impairments who sometimes have trouble seeing the transition between curb ramp and street have usable vision. This appendix discusses some of the major causes of visual impairments in the United States along with the mobility challenges faced by people with visual impairments.

Low Vision
Low vision is best understood as being along a continuum that is often dynamic because of changes in vision status as well as the effects of the environment (illumination, contrast). The most common vision measure, known as visual acuity, ranges from the standard of 20/20 (normal vision) to complete blindness (no light perception). Very low visual acuity is sometimes characterized clinically by the ability to “count fingers” or to see “hand waving” or “light projection” (ability to locate the direction of a light source). Standard measures of visual acuity refer to the ability to see small, high-contrast visual stimuli. Often people with visual impairments have especially reduced sensitivity for low contrast stimuli of many different sizes. Among practitioners, low vision is commonly thought of as best corrected visual acuity in the better eye of less than 20/70, although the definition given by the National Eye Institute (NEI) includes people with somewhat better acuity. Low vision is operationally defined by NEI as a best-corrected visual acuity of less than 20/40 in the better-seeing eye, excluding those who meet the definition for being legally blind (see below).

Approximately 75 percent of persons with low vision in the United States are elderly, and many of the conditions that cause low vision become increasingly common with age. For instance, while less than one percent of people between the ages of 60 and 69 have low vision, nearly 17 percent of people age 80 and older do.18

Legal Blindness
Legal blindness is defined in the U.S. as having best-corrected visual acuity of less than 20/200 in the better-seeing eye or an effective visual field of less than 20 degrees. Functionally, NEI defines low vision as “a visual impairment, not corrected by standard eyeglasses, contact lenses, medication, or surgery, that interferes with the ability to perform everyday activities.”

Approximately 2 percent of Americans age 40 and older have low vision and an additional 0.8 percent are legally blind.19 In 1994-95, approximately 1.3 million Americans reported legal blindness. Of these individuals it is estimated that 80 percent had some “useful vision” while the other 20 percent had only light perception or were totally blind (no light perception).20

People who are legally blind, while significantly impaired, may also be quite visually functional. For example, children with 20/200 visual acuity will typically read print, not Braille. Although persons with low vision and moderate levels of legal blindness have remaining functional vision, they also have a serious loss of vision that affects their visual independence. For example, common issues affecting mobility are problems identifying curbs or stairs and a fear of falling. Changes in illumination that require the eye to light adapt, also be a major problem.

Conditions that Cause Low Vision and Blindness
Low vision has a variety of causes. Among the most common in the United States are age-related macular degeneration (AMD), cataract, glaucoma, and diabetic retinopathy. The following sections summarize these four common conditions and a fifth condition, retinitis pigmentosa which causes blindness. Some implications of these conditions for the visual detection of detectable warnings are discussed. It is important to note that most conditions affecting the visual system occur in varying degrees of severity. Some become progressively more severe over time. Therefore, many people who have symptoms of, or have been diagnosed with any of the conditions below do not necessarily have low vision.

Macular degeneration
Macular degeneration is a dysfunction of the macular region of the retina that affects vision in the center of the visual field. Symptoms of macular degeneration include blurring, dimness, or a blind spot in the center of the visual field.21 Macular degeneration is incurable. It typically progresses slowly and can lead to low vision or blindness in its late stages. There are multiple types of macular degeneration, but the most common is age-related macular degeneration (AMD). More than 1.7 million people age 40 and older currently have late-stage AMD, representing about 1.5 percent of people in this age category.22 AMD rarely causes substantial vision loss among people below the age of 50, but becomes increasingly common, and often more severe, with increasing age. AMD has 2 forms, wet and dry, with dry accounting for approximately 95 percent of all cases. The wet form results in a sudden and dramatic loss of vision while the dry form is slow and gradual.

People with macular degeneration have varying effective fields of view. The affected area may be no more than a small spot in the center of vision or may affect most or all of the visual field. The vision loss within that field may range from slight blurring or dimness to complete lack of vision. A person with AMD will have difficulty with fine details and low contrast. In AMD, visual acuity can be reduced to approximately 20/400, but a person affected will not become complete blind from this condition.

To be visible to people with macular degeneration, detectable warnings must be visible to people who may only be able to use their peripheral vision. Peripheral vision is not as acute for seeing fine details as central, or foveal, vision. However, peripheral vision is sensitive to contrast, motion, and coarser (larger) features. People with macular degeneration may also benefit from adequate street lighting at night.

Glaucoma is a degeneration of cells in the optic nerve that is generally associated with high fluid pressure within the eye. The condition typically develops slowly, beginning with a loss of vision in the periphery. The progression of glaucoma can be slowed or stopped with medical treatment, but vision loss prior to treatment cannot be repaired.23 About 2.2 million people age 40 and older have glaucoma, representing nearly 2 percent of people in this age category.24 Glaucoma is rare among people below the age of 50, but becomes more common with increasing age.

Glaucoma typically begins with a minor loss of vision in the periphery. As the condition progresses, the vision loss becomes more severe in the periphery and the visual field becomes smaller. If it is not treated, glaucoma can lead to “tunnel vision” and then ultimately to blindness. With early detection and treatment, however, the progression of glaucoma can usually be slowed or stopped after minimal peripheral loss of vision. In most cases of treated glaucoma, enough central, acute vision is preserved that these individuals do not have any special needs for detectable warning appearance. However, individuals with more progressed cases may have special needs for detectable warnings. For individuals who have lost most peripheral vision, it is important that detectable warnings be located where individuals are likely to focus. For individuals who have begun to lose vision in the center of the visual field, detectable warnings should have high contrast.

Diabetic retinopathy
Diabetic retinopathy is a condition affecting people with diabetes (Type I and Type II) in which blood vessels in the retina become damaged and can cause loss of vision. It is a leading cause of visual impairment among working-age Americans. Diabetic retinopathy is a progressive disease that may lead to total blindness. The U.S. Centers for Disease Control and Prevention (CDC) estimate that nearly 16 million people in the United States have diabetes, but that approximately one in three diabetics has not been diagnosed.25 About one in three diabetics age 18 and older have signs of diabetic retinopathy; this represents more than 5 million people, or 2.5 percent of this population.26 However, only about 1 in 12 people age 40 and older with diabetes has vision-threatening retinopathy.27 The likelihood of diabetic retinopathy increases over time from the onset of diabetes. Most people who have juvenile-onset diabetes are eventually affected by diabetic retinopathy.28 The onset and progression of diabetic retinopathy can be controlled, but not stopped, by controlling blood sugar, blood pressure, and through laser treatment.29

In its early stages, diabetic retinopathy can cause transient spots on the visual field. As it progresses, vision can become blurred and irregular blind spots can develop. These conditions can vary and sometimes improve over time. In its late stages, visibility may be reduced to light sensitivity and complete blindness may result. Although the majority of people with diabetic retinopathy do not have low vision, those who do may have special needs for the visibility of detectable warnings. People with blurred vision may benefit from detectable warnings that have good contrast against the surrounding area and that do not depend on small details or different colors used within the detectable warning. People with blind spots may have varying degrees of difficulty seeing in any part of the visual field and may benefit from large detectable warnings with high contrast.

Cataract is a clouding, or opacity, of the eye’s lens that causes a blurring of the visual field. Although cataract can be present from birth in rare cases, the first symptoms typically do not occur until middle age or later. More than 20 million people age 40 and older have cataract in one or both eyes, representing about 17 percent of people in this age category.30 However, the condition develops slowly and is treatable with minor surgery (including lens replacement), so in the United States people with cataract may have somewhat impaired vision before surgery is performed, but usually regain good visual function following cataract removal.

Cataract causes a blurring of vision across the entire visual field. The effects are similar to those of myopia, except that unlike myopia, the lack of resolution is consistent regardless of the distance of the visual target from the eye. Cataract may also cause a dulling of color perception, and sensitivity to glare. It may affect one or both eyes. In its early stages the condition may not be noticeable, but if left untreated, it can ultimately lead to low vision or blindness. In order to be visible to people with cataract, detectable warnings must be large enough to appear as distinct objects and must also have high contrast against the pavement. Small borders around the detectable warning or different colors used within the detectable warning may not be detectable because the lack of resolution caused by cataract may cause fine details to blur into the surround. People with cataract may also benefit from adequate street lighting at night, although glare can be a problem.

Retinitis pigmentosa
Retinitis pigmentosa (RP) is group of several inherited diseases with common attributes. In RP degeneration of the retina leads to vision loss. It is a condition that affects about 100,000 Americans. There is no cure or proven treatment for RP. The rod photoreceptors typically begin to degenerate before the cones do, so night blindness is often the first symptom. As the disease progresses, vision deteriorates beginning in the mid-periphery and progressing to the center of the visual field and outward to the far periphery. Unlike most diseases affecting vision, the first signs typically appear between childhood and young adulthood and most people with RP are legally blind by the age of 40.31

The deterioration of night vision is often the first symptom of RP. Aids such as light-gathering scopes and flashlights can help in dark conditions. People with RP may also experience substantial glare in daylight and under artificial lighting. Specially filtered lenses can help to reduce glare. The progression of RP varies depending upon the particular form of the disorder. Typically vision is lost in the periphery and the visual field shrinks over time, causing tunnel vision and sometimes eventually blindness. Rarely, RP can progress such that central vision is affected first. This form is called “inverse RP.” In order to be visible to pedestrians with RP, detectable warnings must be visible under conditions of night blindness and daylight glare. Light, bright-colored detectable warning surfaces may be most visible at night. High contrast may be most effective to overcome problems with glare. Aside from these lighting issues, the progression of vision loss in RP is similar to that of glaucoma. In both conditions, vision loss begins in the periphery and progresses toward center, but acuity in the center of the remaining visual field typically remains sharp. RP, unlike glaucoma, cannot be treated, and is more likely than glaucoma to cause low vision.

Other causes of visual impairment
Visual impairment also can result from other conditions, such as traumatic eye or brain injury, Albinism, cancer of the eye (retinoblastoma, choroidal melanoma), retinopathy of prematurity, ocular histoplasmosis, and many other conditions affecting the eye, optic nerve, or visual centers in the brain. The variety of visual abilities among pedestrians is extremely diverse.

Color blindness
Color blindness, though typically not a low vision condition, affects the ability to discriminate colors, and can alter the conspicuity of visual signals such as detectable warnings which depend on color and luminance differences. In nearly all cases of inherited or acquired color vision deficiencies, the term “colorblind” is a misnomer because it suggests that affected individuals do not see colors. In fact, this is true for only a tiny fraction of individuals affected. Color blindness (color vision impairment) by itself rarely causes any navigational challenges for pedestrians. Color vision impairments may be inherited, in which case they are usually not associated with any other aspect of visual impairment, or they may be acquired as a result of exposure to toxic substances or certain drugs. Acquired color vision disturbances also may be caused by diseases of the eye or neural pathways, in which case there may or may not be other indications of visual impairment.

Inherited color vision deficiencies are quite common, affecting approximately eight percent of men in Europe and North America and approximately 0.4 percent of women. The difference in incidence between the sexes reflects the sex-linked inheritance pattern of the various forms of red-green deficiencies (i.e., “protan” and “deutan”), which are the most common. A third form, “tritan” defects have a different inheritance pattern and very low incidence. Tritanopia involves a loss of sensitivity to short wavelength light and characteristic color confusions between colors such as violet and yellow. There are two main forms of complete color blindness (achromatopsia) and these are very rare. The incidence is less than 1 in 30,000 for typical achromatopsia, which is characterized by poor visual acuity (20/200), and no evidence of functioning cone photoreceptors. Persons with atypical achromatopsia (incidence estimated at 1 in 10 million) retain normal visual acuity.32

The majority of individuals who have any of the inherited variant forms of color vision have essentially normal visual function. They have good visual acuity (perhaps corrected by lenses), they read without difficulty, they can avoid hazards in the environment using visual cues, and have no need for detectable warnings unless their vision system has been altered by injury or by a disease process. Combinations of colors which are distinctly different for most people may look similar or possibly even identical to a person with a color vision impairment. However, for most color combinations, a detectable warning surface will be seen as being distinctly different from the surrounding surface if there is sufficient luminance contrast between the two surfaces. Providing adequate luminance contrast between two surfaces is important to ensure that the surfaces can be distinguished by the greatest number of pedestrians who have atypical forms of color vision and other forms of visual impairment. Also, differences in pattern (domes), texture, and gloss of the warning surface and the adjacent pavement may affect the conspicuity of the detectable warning surface for those pedestrians who have color vision impairments.

How color vision impairments affect visibility of detectable warnings: Detectable warnings must provide visual contrast with their surround. This can be accomplished either by providing light-on-dark contrast (warning surface lighter than surround) or by providing dark-on-light contrast (warning surface darker than surround). The measurement of luminance contrast is based on a standard spectral sensitivity function for human observers (built into the sensitivity of photometers). Individuals who have visual impairments may have spectral sensitivity functions which differ somewhat from the standard spectral sensitivity function. This means that objective measurements of luminance (and luminance contrast) will not precisely characterize the visual difference between two surfaces which differ in color. For most types of visual impairment, providing a moderately high level of luminance contrast between two differently colored surfaces is still a good way to increase the probability of visual detection. However, there are some people who have relatively reduced sensitivity to light in one portion of the visual spectrum, and for these people, standard measures of luminance contrast may overestimate the perceptual difference between surfaces which strongly reflect light in the region of the spectrum where they have reduced sensitivity. People with a form of color vision call protanopia have a markedly reduced sensitivity to long wavelength light. Other people who have other forms of color vision such as tritanopia or deuteranopia have reduced sensitivity to short or middle wavelength light. Many elderly people who have yellowing of the lens or cataract also will have a reduced sensitivity to short wavelength light.

In the case of protanopes, luminance measurements will tend to overestimate the visibility of red surfaces, which will generally appear darker to them than to most people. When choosing the color of detectable warnings, it may be important to consider whether the detectable warning surface is intended to be seen as being lighter or darker than the sidewalk. Surfaces which reflect light predominately at long wavelengths (and appear red) and surfaces which reflect light predominately at short wavelengths (violet or blue) may work well if they are used as the darker surface in a contrasting pair, especially if they are paired with a paving material which reflects light well at all wavelengths such as concrete.33 Red and blue warning surfaces could be difficult to distinguish for some people if they are used as the lighter surface in a contrasting pair. Other useful advice for choosing color combinations suitable for people with partial sight and color deficiencies can be found on the website for Lighthouse International34 and elsewhere.35

Federal yellow, although highly visible to most people, may be seen as being very similar to white or light gray by people who are insensitive to short wavelength light. People who have tritan color defects are less sensitive to short wavelength light and make characteristic color confusions between yellow, white, and violet surfaces. They also may confuse certain blues and greens. Although tritanopia is a rare condition, a similar loss of short wavelength sensitivity may occur with certain acquired “blue” color vision defects.36 Also, a relative loss of short wavelength sensitivity (in addition to an overall decreased sensitivity to light) is very common with natural yellowing of the lens with age or with cataract formation. Many artificial illuminants (e.g. tungsten filament lamps) produce relatively little short wavelength light, making discrimination between white and yellow surfaces and between blue and black surfaces especially difficult for older adults at night.