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Handbook for Designing Roadways for the Aging Population

APPENDIX A

Supplemental Technical Notes

Aging And Driver Capabilities

Many aspects of sensory and cognitive function needed to drive safely deteriorate in later adulthood. In fact, recent data indicate that aging adults are in the highest risk category for crashes when figures are based on crashes per number of miles driven. Among the senses, the importance of vision is paramount. To respond appropriately to all manner of stimuli in the roadway environment, a driver must first detect and recognize physical features of the roadway, traffic control devices, other vehicles, non-motorized users, and a wide variety of other objects and potential hazards of a static and dynamic nature. On rare occasions, critical information concerning the presence or position of traffic is conveyed to a road user solely through an auditory signal; in the vast majority of cases, however, the visual system is preeminent at this (input) stage of processing.

Age-related changes in the lens of the eye, combined with pathology (for example, glaucoma, cataracts, diabetic retinopathy, and macular degeneration), result in the diminished capabilities that are described below.

Reductions in Acuity

This is the ability needed to discriminate high contrast features; it is necessary for reading information on road signs. Visual acuity of 20/40 with or without corrective lenses for both eyes or one blind eye is the predominant minimum standard for driver licensing for passenger car drivers throughout the U.S. However, there are an increasing number of states (including Pennsylvania, Maryland, New Jersey, Florida, Illinois, and others) that will grant a restricted license to low-vision drivers with acuities as poor as 20/70 to 20/100. Restrictions may include daytime only, area, and speed limitations. Added to reductions in acuity, aging is also associated with yellowing of the eyes’ lenses and increased density (or thickening). This affects the way color is perceived and also reduces the amount of light that reaches the retina, which makes seeing in low light conditions more difficult.

Reductions in Contrast Sensitivity

This is the ability needed to detect low-contrast features; it is necessary to see worn lane lines, detect (non retroreflectorized) curbs and median boundaries, and see other road users at dusk. Some people have 20/20 acuity but still have “cloudy” or washed-out vision. Contrast sensitivity makes it possible to distinguish an object from its background. It begins to decline after about age 40, as a result of normal aging. Individuals age 61+ have an increasing risk for the development of cataracts and other sight-threatening or visually disabling eye conditions that reduce contrast sensitivity. Many people with reductions in contrast sensitivity are not aware that their vision is impaired, and contrast sensitivity is not a standard test in most DMVs for licensing.

Reductions in Visual Field

This is the ability to see objects in the periphery; it is necessary for detecting signs, signals, vehicles, pedestrians, cyclists, etc., outside of a limited field of view directly ahead. A limitation in visual field size is a physiological limitation—the person’s visual system is not capable of detecting a stimulus outside of his or her visual field.

Restrictions in the Area of Visual Attention

This is the ability to see potential conflicts in the periphery, and to discriminate relevant from irrelevant information; it is necessary for responding quickly and appropriately to a constantly changing traffic scene. Sometimes termed “useful field of view,” “functional field of view,” or “attentional window,” this refers to a subset of the total field of view. Restrictions in the area of visual attention can lead to “looked but didn’t see” crashes, where stimuli can be detected, but cannot be recognized and understood sufficiently to permit a timely driver response. As such, this term represents a limitation at the attentional stage of visual information processing, rather than a physiological limitation.

Increased Sensitivity to Glare

This refers to the ability to see in the presence of oncoming headlights, at night, or in the presence of sun glare in daytime. Glare introduces stray light into the eye; it reduces the contrast of important safety targets.

Slower Dark Adaptation

This is the ability needed to see targets when moving from areas of light to dark, which may occur at highway interchanges or moving from commercialized areas to noncommercialized areas.

Decreased Motion Sensitivity

This ability is needed to accurately estimate closing speeds and distances; it is necessary, for example, for judging gaps to safely perform left turns at intersections with oncoming traffic, to cross an intersecting traffic stream from a minor road or driveway, or to merge with traffic.

Hearing Sensitivity

While driving is primarily a visual task, sounds sometimes give drivers the first awareness that a problem or condition demanding a timely driver response exists, including not only emergency sounds such as sirens, but also sounds that signify problems with brakes, tires, or other mechanical problems. In addition, it is to be expected that hearing will play an increasingly important role in the future, as new technologies are introduced in vehicles to provide aging drivers with navigational assistance or early warning of hazards. Pedestrians with visual impairments, regardless of age, also depend on auditory cues to detect approaching traffic at intersections, and rely on auditory signals at crosswalks if they are present. A decreased sensitivity to sound, particularly at higher frequencies, accompanies normal aging.

Compounding the varied age-related deficits in visual performance that are a part of normal aging, an overall slowing of mental processes occurs as individuals continue to age into their seventies and beyond. Declines have been demonstrated in a number of specific mental activities that are related to driver and pedestrian safety, such as attentional, decisional, and response-selection functions. These are described below.

Selective Attention

This refers to the ability to filter out less critical information and continuously re-focus on the most critical information (for example, detecting a lane-use restricted message on an approach to a busy intersection; detecting a pedestrian crossing while watching oncoming traffic to locate a safe gap).

Divided Attention

This refers to the ability to perform multiple tasks simultaneously and process information from multiple sources (for example, lane-keeping, reading signs, noticing traffic signals and changing phases, while maintaining a safe headway with other traffic during an intersection approach).

Perception-Reaction Time

This is the time required to make a decision about what response is appropriate for specific highway design elements and traffic conditions, and then make a vehicle control movement such as steering and/or braking. As the overall speed of mental processing of information slows with aging, PRT increases. As the complexity of the driving situation increases, PRT increases disproportionately for aging motorists.

Working Memory

This refers to the ability to store, manipulate, and retrieve information for later use while driving (e.g., carrying out a series of navigational instructions while negotiating in heavy traffic; or remembering, integrating, and understanding successive phases of a changeable message sign).

Finally, it has been well established that physical capabilities decline as a function of age and also as a function of general health. Aging (as well as disease and disuse) brings about changes in the components and structure of the cartilage near the joints, underlying bones, ligaments and muscles. These changes impair the ability of the musculoskeletal system to perform driving acts. The physical capabilities (motor functions) needed for safe and effective vehicle control are described below.

Limb Strength, Flexibility, Sensitivity, and/or Range of Motion

These abilities are needed to quickly shift (the right foot) from accelerator to brake pedal when the situation demands, to apply correct pressure for appropriate speed control, and for arm movements to safely maneuver the car around obstacles.

Head/Neck and Trunk Flexibility

A key ability of a driver is to rapidly glance in each direction from which a vehicle conflict may be expected in a given situation; this includes the familiar “left-right-left” check before crossing an intersection, as well as looking over one’s shoulder before merging with traffic or changing lanes.

Page last modified on May 24, 2016
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