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Publication Number: FHWA-RD-01-051
Date: May 2001
Guidelines And Recommendations To Accommodate Older Drivers and Pedestrians
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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 older 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, pedestrians, 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, for example for 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 non–commercialized 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.
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.
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).
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 (PRT)
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 older motorists.
This refers to the ability to store, manipulate, and retrieve information for later use while driving (for example, 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, and apply correct pressure for appropriate speed control; also, 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.
DRIVER LICENSE RENEWAL REQUIREMENTS
State license renewal requirements for passenger car drivers in the United States are presented below. Many States allow mail–in license renewal, although a subset of these prohibit mail–in renewals for drivers over a certain age. On the other extreme, Florida requires in–person renewal at every third cycle, which means that a driver with a clean record will not step foot into a DMV for 18 years (or 12 years for an unclean record). Petrucelli and Malinowski (1992) state that "the examiner's personal contact with the applicant is the only routine opportunity to detect potential problems of the functionally impaired driver." There are also differences in license renewal testing requirements (vision, written knowledge, and on–road driving) across the United States. General visual acuity requirements for driver licensing are included in this table; however, most States also have a visual field requirement that is not included in this table. Specific driver licensing requirements may be obtained by accessing each State's Department of Motor Vehicles Web site.
MEASURING THE VISIBILITY OF HIGHWAY TREATMENTS
The visibility of highway treatments providing guidance information to motorists is critical, particularly for nighttime operations. Guidance information is needed sufficiently in advance of any change in roadway heading, to allow the driver to plan and execute steering and speed control movements smoothly as needed for path maintenance. Taking into account the diminished visual, attentional, and perceptual–cognitive abilities associated with normal aging as documented in the Highway Design Handbook for Older Drivers and Pedestrians, a 5–s preview distance (at operating speeds) is regarded as the minimum for which visibility requirements should be established, and for high–speed operations a preview distance or 7 to 10 seconds or more may be advisable.
Treatments rendered visible by reflected light include all non–internally illuminated targets, such as pavement markings, raised pavement markers, vertical (post–mounted) delineators, and highway signs. At nighttime, these treatments are illuminated by vehicle headlights, and light is returned (reflected) principally back in the direction of the driver's eye; as illustrated in the drawing to the right, this property denotes the characteristic of retroreflectivity. According to the MUTCD, markings that must be visible at night should be retroreflective unless ambient illumination assures adequate visibility. The recommendations contained within the Highway Design Handbook for Older Drivers and Pedestrians are intended to improve the visibility of retroreflectorized pavement markings used to delineate lane and roadway boundaries, curbs, medians, and other raised surfaces, and to channelize traffic in the vicinity of intersections.
General principles of retroreflection, as well as driver visibility needs, are discussed at length in the FHWA Roadway Delineation Practices Handbook (Migletz, Fish, and Graham, 1994), and the interested reader is encouraged to consult that resource. Before turning to measurement techniques, however, several key points deserve emphasis.
First, the human visual system is capable of discriminating an object against its background only when a threshold level of contrast has been reached. While color contrast is important in certain contexts, it is the relative brightness of the visual target (e.g., pavement striping) against the surrounding area (the road surface) that is most critical. The brightness of an object rendered visible by reflected light is described by its luminance (L) level. Contrast (C) is commonly defined as the ratio of an object's luminance minus the luminance of the surrounding area, relative to the surrounding area alone, and is thus calculated according to the formula:
Luminance contrast, dependent as it is on reflected light, varies according to many factors. Some relationships affecting contrast thresholds for target detection are crosscutting, however. The human visual system is less sensitive to contrast as the ambient light level decreases; and, the human visual system is less sensitive to contrast as a consequence of normal aging. Therefore, moving from daylight through twilight and dusk to nightfall, more contrast is required to see a given target; and, this increment is significantly greater for older drivers than for younger drivers. This means that the contrast of critical safety targets such as lane and road boundaries must be maintained at higher levels to accommodate the needs of older drivers, especially at night.
Considerable research has been conducted by FHWA and others to develop specifications for retroreflective materials to return sufficient light to a driver's eyes (from a target at a specified distance and angular relationship to the driver, and illuminated by a specified light source) to ensure a contrast level above the threshold for detection (Ziskind, Mace, Staplin, Sim, and Lococo, 1991; Mercier, Goodspeed, Simmons, and Paniati, 1995; Zwahlen and Schnell, 1999, 2000). The retroreflective performance of pavement markings, which is a property of the materials from which they are fabricated, is measured in the (metric) units of millicandela per square meter per lux (mcd/m2/lx). This measure also denotes the coefficient of retroreflected light (RL). Higher values of RL for a material indicate higher (installed) brightness levels when viewed by an observer/driver at a specified angular relationship with the light source and target. Two angles are key to this relationship, the angle between the light source, the observer, and the target surface, and the angle between the incident light path and a reference axis normal to the surface of the target. These are, respectively, labeled the observation angle and the entrance angle, as represented in the drawing below:
For entrance angles less than 30, RL is much more sensitive to the observation angle. The observation angle is a function of the distance a vehicle is from the target illuminated by its headlights, and the height of both the headlights and the driver's eyes above the road surface. For an assumed driver eye height of 1.45 m (57 in), headlight height of 0.61 to 0.71 m (24 to 28 in), and detection distance of approximately 80 m (260 ft) –– chosen to afford a 5 s preview at a speed of 56 km/h (35 mi/h) –– the observation angle is 1. In fact, the observation angles for pavement treatments for the full range of road types and operating speeds of interest fall within a one–degree span, from 0.5 to 1.5. Since driver eye height and headlight position do not change, the critical variable is the preview distance at which a target must be visible to the motorist for safe vehicle control.
Retroreflective materials used for pavement treatments are designed to return enough light from headlight illumination to a driver under a defined viewing geometry, as noted above, that their contrast is well above threshold. Specifically, the performance requirement for a given material is defined by the amount that the contrast obtained under a set of reference viewing conditions exceeds threshold contrast. This performance requirement is confirmed through laboratory and or field measurements, using an instrument (a retroreflectometer) with an internal light source and a means of control over the entrance and observation angles when the instrument is applied to the to–be–measured surface. Such measurements yield the amount of light (luminance intensity) that is reflected in the desired direction. If the performance specification for the material is met, it is assumed that a level of contrast resulting in a high probability of detection will also be obtained.
Emerging retroreflectivity standards for various highway signing and marking applications from FHWA hold the promise of significantly improving the visibility of these treatments, if extended to include maintained levels of performance as well as a specification for performance at the time of installation. Even with this development, however, there are concerns with the measurement of retroreflectivity, concerns that are serious enough that a supplementary approach has been recommended in this Handbook.
One concern is with the required precision of measurement using a retroreflectometer. As stated above, retroreflectivity is quite sensitive to small changes in observation angle. Field experience by the Handbook authors with portable retroreflectometers indicates that adjustments in this measurement parameter can be both unreliable and unstable. The calibration of the unit also must be checked periodically to insure valid measurements. More sophisticated, mobile measurement systems have been developed, but these are expensive and may not be available in a local jurisdiction.
Another concern with relying solely on the measurement of retroreflectivity level, is that it is a mediating variable from which inferences about visibility are made, rather than a direct measure of available contrast. The seminal FHWA study in this area concluded, "the practical value of guidelines [for minimum visibility requirements for traffic control devices] will be determined as much as anything else by their simplicity" (Ziskind et al., 1991). It is the contrast of the treatment, when viewed by a driver under the particular conditions of interest, that is fundamental to its visibility and probability of detection. Therefore, if it is feasible to directly measure luminance contrast, this would be a preferred practice for ensuring maintained levels of visibility to accommodate the needs of older drivers. A field methodology for such measurements is diagrammed on the following page using, as an example, an observation angle of 1.
Using a hand–held light meter, or photometer, a technician can obtain luminance readings from a pavement treatment and from the adjacent roadway surface (background), then perform the contrast calculation shown on page 75. Several suitable instruments offering the convenience of through–the–lens aiming are commercially available at a cost of less than $3,000.
Photometric measurements should be obtained under the conditions of interest. For example, if the question is whether a treatment provides a desired level of contrast at a 5–s preview distance under low beam headlight illumination at night, these are the conditions under which luminance measurements should be obtained. The technician operating the photometer could be located whichever is most convenient, either in the vehicle or outside, provided that a large enough target area is viewable using the smallest aperture on the photometer. If positioned outside, as diagrammed above, care should be taken not to interpose oneself directly between the light source (headlights) and the to–be–measured pavement treatment. However, because light is reflected in a cone from a given point on the retroreflective surface, the technician may move laterally a small distance and still obtain valid measurements. And because the intensity of light reflected from the treatment (i.e., luminance) will be the same at any measurement distance, the only essential requirement is to select x and y values using the formula arctan y/x that afford the desired observation angle. This means that as one moves nearer the treatment, the photometer must be held somewhat closer to the pavement surface to preserve the observation angle. Observation angles affording a 5–s preview distance at varying speeds are:
With the information above, the vertical distance above the pavement (y) at which the photometer should be held is easily calculated for a given longitudinal separation (x) from the treatment, for a constant observation angle.
In summary, emerging retroreflectivity standards are expected to serve as a useful metric to insure adequate visibility of highway treatments at the time of installation. It is the maintained visibility of these treatments that will most important for safe operation, however. To confirm that a sufficient level of luminance contrast to accommodate older drivers is afforded by a treatment under a specified operating condition, field measurements using the methodology outlined above are recommended.