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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.

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 (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.

Working Memory

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.

 

State 2001 Licensing Renewal Requirements and Distinctions for Older Drivers
Alabama 4–year renewal cycle (in–person). No tests for renewal. Minimum acuity 20/60 in one eye with/without corrective lenses. May not use bioptic telescopic lens to meet acuity standard. No special requirements for older drivers.
Alaska 5–year renewal cycle (mail–in every other cycle). No renewal by mail for drivers age 69+ and to drivers whose prior renewal was by mail. Vision test required at in–person renewal. Minimum 20/40 in one eye for unrestricted license. 20/40 to 20/100 needs report from eye specialist; license request determined by discretion. May use bioptic telescopic lens under certain conditions.
Arizona 12–year renewal cycle. At age 65, reduction of interval to 5 years. New photograph and vision test at renewal; no renewal by mail after age 70 (available to active duty veterans and dependents only). Minimum acuity 20/40 in one eye required; acuity of 20/60 restricted to daytime only. May not use bioptic telescopic lens to meet acuity standard
Arkansas 4–year renewal cycle. Vision test required at renewal, with minimum 20/40 required for unrestricted license. Acuity of 20/60 restricted to daytime only. Bioptic telescopes permitted under certain circumstances. No special requirements for older drivers.
California 5–year renewal cycle with vision test and written knowledge test required. No renewal by mail at age 70. Minimum visual acuity is 20/200 (best corrected) in at least one eye, as verified by an optometrist or ophthalmologist. Bioptic lenses are permitted for driving, but may not be used to meet 20/200 acuity standard.
Colorado 5–year renewal cycle (mail–in every other cycle). Vision test required at renewal. Minimum acuity must be 20/70 in the better eye if worse eye is 20/200 or better; 20/40 if worse eye is worse than 20/200. Bioptic telescopes are permitted to meet acuity standard. No renewal by mail for drivers age 65+.
Connecticut 4–year renewal cycle (mail–in every other cycle). Vision test required at in–person. 20/40 required in better eye for unrestricted license; 20/50 to 20/70 restricted license; under some circumstances, a license may be issued when acuity is 20/200. No license may be issued to drivers using telescopic aids. Reduction of interval to 2 years may be requested by drivers age 65+.
Delaware 5–year renewal cycle (in–person). No tests required for renewal. Minimum acuity 20/40 for unrestricted license; restricted license at 20/50; beyond 20/50 driving privileges denied. Bioptic telescopes treated on case–by–case basis. No special requirements for older drivers.
District of Columbia 4–year renewal cycle (in–person). Unrestricted license for 20/40 acuity; 20/70 in better eye requires 140 visual field for restricted license. At age 70, vision test required and physician signature attesting to physical and mental capability to drive; a medical report plus reaction test may also be required. At age 75 written knowledge and road tests may be required.
Florida 6–year renewal cycle for clean driving record; 4–year renewal cycle for unclean record. In–person renewal required every 3rd cycle. Vision test at in–person renewal. Must have 20/70 in either eye with or without corrective lenses. Monocular persons need 20/40 in fellow eye. Bioptic telescopes are not recognized to meet acuity standard. No special requirements for older drivers.
Georgia 4–year renewal cycle (in–person). Vision test required for renewal (within prior 6–month period). Acuity 20/60 in either eye with or without corrective lenses. Bioptic telescopes permitted for best acuity as low as 20/200, with restrictions. No special requirements for older drivers.
Hawaii 6–year renewal cycle for drivers ages 18 to 71 (in–person). Vision test required, with 20/40 standard for better eye. Bioptic telescopes permitted for driving, but not for passing vision test. Reduction of interval to 2 years for drivers age 72+.
Idaho 4–year renewal cycle (mail–in every other renewal). Vision test required: 20/40 in better eye for no restrictions; 20/50 – 20/60 requires annual testing; 20/70 denied license. Use of bioptic telescopes is acceptable, but acuity must reach 20/40. Driving test may be required if examiner thinks it is needed. No renewal by mail after age 69.
Illinois 4–year renewal cycle for ages 21 to 80 (mail–in every other cycle for drivers with clean records and no medical report review requirements). Vision test at in–person renewal: 20/40 in better eye for no restrictions; 20/70 in better eye results in daylight only restriction. May have 20/100 in better eye and 20/40 through bioptic telescope. Written test every 8 years unless clean driving record. From ages 81 to 86, reduction of interval to 2 years. At age 87, reduction of interval to 1 year. No renewal by mail, vision test required, and on–road driving test required at age 75+.
Indiana 4–year renewal cycle (in–person). Vision screening at renewal, including acuity and peripheral vision. 20/40 in better eye for no restriction; restricted license for 20/50. Bioptic telescope lenses permitted for best acuity as low as 20/200, with some restrictions, if 20/40 achieved with telescope. At age 75 renewal cycle reduced to 3 years. (Mandatory drive test for persons age 75+ eliminated 1/19/00). Drive test required for persons with 14 points or 3 convictions in 12–month period.
Iowa Renewal cycle of 2 years or 4 years at driver's option. Vision screening at renewal: 20/40 in better eye, with or without corrective lenses; 20/50 in better eye results in restricted license for daylight only; 20/70 in better eye results in restricted license for daylight only up to 35 mi/h. Bioptic telescopes are not permitted to meet acuity requirement. At age 70, renewal cycle is 2 years.
Kansas 6–year renewal cycle for ages 16–64 (in–person). Vision and knowledge test at renewal. Minimum acuity: 20/40 better eye; 20/60 better eye with doctor report; worse than 20/60 must demonstrate ability to operate vehicle safely and have safe record for 3 years. At age 65, renewal every 4 years.
Kentucky 4–year renewal cycle (in–person). No tests required for renewal. Minimum visual acuity 20/200 or better with corrective lenses in better eye; 20/60 or better using a bioptic telescopic device. No special requirements for older drivers.
Louisiana 4–year renewal cycle (mail–in every other cycle). Vision test at renewal. Minimum acuity 20/40 in better eye for unrestricted; 20/50 – 20/70 with restrictions; 20/70 – 20/100 possible restricted license; less than 20/100 in better eye – referred to Medical Advisory Board (MAB). No renewal by mail to drivers over age 70, or those with a conviction of moving violation in 2–year period prior to renewal.
Maine 6–year renewal cycle. At age 65, renew every 4 years. Vision screening test at renewal for age 40, 52, and 65; every 4 years after age 65. Minimum acuity 20/40 better eye without restrictions; 20/70 better eye with restrictions.
Maryland 5–year renewal cycle. Vision tests required for renewal (binocular, acuity, peripheral). Minimum acuity of at least 20/40 plus continuous field of vision at least 140 in each eye for unrestricted license; at least 20/70 in one or both eyes for restricted, but requires continuous field of view of at least 110 with at least 35 lateral to the midline of each side; 20/70–20/100 requires special permission from MAB. Medical report required for new drivers over age 70. (Maryland law specifies that age alone is not grounds for re–examination of older drivers).
Massachusetts 5–year renewal cycle (in–person). Vision screening at renewal: 20/40 better eye for unrestricted; 20/70 better eye for restricted; 20/40 through telescope, 20/100 through carrier. No special requirements for older drivers (Massachusetts law prohibits discrimination by reason of age for licensing issues.)
Michigan 4–year renewal cycle (mail–in every other cycle if free of convictions). Vision and knowledge test at renewal. Minimum acuity 20/40 better eye for unrestricted; 20/70 better eye with daylight only restriction; 20/60 if progressive abnormalities or diseases of the eye. No special requirements for older drivers.
Minnesota 4–year renewal cycle. Vision test at renewal: 20/40 in better eye for no restrictions; 20/70 in better eye for speed limit restrictions; 20/100 better eye referred to driver evaluation unit. No special requirements for older drivers. (Minnesota law specifies that age alone is not justification for reexamination.)
Mississippi 4–year renewal cycle (in–person). Vision test at renewal: 20/200 best corrected without telescope; 20/70 with telescope. No special requirements for older drivers.
Missouri 3–year renewal cycle (in–person). Vision test and traffic sign recognition test required at renewal. Minimum acuity: 20/40 in better eye for unrestricted; up to 20/160 for restricted. No special requirements for older drivers.
Montana 8–year renewal cycle for ages 21–67. Vision test at renewal: 20/40 in better eye for no restrictions; 20/70 in better eye with restrictions on daylight and speed; 20/100 in better eye possible restricted license if need is shown. For ages 68–74, renewal cycle reduced to 1–6 years. At age 75, renewal cycle reduced to 4 years.
Nebraska 5–year renewal cycle. Vision test at renewal: Knowledge test if violations on record. Acuity 20/40 required in better eye, but 17 restrictions are used, depending on vision in each eye. No special requirements for older drivers.
Nevada 4–year renewal cycle (mail–in every other cycle, if qualified). Minimum acuity 20/40 in better eye. Bioptic telescopes permitted to meet acuity standard: 20/40 through telescope, 20/120 through carrier, 130 visual field. Vision test and medical report required to renew by mail at age 70
New Hampshire 4–year renewal cycle (in–person). Vision test at renewal: 20/40 better eye for unrestricted; 20/70 in better eye with restrictions. At age 75, road test required at renewal.
New Jersey 4–year renewal cycle (10–year in person digitized photo licenses will be implemented in 2003). Periodic vision retest: 20/50 better eye; 20/70 in better eye with restrictions. Bioptic telescope permitted to meet acuity standard. No special requirements for older drivers.
New Mexico 4– or 8–year renewal cycle. Drivers may not apply for 8–year license if they will reach the age of 75 during the last 4 years of the 8–year period. Vision test required for renewal; knowledge and driving test may be required Minimum acuity: 20/40 better eye; 20/80 better eye with restrictions.
New York 5–year renewal cycle. No tests for renewal. Minimum best corrected acuity 20/40 in one eye; 20/40 – 20/70 best corrected one eye requires minimum 140 horizontal visual field; 20/80 – 20/100 best corrected in one eye requires minimum 140 horizontal visual field plus 20/40 through bioptic telescopic lens. No special requirements for older drivers.
North Carolina 5–year renewal cycle (in–person). Vision and traffic sign recognition tests required for renewal. Acuity 20/40 in better eye required for unrestricted; 20/70 better eye with restrictions. Bioptic telescopes are not permitted for meeting acuity standard, but are permitted for driving. No special requirements for older drivers.
North Dakota 4–year renewal cycle. Vision test required for renewal: 20/40 better eye for unrestricted; 20/70 in better eye with restrictions. Bioptic telescopes permitted to meet acuity standard: 20/130 in carrier, 20/40 in telescope, full peripheral field. No special requirements for older drivers.
Ohio 4–year renewal cycle. Vision test required for renewal: 20/40 better eye for unrestricted; 20/70 better eye with restrictions; bioptic telescopes permitted to meet acuity standards. No special requirements for older drivers.
Oklahoma 4–year renewal cycle (in person). No tests for renewal. Minimum acuity: 20/40 better eye for unrestricted; 20/100 better eye with restrictions. Bioptic telescopes not permitted to meet acuity standard, but may be used for driving. No special requirements for older drivers.
Oregon 8–year renewal cycle (mail–in every other cycle). Vision screening test once every 8 years at age 50+. Minimum acuity: 20/40 better eye for unrestricted; 20/70 better eye with restrictions. Bioptic telescopes not permitted to meet acuity standard, but may be used for driving.
Pennsylvania 4–year renewal cycle. Drivers age 65+ may renew every 2 years. Random physical examinations for all drivers age 45+; most selected are over age 65. Minimum acuity: 20/40 better eye for unrestricted; up to 20/100 combined vision with restrictions. Bioptic telescopes not permitted to meet acuity standards, but may be used for driving.
Rhode Island 5–year renewal cycle. Vision test required for renewal: 20/40 better eye. At age 70, renewal cycle reduced to 2 years.
South Carolina 5–year renewal cycle (in–person). Renewal by mail if no violations in past 2 years, and license is not suspended, revoked, or canceled. Vision test and knowledge test required if > 5 points on record. Minimum acuity: 20/40 better eye for unrestricted; 20/70 in better eye if worse eye is 20/200 or better; 20/40 if worse eye is worse than 20/200. Bioptic telescopes not permitted to meet acuity standard, but may be used for driving. No special requirements for older drivers.
South Dakota 5–year renewal cycle. Vision test required for renewal: 20/40 better eye for unrestricted; 20/60 better eye with restrictions. No special requirements for older drivers.
Tennessee 5–year renewal cycle (mail–in every other cycle). Minimum acuity: 20/30 better eye; 20/70 better eye with restrictions; 20/200 better eye requires bioptic telescopes with 20/60 through the telescope. Bioptic telescopes are permitted to meet standard. No tests required for renewal. No special requirements for older drivers.
Texas 6–year renewal cycle (effective 01/01/02; staggered 4 to 6 years until 2002). Vision test required for renewal: 20/40 better eye; 20/70 better eye with restrictions. Bioptic telescopes are permitted to meet acuity standard, and driver must pass a road test. No special requirements for older drivers.
Utah 5–year renewal cycle (mail–in every other cycle if: no suspensions, no revocations, no convictions for reckless driving and no more than 4 reportable violations). Vision test required for drivers age 65+, every renewal. Minimum acuity: 20/40 for unrestricted; 20/100 in better eye with restrictions. Bioptic telescopes are not permitted to meet acuity standard.
Vermont 2–year or 4–year renewal cycle. Minimum acuity: 20/40 in better eye; bioptic telescopes are permitted to meet visual acuity standard, and driver must pass road test. No tests for renewal. No special requirements for older drivers.
Virginia 5–year renewal cycle (mail–in every other cycle unless suspended or revoked, 2+ violations, seizures/blackouts, DMV medical review indicator on license, failed vision test). Vision test required for renewal. Minimum acuity: 20/40 better eye for unrestricted; 20/200 with restrictions; bioptic telescopes are permitted with 20/200 through carrier, 20/70 through telescope. Knowledge and road test required if 2+ violations in 5 years. No special requirements for older drivers.
Washington 4–year renewal cycle (in–person). Vision test required for renewal. Minimum acuity 20/40 better eye; 20/70 better eye with restrictions. Bioptic telescopes are permitted to meet acuity standards. Other tests may be required if License Service Representative deems it necessary. No special requirements for older drivers.
West Virginia 5–year renewal cycle. Minimum acuity: 20/60 better eye; if worse than 20/60, optometrist or ophthalmologist must declare ability to be safe. Bioptic telescopes are not permitted to meet acuity standard, but may be used for driving. No tests required for renewal. No special requirements for older drivers.
Wisconsin 8–year renewal cycle. Minimum acuity: 20/40 better eye; 20/100 better eye with restrictions. Bioptic telescopes are not permitted to meet acuity standards, but may be used for driving. Vision test required for renewal. No renewal by mail at age 70+.
Wyoming 4–year renewal cycle (mail–in every other cycle). Vision test required for renewal (for both mail in and in person). Minimum acuity; 20/40 better eye; 20/100 better eye with restrictions. Bioptic telescopes are permitted to meet acuity standard. No special requirements for older drivers.

 

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 drawing illustrating retroreflectivityreturned (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:

equation for contrast

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:

 

drawing of angles between observer and retroreflector

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.

drawing of person using photometer

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:

 

•  1.0 at 56 km/h (35 mi/h)   •  0.7 at 89 km/h (55 mi/h)  
•  0.8 at 72 km/h (45 mi/h)   •  0.6 at 105 km/h (65 mi/h)  

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.

 

FHWA-RD-01-051

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