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Federal Highway Administration > Publications > Public Roads > Vol. 62· No. 6 > Designing Highways with Older Drivers in Mind

May/June 1999
Vol. 62· No. 6

Designing Highways with Older Drivers in Mind

by Elizabeth Alicandri, Mark Robinson, and Tim Penney

The graying of American society is clearly documented by demographic data. Each year, the average age of Americans increases as the baby boomer generation leaves middle age and heads toward older adulthood. By the year 2020, adults aged 65 and older are projected to make up 20 percent of the population. Within this age group, the number of people aged 75 and older is expected to double by 2000.1

Because of the large variance in the effects of aging, chronological age is not necessarily a good indication of capabilities. It is, however, an easily measured and often used variable. For research purposes, older people are often categorized into separate groups -- young-old (ages 65 to 74) and old-old (ages 75 and above) -- in an attempt to account for the wide range of capabilities inherent in the older population.2

Studies have shown that older drivers have high rates of accidents, injuries, and fatalities on a per-mile-driven basis. Driving, however, is very important to many older persons because it is a critical factor in enabling them to maintain an independent lifestyle. Solutions to the problem of older driver mobility and safety must and can be developed through application of findings in human factors research.

Demographic Changes

At the beginning of the 20th century, approximately 4.1 percent of the population (3 million people) was more than 65. Currently, persons more than 65 are approximately 12.5 percent of the population (31 million people). Figure 1 graphically shows the graying of America.

The large increase in the percentage of the population older than 65 is due to two phenomena. The first is the ever-improving health care system in the United States. The second is the effect of the baby boomers, the large number of children born in the generation following World War II. The aging of the baby boomers is largely responsible for the dramatic shifts in the population distribution.

Age-Related Changes That Affect Driving

Aging affects a wide variety of skills that are critical to safe driving.

In biological terms, aging is defined as the combination of changes in an organism that occur inevitably and irreversibly with the passage of time.3 These changes are manifested in deterioration in physical, sensory, and cognitive skills. Some changes are due strictly to aging itself. Others are due to the propensity for certain diseases (arthritis, glaucoma, etc.) to strike at later ages. These losses are usually gradual and proceed at different rates for each person. Many people do not experience declines until very old age, and most learn to adjust to the limitations imposed by these changes.

Physical changes related to aging include reduced muscle mass and decreases in the efficiency of the circulatory, cardiac, and respiratory systems. Reductions in strength, flexibility, and range of motion caused by arthritis or other conditions can negatively impact driving. For example, difficulties in turning the head, due to reduced flexibility in the neck, make merging into traffic and dealing with skewed intersections (where the angle of intersecting roads is less than perpendicular) problematic for older drivers.

Driver fatality rates, 1996. Driver fatality rates, 1996 (National Highway Traffic Safety Administration).

Fatal crashes by age and sex.

Driver involvement rates in fatal crashes by age and sex, 1997 (National Highway Traffic Safety Administration).

Sensory and perceptual changes affect all senses, but visual changes are the most critical to driving, which is considered to be a predominantly visual task because up to 90 percent of the information required is obtained visually.4 Many visual functions -- including static and dynamic visual acuity, contrast sensitivity, and glare sensitivity -- deteriorate with age.

A decrease in visual acuity can cause difficulty in reading signs. The current standard for sign legibility -- 50 feet (15.24 meters) of legibility distance for each inch (2.54 centimeter) of letter height -- presumes visual acuity of 20/25. This is not only higher than the visual acuity standard for licensing in most states, which is usually 20/40, but exceeds the visual capabilities of 40 percent of drivers older than 65. Reductions in contrast sensitivity start at around 40 years of age.

Reduced contrast sensitivity can affect the detection of pedestrians in low-light situations and the detection of worn lane lines. Glare sensitivity makes driving at night or entering and exiting tunnels very difficult, and this condition is exacerbated by cataracts.

Cognitive changes (attention and reaction time) are perhaps the most varied with age. Working memory, selective attention, and processing speed are most often affected in normative aging. Deteriorating working-memory functions make it difficult to process information if the density of signs is excessive. Memory issues also make phased variable message panels harder to interpret. Selective attention problems make it difficult for older drivers to pick out the most critical information when they are confronted with a wide array of signs. Processing speed affects perception-reaction time, particularly in situations where the response requires choosing between alternatives (i.e., brake or steer).

Description of the Problem

Personal vehicles remain the transportation mode of choice for almost all Americans, including older people. As shown in figures 2 and 3, accident rates on a per capita basis go down with age, but the fatality rates on a per-mile-driven basis are highest for the youngest and oldest drivers. In fact, adjusted for mileage, drivers older than 85 have nine times the fatality rate of drivers aged 25 to 69.5 Although people limit their actual miles driven as they age and even though those miles are often logged at the safest times -- in daylight during non-rush-hour traffic -- their rate of accidents per mile is extremely high.6

FHWA's Older Driver Program

In 1989, the Federal Highway Administration (FHWA) enacted a High-Priority National Program Area entitled, "Improved Highway Travel for an Aging Population." This ambitious human factors research program was designed to analyze highway design standards to determine if they accommodate the needs and capabilities of older drivers. Ensuring that design standards meet the needs of all segments of the driving population will reduce the number of accidents for all drivers and allow older persons to remain mobile throughout their life spans.

"Human factors" uses information about human characteristics and behavior to optimize human performance within a system. In this case, human factors research is used to assess age-related declines in abilities that are critical to the driving task and to develop and test highway engineering enhancements that improve highway safety for everyone, especially older drivers.

The FHWA program focuses on developing a clear understanding of older driver needs and capabilities, analyzing current highway design standards, and identifying and implementing practical solutions through the development of revised guidelines and standards for traffic control devices, geometric design, and traffic operations. The research phase of the program was initially built on the recommendations in the Transportation Research Board's Transportation in an Aging Society. Older and young/middle-aged drivers were included in all the FHWA studies, and the results showed that changes that help older drivers also make the system easier for all drivers.

The program is now in an implementation phase in which the findings of the research are being turned into products.

At-Grade Intersections (54) Interchanges (13) Roadway Curvature and Passing Zones (12) Construction/Work zones (17)

Intersecting angle

Receiving lane width

Channelization

Intersection sight distance

Left-turn lane geometry, signing and delineation

Edge treatments/delineation of curbs, medians, and obstacles

Curb radius

Traffic control for left-turn movements

Traffic Control for right-turn/right-turn-on-red movements

Street-name signage

One-way/wrong-way signage

Stop-and yield-controlled intersection signage

Devices for lane assignment

Traffic signal performance

Fixed Lighting installations

Pedestrian control

Exit signing and exit ramp gore delineation

Acceleration/deceleration lane design

Fixed lighting installations

Traffic control devices for prohibited movements on ramps Pavement markings and delineation on horizontal curves

Pavement width on horizontal curves

Crest vertical curve length and advance signing for sight-restricted locations

Passing zones on two-lane highways. Advance signing for lane closures

Variable message signing practices

Channelization practices

Delineation of crossovers

Temporary pavement markings

Products and Implementation

Older Driver Highway Design Handbook

The Older Driver Highway Design Handbook (Publication No. FHWA-RD-97-135) summarizes the results and findings of the FHWA older driver program. It is designed to provide guidance to highway designers and traffic engineering professionals in accommodating the needs of older drivers while enhancing the safety of all road-users. The handbook supplements the standards and guidelines in existing highway design manuals -- including state design manuals, the American Association of State Highway and Transportation Officials (AASHTO) "Green Book," and the Manual on Uniform Traffic Control Devices (MUTCD) -- but does not constitute a new standard of required practice. The handbook provides recommendations for geometrics, signing, and pavement markings in four major areas of highway design and traffic engineering:

  • At-grade intersections.
  • Interchanges.
  • Roadway curvature and passing zones.
  • Construction/work zones.

A more detailed description of the recommendation categories found in each of the four major areas is presented in table 1. Numbers shown in parentheses in table 1 indicate the number of recommendations in that major area.

Normal contrast sensitivity. Vision impaired by cataract condition.
Older drivers who have diminished visual acuity and contrast sensitivity have difficulty, particularly in low-light conditions, detecting and reading traffic signs, reading variable message signs, and detecting pavement markings and downstream channelization devices. This is a road sign as seen by a driver with normal contrast sensitivity.
This is the view of a driver with a cataract condition.

For ease of use, the recommendations and the rationale/supporting evidence for the recommendations have been separated and make up the two major sections of the handbook. This separation allows practitioners to examine the recommendations without having to wade through detailed background information, but the handbook still provides that information for more in-depth reading and reference.

The recommendations are also available in a shorter version, Older Driver Highway Design Handbook: Recommendations and Guidelines (Publication No. FHWA-RD-99-045). This compact version of the handbook contains all of the recommendations without the supporting evidence. It is designed for ease of use by the highway designer. Ideally, each member of the design team would have their own compact version for day-to-day use, and the team would keep a copy of the full version for reference when more information is desired.

The target audiences for both the full and compact versions of the handbook are practicing highway design engineers; traffic engineers; and highway safety specialists in state, county, and city highway agencies or in private consulting firms. Based on the organizational affiliations of people who have requested copies of the handbook, it is also of interest to research organizations, universities, accident investigators, and associations that represent the interests of aging and retired people.

Older Driver Highway Design Workshops

While the development of the Older Driver Highway Design Handbook is a significant accomplishment, FHWA understands the need to do more than merely deliver a publication. It is necessary to encourage the implementation of the recommendations to improve the safety of the highway system.

Signal timing should be set to accommodate older pedestrians.
To accommodate the shorter, stride and slower gait of less capable (15th percentile) older pedestrians and their exaggerated "start-up" time before leaving the curb, pedestrian control signal timing should be based on an assumed walking speed of 0.85 meters per second (2.8 feet per second).

To bridge the gap from delivery of findings to implementation and awareness, FHWA has developed a workshop, as a companion for the Older Driver Highway Design Handbook, to be presented to FHWA division offices, state departments of transportation (DOTs), and local officials. The workshops are designed for state and local design and traffic engineers and for the staffs of FHWA division offices and resource centers to introduce them to the changes that occur with aging and to familiarize them with the recommendations contained in the handbook. The workshops are part of a continuing effort by FHWA to improve the safety and mobility of older drivers. They represent another mechanism to deliver the results of the extensive research that has been conducted in the area of older drivers to those who can put the findings to good use.

The workshop draws on the expertise of instructors whose disciplines include human factors research as well as highway engineering and employs methods that literally let highway designers and traffic engineers see through the eyes of the older driver. They are conducted by two-person teams of instructors consisting of a research psychologist with in-depth knowledge of older driver needs and capabilities and an engineer familiar with day-to-day issues faced by state DOTs and local agencies.

Participants play a critical role in the workshop by sharing their expertise and knowledge with the group. This format and approach provide opportunities for design and traffic engineers to discuss issues with instructors and to exchange ideas with other participants who know about designing and operating freeways and local systems. The workshop is not just a review of technical design requirements; it provides a quick study in demographic changes and the needs and capabilities of older drivers. The workshop creates a compelling argument for explicit consideration of older driver needs in designing new facilities or reconstructing existing ones.

The workshop includes three technical modules: age-related diminished capabilities, handbook organization and content, and case studies. Each module builds upon the information in previous modules. The first module, age-related diminished capabilities, reviews the changes that occur with age and discusses generically how current highway design standards should be modified to meet these changes. The second module, handbook organization and content, reviews in detail the specific recommendations for meeting older road-user needs that are included in the handbook. The final module provides an opportunity for participants to apply the guidelines in a series of case studies. Time is provided in this last module to discuss local conditions and problems.

To assist drivers, a street-name sign should accompany every intersection warning sign.
A street-name sign should accompany every intersection warning sign to assist drivers, particularly diminished-capacity drivers, as they approach and travel intersections.

The module on age-related diminished capabilities familiarizes participants with the problems faced by older drivers, such as diminished visual, cognitive, and physical capabilities, and how these diminished capabilities impact the safety of older road users. Visual demonstrations of changes that occur with aging provide participants with a sense of how some older drivers see the world. Other props and exercises are included to help participants understand the effects of age-related diminished capabilities on driving-related skills. By allowing traffic engineers and designers to "walk a mile" in the shoes of older drivers, they can better appreciate the need for changes to the system.

The module on handbook organization and content demonstrates solutions in the form of specific highway design changes. The module presents in depth each of the recommendations made in the handbook and examples of where they have been used. This module is an important interactive part of the workshop and allows participants to relate the recommendations to real-life operations, design, and maintenance issues.

The case study module provides participants with the opportunity to examine a roadway design problem, identify areas where older drivers would be affected, and apply the concepts and specific recommendations of the Older Driver Highway Design Handbook in developing solutions. When feasible, one case study focuses on a local problem specific to the area where the workshop is being held. Participants leave the workshop with an increased awareness of the issues faced by older road users and of the need to address these issues and with a powerful tool -- the handbook -- to assist them in their day-to-day duties.

Ongoing Activities and Summary

The research portion of the FHWA older driver program is ongoing. One impact of the program has been to ensure that all human factors research at FHWA, in both highway safety and intelligent transportation systems, includes older road users as subjects. This allows for continual updating of the knowledge base for meeting older driver needs. A follow-on project, which will update the existing handbook, is also underway. This update will incorporate research findings that were published after the literature review for the handbook was completed and recommendations learned from the workshops. It may also include new sections.

The extended version of the Older Driver Highway Design Handbook with the supporting evidence and rationale is being prepared for the Internet. This will allow a broader range of people to access this information and will facilitate the possible production of a CD-ROM version of the handbook.

In the spirit of intermodal cooperation, FHWA is participating with the National Highway Traffic Safety Administration in a series of forums on "Aging Drivers: Safe Mobility for Life." These multidisciplinary meetings are being held across the country and will inform local transportation professionals about older person transportation issues and what can be done to resolve them.

The FHWA older driver program is reaching out to practicing traffic engineers and highway designers to provide them with the knowledge of the wide variety of countermeasures that can be implemented to better meet the needs of older road users. While the guidelines are specifically geared toward improving the safety and mobility of older drivers, the benefits of these changes will be experienced by the entire driving public.

References

1. Transportation in an Aging Society, Transportation Research Board, National Research Council, Washington, D.C., 1988.

2. B. Neugarten. "The Future and the Young-Old," The Gerontologist, Vol. 15, 1975.

3. A. Spence. The Biology of Aging, Prentice-Hall Publishing Co., Englewood, N.J.,1989.

4. R. Dewar. "Driver and Pedestrian Characteristics," Institute of Transportation and Traffic Engineering Handbook, Prentice-Hall Publishing Co., Englewood, N.J., 1992.

5. Traffic Safety Facts 1997: Older Population, National Highway Traffic Safety Administration, U.S. Department of Transportation, Washington, D.C.

6. T. Planek and R. Overland. "How Aging Affects the Driver," Traffic Safety, Vol. 73, No. 2, 1973.

7. M. Mattson. Atlas of the 1990 Census, MacMillan Publishing Co., New York, N.Y., 1992.

Elizabeth Alicandri is an engineering research psychologist and the manager of the Human Factors Laboratory in the Traffic and Driver Information Systems Division of FHWA's Office of Safety and Traffic Operations Research and Development at the Turner-Fairbank Highway Research Center. She has participated in human factors highway safety research since 1984. She has a bachelor's degree in psychology from Georgetown University and is currently a master's degree candidate in traffic engineering at the University of Maryland.

Mark Robinson is a traffic engineer with Science Applications International Corporation. He works as an on-site contractor at the Turner Fairbank Highway Research Center. He has a bachelor's degree and master's degree in civil engineering from Queen's University and is registered as a professional engineer in Ontario.

Tim Penney is a research highway engineer at the Turner Fairbank Highway Research Center. He has also had field assignments in the Texas, Colorado and California divisions of FHWA. He received a bachelor's degree in civil engineering from Oklahoma State University and is a registered professional engineer.

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