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Publication Number: FHWA-RD-97-135
Date: January 1998
Older Driver Highway Design Handbook
The proportion of the driving population over 65 is growing significantly. Older motorists can be expected to have problems driving given the known changes in their perceptual, cognitive, and psychomotor performances, presenting many challenges to transportation engineers, who must ensure system safety while increasing operational efficiency.
This Older Driver Highway Design Handbook provides practitioners with a practical information source that links older road user characteristics to highway design, operational and traffic engineering recommendations by addressing specific roadway features. This handbook supplements existing standards and guidelines in the areas of highway geometry, operations, and traffic control devices.
The information in this handbook should be of interest to highway designers, traffic engineers, and highway safety specialists involved in the design and operation of highway facilities. In addition, this handbook will be of interest to researchers concerned with issues of older road user safety and mobility.
Copies of this report can be obtained through the FHWA Research and Technology Report Center, 9701 Philadelphia Court, Unit Q, Lanham, Maryland 20706, telephone: (301) 577-0818, fax: (301) 577-1421, or the National Technical Information Service, 5285 Port Royal Road, Springfield, Virginia 22161, telephone: (703) 487-4650,
fax: (703) 321-8547.
A. George Ostensen
Director Office of Safety
and Traffic Operations Research
This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof. This report does not constitute a standard, specification, or regulation.
The United States Government does not endorse products or manufacturers. Trade or manufacturers' names appear herein only because they are considered essential to the object of this document.
At the outset of Handbook development, it was recognized that many traffic engineers have already identified for themselves one or more priority issues where age-related declines in driver performance capability define the need for modifications or enhancements of current practice. This Handbook was therefore built upon the results of a user requirements analysis, in which 94 practitioners from five national committees provided detailed feedback indicating how the highway design and engineering community could most effectively use older driver data in design, operational, and safety decisions. A two-stage review process incorporating a lengthy and detailed survey was undertaken to yield consensus regarding the most useful contents and format for the Handbook. Participating committees in the user requirements analysis included the American Association of State and Highway Transportation Officials (AASHTO) Subcommittee on Design; the National Committee on Uniform Traffic Control Devices; the AASHTO Standing Committee on Highway Traffic Safety; the Transportation Research Board (TRB) Committee on Geometric Design (A2A02); and the TRB Committee on the Operational Effects of Geometrics (A3A08). The conscientious response by the practitioners contacted through these committees is gratefully acknowledged.
As Handbook development proceeded, a more rigorous requirement for review and criticism of draft recommendations and supporting materials was defined. Specifically, a need was identified to determine the utility of the Handbook for its intended users—practicing engineers at the State and local levels. The critical review by a panel of individuals who are presently engaged in engineering practice or have recent past experience as practicing engineers was solicited, with the active support of three key committee chairmen: Mr. Thomas Warne, AASHTO Subcommittee on Design; Mr. Richard Weaver, AASHTO Subcommittee on Traffic Engineering; and Mr. Ken Kobetsky, National Committee on Uniform Traffic Control Devices.
Members of the review panel, which represented to a roughly equal degree the design and operational sides of the highway engineering community, were asked to: (1) apply draft recommendations for one or more design elements from the Handbook in case studies involving real-world engineering problems where older driver performance has been (or could be) an important variable; (2) provide structured responses using rating scales to identify needed changes in the information presented in the Handbook; and (3) provide open-ended responses and edits of Handbook material as deemed necessary to improve its accuracy, accessibility, or presentation. A frank discussion of the relevance of each recommendation reviewed by panel members was requested, in the sense of whether it contributed to an improved solution to the problem under study and would be consulted freely for applications apart from this research, or whether the practitioner deemed it irrelevant or confusing and would not be likely to consult this reference in the future. It is only as a result of the thoughtful responses of the individuals listed on the following page that revision of the earlier draft into a completed document was accomplished.
Special thanks are extended to the practitioners who provided critical review of Handbook materials. The following persons provided thoughtful comments—expressing both agreement and disagreement—on a draft version of this document:
The increasing numbers and percentages of older drivers using the Nation's highways in the decades ahead will pose many challenges to transportation engineers, who must ensure system safety while increasing operational efficiency. The 65 and older age group, which numbered 33.5 million in the United States in 1995, will grow to more than 36 million by 2005 and will exceed 50 million by 2020, accounting for roughly one-fifth of the population of driving age in this country. In effect, if design is controlled by even 85th percentile performance requirements, the "design driver" of the early 21st century will be an individual over the age of 65.
There are important consequences of the changing demographics in our driving population. Traffic volumes will increase, problems with congestion will become more widespread, and the demands on drivers will grow significantly beyond present-day operating conditions. At the same time, a steadily increasing proportion of drivers will experience declining vision; slowed decisionmaking and reaction times; exaggerated difficulty in dividing attention between rapidly shifting sources of potential conflicts and other traffic information; and reductions in strength, flexibility, and overall fitness.
A premise for development of the Older Driver Highway Design Handbook is that practitioners, while generally aware of the current number and projected increases in the number of older drivers, do not presently have access to any practical information source linking the characteristics of these highway users to design, operational, and traffic engineering recommendations keyed to specific roadway features. This Handbook has accordingly been developed to supplement existing standards and guidelines in the areas of highway geometry, operations, and traffic control devices.
The specific roadway features singled out for attention in this Handbook represent four broad site types identified either directly or indirectly in recent accident analyses as most problematic for older drivers. A top priority is at-grade intersections, reflecting older drivers' most serious accident problem area as documented in recent analyses (Council and Zegeer, 1992; Staplin and Lyles, 1991; Stamatiadis, Taylor, and McKelvey, 1991). Next, older driver difficulties with merging/weaving and lane changing operations focus attention on interchanges (grade separation). Finally, roadway curvature and passing zones plus highway construction/work zones are included for two reasons: (1) heightened tracking (steering) demands may increase the driver's workload, and (2) there is an increased potential for unexpected events requiring a swift driver response.
These classes of highway features define the primary organizing principle for the main body of the Handbook. Recommendations are presented initially in a brief section, followed by a more lengthy section presenting the Rationale and Supporting Evidence. Within each of these two major Handbook sections, material is organized in terms of four subsections, corresponding to the classes of highway features noted above. Then, for each class of highway feature, Handbook materials are organized according to a unique set of geometric, operational, and traffic control design elements. The Handbook concludes with an integrated glossary providing definitions of selected terms, including acronyms and abbreviations; a reference list; and an index containing terms that provide reliable guidance to help locate Handbook entries pertaining to a particular design element.
The recommendations in this Handbook are based on supporting evidence drawn from a selected set of research findings. The results of field studies employing older drivers were always given precedence, followed by laboratory simulations or modeling efforts where both age and some aspect of highway design, operations, or traffic control were included as study variables. More general findings on the effects of aging, independent of driver performance research per se, may also be cited, but only where there is an indisputable logic extending a given finding to the highway context. A broader discussion of issues related to aging and driving can be found in the Transportation Research Board's Special Report 218 (1988).
It is important to emphasize that Handbook recommendations, as well as the evidence cited to support them, relate to demonstrated performance deficits of normally aging drivers. Thus, diminished driver capabilities that result from the onset of Alzheimer's disease and related dementias, which are believed to afflict over 10 percent of those age 65 and older and nearly 50 percent of those age 85 and older, are not the current focus.
Finally, the recommendations presented in this Handbook do not constitute a new standard of required practice for the included highway design elements. Questions related to when and where to apply each Handbook recommendation remain at the discretion of the practitioner. This document may be useful as a "problem solver" at older driver accident sites, or it may be applied preemptively to enhance safety wherever there are large numbers of older drivers in the traffic stream in a given jurisdiction. As a practical matter, it is recognized that the application of Handbook recommendations may be limited to the design of new facilities and to planned highway reconstruction projects. Furthermore, the recommendations contained herein seek to avoid "optimum" solutions that may be unattainable using current materials or practices or that will result in situations where extreme costs are incurred for small anticipated gains in system safety. Ultimately, the contents of this Handbook are intended to provide guidance which—based on the current state-of-the-knowledge of the special needs of normally aging seniors—can be expected to significantly enhance the safety and ease of use of the highway system for older drivers in particular, and for the driving population as a whole.
Loren Staplin, Ph.D.
Kathy H. Lococo
Stanley R. Byington
AADT annual average daily trafficback to top
AASHTO American Association of State and Highway Transportation Officialsback to top
ATSSA American Traffic Safety Services Associationback to top
CIE commission Internationale de l'Eclairageback to top
CIL complete interchange lightingback to top
CSSB concrete safety shaped barrierback to top
DSD decision sight distanceback to top
FHWA Federal Highway Administrationback to top
fL footlambertback to top
IIHS Insurance Institute for Highway Safetyback to top
ISD intersection sight distance back to top
ISBL in-service brightness levelback to top
ISTEA Intermodal Surface Transportation Efficiency Actback to top
ITE Institute of Transportation Engineersback to top
MRVD minimum required visibility distanceback to top
MSC merge steering controlback to top
MUTCD Manual on Uniform Traffic Control Devices for Streets and Highways
NHTSA National Highway Traffic Safety Administrationback to top
NTOR no turn on redback to top
NTSB National Transportation Safety Boardback to top
PMD post-mounted delineatorback to top
PRT perception-reaction timeback to top
RPM raised pavement markersback to top
RTOR right turn on redback to top
SCL speed-change laneback to top
SSD stopping sight distanceback to top
STV small target visibilityback to top
TRB Transportation Research Boardback to top
TVA transient visual adaptation back to top
VMS variable message sign back to top
Topics: research, safety, human factors, older road user, design
Keywords: research, safety, highway design, highway operations, driver age, driver performance, human factors, vision, attention, perception, cognition, memory, physical ability, risk perception, hazard perception
TRT Terms: Roads--United States--Design and construction, Older automobile drivers, Roads--United States--Visibility, Roads--United States--Safety measures, Traffic signs and signals--United States, Highway safety, Highway design, Aged drivers, Human factors