Section 1: Introduction
Motor vehicle crashes are a leading cause of death in the United States. Data from the National Center for Health Statistics of the U.S. Department of Health and Human Services reveal that, in 1997, motor vehicle crashes resulted in 42,340 deaths, ranking eighth behind heart disease, cancer, and stroke as a leading cause of death (National Highway Traffic Safety Administration [NHTSA], 2000). The causes of motor vehicle crashes are varied, including road design, vehicle design, and traffic volume. However, it has been estimated that as much as 90 percent of highway crashes are due to human error (Tignor, 2000). Although data on the overall contribution of medical conditions to motor vehicle crashes are unavailable, it is reasonable to assume that medical conditions that affect functional capabilities (e.g., sensory, motor, or cognitive functioning) play a major role.
This report, entitled Medical Conditions and Driving: A Review of the Scientific Literature, provides a comprehensive and integrative review of past and current research (to the year 2000) on the effects of medical conditions on driving performance. The report is divided into 15 sections (Introduction, Vision, Hearing, Cardiovascular Diseases, Cerebrovascular Diseases, Peripheral Vascular Diseases, Diseases of the Nervous System, Respiratory Diseases, Metabolic Diseases, Renal Diseases, Musculoskeletal Disabilities, Psychiatric Diseases, Drugs, The Aging Driver, and The Effects of Anesthesia and Surgery). Each section is divided into subsections, with a brief overview of the condition/illness, information on prevalence, a review of the literature relevant to driving, followed by current fitness-to-drive guidelines from Australia and Canada for the condition/illness. The guidelines from Canada (Canadian Medical Association [CMA], 2000) and Australia (Austroads, 1998) have been reproduced with permission. Sincere appreciation is extended to both the CMA and Austroads for allowing the reproduction of their guidelines.
Medical Conditions that Serve as ‘Red Flags’ for Driving Impairment
A number of medical conditions may result in functional impairments that negatively affect driving performance. The effects can result in functional impairments that are either acute or chronic. The distinction between acute and chronic becomes critical in terms of assessment for fitness-to-drive and for licensing decisions (A. Dobbs, personal communication).
With acute effects (e.g., an epileptic seizure, a hypoglycemic reaction), the event is, most often, sporadic and unpredictable. There is no question that when the event occurs, the individual is not competent to drive. The difficulty, in terms of licensing decisions, is that the occurrence of the event is unpredictable. This means that decisions about the individual’s safety to drive cannot be based on direct measurement. Therefore, decisions about continued driving and/or potential restrictions on driving activities may need to be based on a consensus of estimated risk (e.g., expert panel decisions, calculated relative risk) to the person and society.
Unlike the acute effects of medical conditions, chronic effects are, by definition, more enduring. In addition, unlike acute effects, chronic effects are relatively predictable and stable. Importantly, the impact of chronic effects on an individual’s driving ability is measurable. Thus, decisions about continued driving can be based on measures of individual performance rather than on estimates of risk. The challenge has been to operationalize performance in a manner that is valid, reliable, and defensible.
Acute and Chronic Effects
Some medical conditions can have both acute and chronic effects. For example, diabetes can have an acute effect (hypoglycemic reaction) and chronic effects (diabetic retinopathy, cardiovascular complications, diabetic neuropathy, etc.). Similarly, cardiovascular disease can be associated with acute effects (myocardial infarction) and with chronic effects (hypertrophic cardiomyopathy, congestive heart failure, etc.).
Research designed to increase our understanding of the effects of medical conditions on driving has increased substantially in recent years. Despite the methodological challenges of conducting research in this area, a substantial body of literature now exists on the relationship between many medical conditions and driving performance. In some instances, there is a clear link between the presence of a medical condition and impaired driving performance, allowing health care professionals to make evidence-based decisions. In other instances, the relationship is less clear.
The medical conditions listed below have been found, through research, to be associated with a higher risk of crash and/or have been associated with cognitive impairment and/or significant functional impairments (visual, motor). The list is confined to those conditions that are chronic. Although there are likely to be other medical conditions that have the potential to adversely affect driving performance, those conditions may not appear on the list because of a lack of research into the effects of the condition on driving performance. Importantly, not everyone with an illness listed below would have their driving reduced to an unsafe level. Rather, the presence of one or more of the illnesses should serve as a ‘red flag’ that driving may be compromised, and that evaluation of driving competence is needed for both personal and public safety.
Summaries of current fitness-to-drive guidelines for medical practitioners from Australia (1998) and Canada (2000) are presented throughout this report. While these guidelines are not controlling on licensing authorities or physicians in the United States or endorsed by NHTSA, they are provided for informational and/or reference purposes.
In considering applications for fitness-to-drive guidelines, it should be noted that licensing activities of state and local motor vehicle agencies in the United States must comply with both the Americans with Disabilities Act of 1990 (‘ADA’) and the Rehabilitation Act of 1973 (‘Rehabilitation Act’). Under these statutes and their implementing regulations, public entities may utilize neutral rules and criteria, such as medical guidelines, even if they screen out, or tend to screen out, individuals with specific medical conditions, provided the criteria are necessary for the safe operation of a program. However, the public entity must ensure that its medical standards are based on real risks, not on speculation, stereotypes, or generalizations about individuals with specific medical impairments. Consequently, the ADA and Rehabilitation Act typically require that a State Department of Motor Vehicles base its licensing decisions not on risk analyses, but on individual fitness-to-drive assessments that examine whether an applicant poses a direct threat to public safety, which cannot be eliminated through auxiliary aids or reasonable modifications of policies, practices, or procedures.