A Review of Driving Risks and Impairments Associated with Attention-Deficit/Hyperactivity Disorder and the Effects of Stimulant Medication on Driving Performance
Russell A. Barkley, PhD; Daniel Cox, PhD
Contact Author:
Russell A. Barkley, PhD
Research Professor of Psychiatry
SUNYUpstateMedicalSchool(Syracuse, NY)
And
Clinical Professor of Psychiatry
MedicalUniversity of South Carolina (Charleston)
Mail to:
1752 Greenspoint Court
Mt. Pleasant, SC 29466
Phone: (843) 670-6127
Fax: (843)971-8323
Email:
Daniel Cox, PhD
Professor
Department of Psychiatric Medicine
University of Virginia Health System
Behavioral MedicineCenter
P.O. Box 800223
Charlottesville, VA 22908
Phone: (434)924-5314
Fax: (434)924-0185
Email:
ABSTRACT
Introduction:ADHD may interfere with driving competence, predisposing those with the disorder to impaired driving performance and greater risk for adverse driving outcomes. Effective treatment may minimize the risk in those with ADHD.
Method:We reviewed the scientific literature on driving risks and impairments associated with ADHD and the effects of stimulants on driving performance.Several lines of evidence were considered, including longitudinal studies and community-derived sample studies. The present review is based on a weekly review by the first author of all journals in the behavioral and social sciences indexed in the publication, Current Contents, spanning the past 15 years and a search of the reference section of all studies found that pertained to driving risks associated with ADHD or to the treatment of ADHD as it relates to driving difficulties.
Results: The review of the scientific literature demonstrated well-documented driving risks and impairments associated with ADHD and the positive effects of stimulant medications on driving performance.
Conclusions:Clinicians should educate patients/caregivers about the increased risk of adverse outcomes among untreated individuals with ADHD and the role of medication in potentially improving driving performance.
Impact on Industry: Owing to the significantly higher risk of adverse driving outcomes, the use of stimulant medications to treat people with ADHD who drive may reduce such safety risks.
KEYWORDS: Attention-deficit/hyperactivity disorder, ADHD, driving, methylphenidate, accident risk
INTRODUCTION
Since the 1970s, the number of deaths due to automobile accidents has been reduced, but in recent years, according to the United States General Accounting Office (United States General Accounting Office, 2003), the decline in fatalities has leveled off.Although the number of fatalities has remained steady, younger drivers (16 to 20 years of age) and older drivers (older than 75 years of age) are involved in a greater number of automobile collisions, especially fatal crashes, than other age groups. (United States General Accounting Office, 2003) There were 42,643 driving fatalities in the United States in 2003 alone. Three categories of factors contribute to automobile accidents: humanfactors, vehicle-related factors, and environmental factors. Human factors are considered to be the most common cause of automobile accidents and include the actions taken by or the condition of the driver, such as speeding, violating traffic laws, drug or alcohol use, errors in decision making, age, and inattention.
Attention-deficit/hyperactivity disorder (ADHD) is a common developmental disorder that comprises symptoms involving poor sustained attention or persistence, distractibility, impaired impulse control, and hyperactivity.(APA, 1994; Barkley,2005a) It affects 8% to 10% of children (AAP, 2000) and 4% to 5% of adults.(Briggs-Gowan,2000; Murphy, 1996a) ADHD is relatively persistent, with clinically important symptoms continuing into adolescence (Barkley, 1990; Weiss, 1993) and early adulthood (Barkley, 2002a) in up to 80% and 66% of diagnosed cases, respectively. ADHD is associated with impairments and adverse outcomes across the life span, having a substantial impact on a variety of domains of adaptive functioning, including family life, social relationships, communityfunctioning, and educational success.(Barkley, 2002b) Children who have been diagnosed with ADHD also demonstrate difficulties in work performance, social functioning, and substance dependence and abuse during early adulthood. (Barkley, 1990; Mannuzza, 1993)
Driving is an activity that contributes substantially to self-sufficiency among adolescents and adults. It provides a means of engagement in most domains of adaptive functioning, including employment, family care, educational pursuits, social engagements, shopping, and entertainment. These activities would be extremely curtailed if an adult were deprived of this privilege, especially in most areas of the United States. Balanced against the various major life activities that driving facilitates is the greater exposure to harm to one’s self, to others, and to property that accompanies operating a vehicle capable of traveling at high speeds. Although speeding is a contributory factor in approximately 15% of all crashes and 30% of all fatal crashes, (United States General Accounting Office, 2003) automobile crashes and harm can also occur while traveling at low speeds.
Driving is a multidimensional activity that involves at least 3 hierarchically organized levels of competency: operational, tactical, and strategic (Figure 1). (Barkley, 2004) During driving, higher levels of competency can harness lower levels for the achievement of larger goals. (Michon, 1979; van Zomeren, 1987) Deficits in lower levels of the hierarchy may have profound effects on higher levels of competency required for driving; however, deficits at higher levels may have little or no influence on lower level competencies and may be undetected by methods aimed at assessing only those lower levels. Therefore, although this widely used, hierarchical control structure delineates 3 different levels of competency and conceptualizes them as distinct, coordination is often required across the operational, tactical, and strategic levels. For practical applications, models that coordinatethe levels of cognitive processing along with operational control are needed to capture the overall behavior of the driver.
Operational competency (level 1) comprises elementary mental functions, such as attention and concentration, reaction time, visual scanning, spatial perception and orientation, visualmotor integration, speed of cognitive processing, motor coordination, and other basic neuropsychological abilities that are inherent in driving. Tactical competency (level 2) includes those behaviors, skills, and decisions that are associated with driving in traffic, such as adaptation of speed to driving conditions, use of headlights to improve visibility, and decisions about whether to pass other vehicles. Strategic competency (level 3) involves decisions and planning abilities that pertain to the reasons the vehicle is being used at a particular time. It includes the goal(s) for the particular driving session, as well as choices regarding the best route, time of day, and trip sequence (subgoals), along with evaluation of general risks (for example, traffic conditions, density, and climate) that pertain to the excursion. A disorder that affects driving skills at any of these 3 levels would produce secondary adverse effects on the various domains of daily adaptive functioning that driving supports, while possibly causing the driver to subject himself or herself or others to harm.
Inattention, particularly in-vehicle distraction, is among the most common contributors to traffic crashes. (Lam, 2002; United States General Accounting Office, 2003) Any disorder that markedly impairs attention and resistance to distraction would elevate one’s risk for crashes and other adverse driving outcomes. Automobile accidents are more common among those with ADHD, and may be associated with a higher rate of fatality. (National Highway Traffic Safety Administration, 1997) Young drivers with ADHD are 2 to 4 times more likely to have traffic accidents (Barkley, 1993, 1996; Cox, 2000), 3 times as likely to have injuries (Barkley, 1996), 4 times as likely to be at fault (Barkley, 1993), and 6 to 8 times more likely to have their license suspended. (Barkley, 1993, 1996) Other conditions that often occur in conjunction with ADHD are associated with additional driving risks, including excessive anger, aggression, and risk taking (Boyd, 1984; Chliaoutakis, 2002; Deffenbacher, 2003; Jonah, 2001; Sammula, 1987); infrequent use of seat belts; greater use of alcohol and drugs; affiliation with peers who tolerate and support drug use; poor parental monitoring and parental stress; and the presence of persistent behavioral or emotional difficulties. (Brown, 1986; Finn, 1986; Shope, 2001, 2003)
This article reviews the available literature on driving risks that are possibly associated with ADHD.Our intent was not to conduct a meta-analysis of the existing literature as there is an insufficient quantity to do so. Such an analysis would certainly have required a more systematic methodology for identifying studies to be included in the statistical analysis and the criteria for their selection. Nevertheless, our search of the literature was reasonably comprehensive given that it was based on a weekly review by the authors of all journals in the behavioral and social sciences indexed in the weekly publication, Current Contents, spanning the past 15 years. We searched for all studies that pertained to driving risks associated with ADHD or participants with high levels of ADHD symptoms (inattention, impulsivity, and hyperactivity) or to the treatment of ADHD as assessed by improvement in driving problems. The reference sections of these papers were also searched for additional studies. Several lines of evidence were considered. First, we review longitudinal studies of children with ADHD that followed subjects into adulthood and examined driving as an outcome. Second, we consider studies that use community-derived samples in which the symptoms of ADHD were examined for any association with driving problems. Third, we discuss research that has focused on clinically referred adults with ADHD. Fourth, we consider several recent, small-scale investigations of driving-related anger and aggression among patients with ADHD. Last, we review the limited research to date on the effects of stimulant medication on driving performance. Consideration is then given to recommendations for addressing the driving risks that appear to be associated with ADHD.
Longitudinal Studies of Hyperactivity/ADHD in Children
Results of longitudinal studies have demonstrated a relationship between ADHD and adverse outcomes in driving (Table 1). Results of an early study performed in the 1970s suggested that, although most adults are involved in at least 1 car accident, those with a childhood history of hyperactivity are likely to have more frequent and more severe crashes (as measured by dollar damage to vehicles). (Weiss, 1979) However, these findings were based primarily on self-report and were not corroborated by official driving records. Furthermore, the impact of coexisting conditions was not assessed. Other disorders, especially conduct disorder (CD) and associated substance use, are likely to coexist with ADHD (Pliszka, 2003; Wilson, 2005)and may increase crash risk.(Malta, 2005)
A number of driving-related problems were identified in a 3- to 5-year follow-up survey of 36 control adolescents and 35 adolescents with ADHD who were between 16 and 22 years of age and had been involved in an earlier study of family functioning.(Barkley, 1993) The survey included ratings of current symptoms of ADHD, oppositional defiant disorder (ODD), and CD; questions regarding the use of safe driving behavior; and questions concerning adverse driving outcomes (for example, crashes, citations). Parents completed surveys about their children with regard to the period since the adolescent began driving. Apparent driving-related problems included driving without a license, license suspension/revocation, traffic citations, and vehicle crashes. Parents of the ADHD group also rated their adolescents as less likely to employ sound driving habits, with 40% of the ADHD group rating driving skills as deficient (≥1.5 standard deviations [top 7th percentile] below the mean score of the control group on a driving skills rating scale). This survey also found that, although the degree of severity of current ADHD symptoms was significantly associated with driving risks, some risks were further associated with the degree of severity of symptoms of ODD and CD.
In a report to the US Department of Transportation’s National Highway Traffic Safety Administration, the presence of severe ADHD in childhood was associated with significantly greater likelihood of traffic citations in later driving years compared with children without ADHD or only mild symptoms.(Lambert, 1995) Members of the ADHD group were also more likely to repeat the same traffic offenses than were those in the comparison groups. The proportions of each group that experienced a crash did not significantly differ between patients with ADHD and the control group;however, severe ADHD was marginally associated with a greater frequency of crashes and with a significantly greater number of fatal crashes. The impact of comorbid disorders was not evaluated.
The most recent longitudinal study of children with hyperactivity/ADHD, which was undertaken to examine driving as an outcome in young adulthood, was conducted by Fischer and colleagues at the Medical College of Wisconsin (Fischer, 2006). A multimethod, multilevel, multisource battery of driving measures was collected during young adulthood (mean age, 20 years) on a large sample of clinically referred hyperactive children (n=147) and community control children (n=71) who were followed for more than 13 years as part of a larger study. (Fischer, 2005) The control group had a greater duration of licensed driving (in years) than the hyperactive group. The study examined correlations between both age and years of licensed driving and all outcome measures. Where these were significant, that demographic factor was used as a covariate in the analysis of that outcome measure in an effort to control for driving exposure as a confounding variable. A greater percentage of children in the hyperactive group than in the control group reported receiving a ticket for reckless driving, driving without a license, having hit-and-run crashes, or having their licenses suspended or revoked. Official driving records confirmed that a greater proportion of the hyperactive group compared with the control group had received traffic citations, and the mean number of citations per individual was greater in young adults with a history of hyperactivity/ADHD. Both assessments from others and self-report ratings of actual driving behavior revealed that less-safe driving practices were used by the hyperactive group. Observations recorded by driving instructors during behind-the-wheel road tests revealed a significantly greater number of errors in driving resulting from impulsiveness in the hyperactive group than in the control group. Additionally, performance on simulated driving tests demonstrated slower and more variable reaction times, more frequent errors of impulsiveness (for example, false alarms, poor rule following), greater steering variability, and a greater number of scrapes and crashes of the simulated vehicle against road boundaries in the hyperactive group than in the control group. The road test and simulator results began to reveal a possible basis for elevated driving risks in formerly hyperactive/ADHD children—impairment in Level I Operational (elementary cognitive) abilities that are necessary for the safe operation of the vehicle.
Noteworthy in this study was that it did not find a greater frequency of crashes or a greater proportion having such crashes in the hyperactive than in the control group.One reason why the groups may not have differed in this study pertains to the fact that up to 34% of hyperactive or ADHD subjects no longer had significantly elevated symptoms (≥98th percentile) and 54% of subjects no longer met full diagnostic criteria for the clinical diagnosis (DSM-III-R) (Barkley, 2002a).The presence of such a high percentage of individuals no longer affected by or diagnosed with the disorder would weaken differences between this group and any control group in comparison to studies in which all participants in the ADHD group were currently clinically diagnosed with the disorder.Another reason may have had to do with the validity of self-reported symptoms in children with ADHD followed to adulthood. The hyperactive/ADHD participants dramatically underreported their current symptoms relative to the reports given by their parents for those same symptoms. (Barkley, 2002a)This problem of underreporting may also have affected the validity of their self-reported driving behavior or their history of adverse driving outcomes.
It is also important to note that the studies reviewed so far were based on children with hyperactivity or ADHD who were referred to clinics and followed into their early driving years. Therefore, they were subject to referral biases and may not accurately represent the general population of adolescents and young adults with ADHD.
Results of longitudinal studies conducted in community-derived samples support the relationship between ADHD and driving impairment observed in the childhood referral population. Attention deficits were assessed in a large sample of 13-year-old children from New Zealand (the Dunedin longitudinal project; N=1265), and driving outcomes were evaluated during a follow-up study, when the children were 21 years of age.(Woodward, 2000)Data collection included reports of teacher- and parent-rated measures of attentional difficulties; incidents of risky driving behavior, including drinking and driving, traffic violations, and involvement in automobile accidents; and measures of potentially confounding factors, including individual, socio-familial, and driving-related factors.Thirteen-year-old children with high levels of attention deficit were at greater risk as adolescents for traffic offenses and vehicular crashes. After various confounding factors were controlled, the associations between attention deficits and increased risk of injury during an accident, driving without a license, and traffic violations were still present.