A review of driving risks and impairments associated with attention-deficit/hyperactivity disorder and the effects of stimulant medication on driving performance

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Abstract

Introduction

Attention-Deficit/Hyperactivity Disorder (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, as well as 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.

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 (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). There were 42,643 driving fatalities in the United States in 2003 alone. Three categories of factors contribute to automobile accidents: human factors, 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 (American Psychiatric Association [APA], 1994, Barkley, 2005a). It affects 8% to 10% of children (American Academy of Pediatrics [AAP], 2000) and 4% to 5% of adults (Briggs-Gowan et al., 2000, Murphy and Barkley, 1996a). ADHD is relatively persistent, with clinically important symptoms continuing into adolescence (Barkley et al., 1990, Weiss and Hechtman, 1993) and early adulthood (Barkley, Fischer, Smallish, & Fletcher, 2002) 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, community functioning, and educational success (Barkley, 2002). Children who have been diagnosed with ADHD also demonstrate difficulties in work performance, social functioning, and substance dependence and abuse during early adulthood (Barkley et al., 1990, Mannuzza et al., 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 three hierarchically organized levels of competency: operational, tactical, and strategic (Fig. 1; Barkley, 2004). During driving, higher levels of competency can harness lower levels for the achievement of larger goals (Michon, 1979, van Zomeren et al., 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 three 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 coordinate the 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, visual-motor 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 (e.g., traffic conditions, density, and climate) that pertain to the excursion. A disorder that affects driving skills at any of these three 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 two to four times more likely to have traffic accidents (Barkley et al., 1993, Barkley et al., 1996, Cox et al., 2000), three times as likely to have injuries (Barkley et al., 1996), four times as likely to be at fault (Barkley et al., 1993), and six to eight times more likely to have their license suspended (Barkley et al., 1993, Barkley et al., 1996). Other conditions that often occur in conjunction with ADHD are associated with additional driving risks, including excessive anger, aggression, and risk taking (Boyd and Huffman, 1984, Chliaoutakis et al., 2002, Deffenbacher et al., 2003, Jonah et al., 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 et al., 1986, Finn and Bragg, 1986, Shope et al., 2003, Shope et al., 2001).

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.

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 one 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, Hechtman, Perlman, Hopkins, & Wener, 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 and Levin, 2005) and may increase crash risk (Malta, Blanchard, & Freidenberg, 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 et al., 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 (e.g., 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 [bottom 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 U.S. 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, Barkley, Smallish, & Fletcher, 2007). 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, Barkley, Smallish, & Fletcher, 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 (e.g., 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 et al., 2002). 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, Fergusson, & Horwood, 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.

In a comparable study that was also conducted in New Zealand (the Dunedin Multidisciplinary Health and Development Study), symptoms of ADHD and other comorbid disorders were assessed at age 15, and their influence on driving offenses was evaluated when the adolescents were between 15 and 18 years of age (Nada-Raja et al., 1997). Attention deficits were associated with increased risks for crashes involving injury, driving without a license, and traffic violations. Associations persisted after conduct problems, driving experience, and sex of subjects were controlled. Notably, CDs were associated with driving risks beyond those associated with the symptoms of ADHD. In addition, ADHD in female subjects was significantly associated with driving offenses and more traffic crashes compared with females with other disorders (CD and anxiety/depressive disorder) or no disorder (P = 0.05).

Both community-based studies present findings that are similar to those derived from follow-up studies of children who were referred to a clinic. Risks for later driving offenses and crashes were increased among children with more severe attention deficits and ADHD symptoms. It is important to note that none of the longitudinal studies confirmed that ADHD was still present at the time of driving assessments, thereby weakening the potential association of ADHD with adverse driving outcomes.

One must take methodology into account when reviewing the results of the longitudinal studies. Four of the studies have data based on self-report. It is possible that individuals with ADHD are more likely to disclose impairments in driving compared with healthy, same-aged controls. In fact, however, those children growing up with ADHD tend to underreport their symptoms (Barkley, Fischer et al., 2002), which may extend to their underreporting of their driving problems as well. This may have affected the self-reported results in the longitudinal studies. Moreover, clinic-referred adults with ADHD have actually been found to overestimate their driving performance relative to a control group (Knouse, Bagwell, Barkley, & Murphy, 2005). This may actually reduce the likelihood of finding group differences. Such limitations in self-awareness suggest that our findings may be conservative estimates of driving problems on self-reported measures of driving. The study by Nada-Raja and colleagues (Nada-Raja et al., 1997) indicated that the data are from official driving records for offences only. The data on crashes are from self-report, and those data indicate no differences for males but an increase for females. Two studies have data based on driving records, but there are limitations regarding the lack of use of comparison groups in these studies (Lambert, 1995, Woodward et al., 2000). Also, driving exposure may be a confounding variable. It may be that individuals with ADHD have greater driving exposure compared with their same-aged peers, and such an increase in driving exposure may account for the reported increases in crashes rather than ADHD itself. In the Milwaukee longitudinal study (Fischer et al., 2007), the hyperactive group had a shorter duration of actual licensed driving (in years) than did the control group despite being older at follow-up than the control group. This would have reduced any differences in driving histories that may have been associated with the hyperactive group. The authors also correlated age and years of licensed driving experience with all outcomes and, where significantly correlated, used that demographic factor as a covariate in their analyses. It is therefore unlikely that differences in driving exposure account for the group differences.

Section snippets

Clinically referred adults diagnosed with ADHD

Data from studies of adults referred to ADHD clinics clearly show that those with ADHD are at increased risk for adverse driving outcomes. Unlike the longitudinal studies, these studies included individuals who demonstrate clinically significant symptoms of ADHD and all received a diagnosis of the disorder. Adverse driving outcomes, such as a greater number of traffic citations, repeated vehicular crashes, more frequent severe crashes, and greater likelihood of license suspension/revocation,

ADHD and driving-related anger, hostility, and aggression

Recent studies have documented a significant relationship between driver emotional status, aggressive driving, and risks for adverse driving outcomes, such as citations and crashes (Chliaoutakis et al., 2002, Dahlen et al., 2005, Porter and Berry, 2001). Irritability (frustration, anger, and aggression) while driving is among the most important predictors of car crashes, apart from the age of the driver (younger drivers have higher crash rates). Many researchers have reported findings that

Effects of alcohol on driving in adults with ADHD

One study examined whether alcohol consumption produces a differentially greater impairment in driving among adults with ADHD in comparison to a community control group (Barkley, 2006). The study compared 56 adults with ADHD (mean age 33 years) with 46 control adults (mean age 29 years) on various adverse events in their driving histories, essentially replicating earlier findings of greater driving risks. It then evaluated the effects of two single, acute doses of alcohol (0.04 and 0.08 blood

Effects of stimulant medication on driving performance

Few reports have examined potential treatments for the effects of ADHD on driving performance. Jerome and Segal reviewed the charts of 1,100 adult patients with ADHD (aged 16 to 52 years) who were prescribed either methylphenidate (MPH) up to 60 mg/day or sustained-release dextroamphetamine sulfate capsules (Dexedrine® Spansule®) up to 40 mg/day (Jerome & Segal, 2001). Patients' self-reported and spouse-reported driving performance (e.g., speeding, wandering attention and distractibility, and

Implications for treatment

In view of the possible pervasive adversities at all levels of driving performance (operational, tactical, and strategic) and the negative driving outcomes that have been demonstrated in adolescents and adults with ADHD, patients with the disorder may be at greater risk for negative driving outcomes. Using three factors (errors, lapses, and violations), Reimer and colleagues ran regression analyses to explore the impact of ADHD status, gender, and age on error, lapse, and violation report

Russell A. Barkley, Ph.D. is a Research Professor of Psychiatry at SUNY Upstate Medical University (Syracuse, NY) and a Clinical Professor of Psychiatry at the Medical University of South Carolina (Charleston), who has authored or edited 20 books and published more than 200 scientific papers and book chapters on ADHD and related topics.

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    Russell A. Barkley, Ph.D. is a Research Professor of Psychiatry at SUNY Upstate Medical University (Syracuse, NY) and a Clinical Professor of Psychiatry at the Medical University of South Carolina (Charleston), who has authored or edited 20 books and published more than 200 scientific papers and book chapters on ADHD and related topics.

    Daniel Cox, Ph.D. is a Professor of Psychiatric Medicine at the University of Virginia Health System (Charlottesville) who has published numerous studies on driving risks associated with various medical conditions and especially those related to ADHD and its medical treatments.

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