There are still some professionals who doubt that ADHD is a “real” disorder, and cite poor parenting practices, for example, as an alternate explanation for the attentional and behavioral problems of ADHD children. However, many, many studies over the last 30 years have consistently shown that both teens and adults with ADHD show difficulties driving. None of those who doubt the diagnosis of ADHD have offered plausible explanations for how parenting practices in young children can affect driving skills 10, 20, or 30 years later. Thus, the data on driving is worth examining carefully.

Studies of ADHD and Driving

Barkley and Cox (2007) review and summarize the research on ADHD and driving:

• The first published study of ADHD and driving appears to be one by Weiss et al. in 1979, which found that adolescents and young adults with ADHD were more likely to be involved in traffic accidents and to do more damage to their cars in these accidents. Barkley found in one study that car repairs after an accident for drivers with ADHD were 2.5 time more expensive than those of drivers without ADHD, and that they were more likely to have their first accident at a younger age as compared to drivers without ADHD.
• Research shows that drivers with ADHD are at a greater risk for traffic accidents; they show four times the risk for an accident compared to drivers without ADHD. Of note, we cannot take this value at face value because it includes the risk associated with ADHD, as well as the risk associated with other risk factors:
  • For example, males are more likely to have accidents, and are more likely to have ADHD. Thus, if we compare drivers with ADHD to a sample of non-ADHD drivers, we would expect the group of drivers with ADHD to have more traffic accidents simply because that group has more male drivers.
  • Other risk factors include age (older drivers are more likely to have accidents when compared to younger drivers) and miles driven per year (those who drive more miles are likely to have more accidents).
• Researchers can statistically adjust risk statistics like this one by removing the risk associated with these other factors. After reducing the risk associated with sex, age, and miles driven per year, ADHD drivers are still at twice the risk for an accident. This would be the portion of their risk that is solely the result of having ADHD.

Seriously? Yes…

• Drivers with ADHD are four times as likely to be judged by others as being at fault for an accident. Studies have also shown that drivers with ADHD themselves report their inattention while driving was the reason in 45% of these cases. Reimer et al. (2005) cite Stuts and Hunter (2003) as estimating that 25-30% of all traffic accidents are attributable to driver inattention, and so this finding would mean that drivers with ADHD are about twice as likely as the average driver to identify themselves as being at fault for an accident.
• Drivers with ADHD are also more likely to be rear-ended in an accidents, and three times more likely to be injured in car crashes. Barkley found that drivers with ADHD were less likely to wear seat belts and more likely to take risks, twice as likely to drive above the speed limit, and more likely allow anger to impact their driving.
• Drivers with ADHD are six to eight times more likely to have their licenses suspended. Barkley found they are more likely to be driving without a license as well, either because their license has been suspended or because they have not yet obtained a driver’s license. In fact, Barkley found that drivers with ADHD were 50% more likely to be driving before obtaining a driver’s license.
• Studies using driving simulators show that drivers with ADHD are more likely to “scrape” or “side swipe” virtual things (such as cars and signs shown in the driving simulator). Barkley notes that driving knowledge explained little of these differences, and drivers with ADHD seem to know as much about driving and traffic laws as drivers without ADHD. Most of the differences between drivers with and without ADHD are explained by attention, motor control while driving, and adherence to driving rules.
• Further, the more severe the symptoms of ADHD, the more likely the driver was to have traffic citations, have repeated citations for the same offense, have traffic accidents, and drive without an active license.

Judgments About Driving

Why would people with ADHD show such driving problems? Part of the answer is driving judgments.

Reimer et al. (2005) used the Driving Behavior Questionnaire, which asks drivers to report their

• driving errors (almost hitting pedestrians or cyclists, for example)
• driving violations (e.g., deliberately violating a traffic law such as passing in the right lane to move past a slower driver)
• driving lapses (brief periods of inattention that caused the driver to miss an exit or turn-off)

They use a sample of 83 drivers, with half meeting criteria for ADHD since childhood, 54% being male, and the average participant’s age being 30-35 years old (they note that most studies use younger drivers, under the age of 30, and so they sought a slightly older sample of drivers for their study).

The researchers found that drivers with ADHD reported more errors and lapses in attention than drivers without ADHD, especially with male drivers and with younger (18-25 years old) drivers. Drivers with ADHD were significantly more likely to report, for example,

• almost rear-ending the car in front of them when they watched for oncoming traffic and did not realize the car in front of them had stopped
• almost hitting cyclists or pedestrians in the crosswalk
• hitting stationary objects while backing-up, and failing to check the rear-view mirror when changing lanes
• chasing a driver who angered them, or passing in the right lane to move past a slower car
• missing exits and turn-offs, and not recalling portions of their driving route immediately after traveling through that portion of their route

Interestingly, they found that older drivers with ADHD reported far fewer violations and errors than did younger drivers with ADHD. This same difference did not appear between older and younger drivers without ADHD; in fact, errors and violations increased as drivers without ADHD got older. Drivers over the age of 40 with ADHD reported the same number of violations as same-age drivers without ADHD.

The authors interpreted this to mean that older drivers with ADHD have made (or, given that the data is self-reported, believe they have made) adjustments to their driving. More specifically, they reported greater adherence to driving laws and better driving judgments.

Drivers with Impairment

Some would note that anyone with a psychiatric diagnosis is more likely to have accidents. Studies show that drivers with psychiatric conditions, including those with anxiety and depression, are as much as six times more likely to have accidents when compared to drivers without a diagnosis. However, drivers with ADHD have the highest risk of all, (Barkley et al., 2005) and this has been repeatedly shown since the 1980’s (Barkley et al., 2005 cite Hechtman, Weiss, Perlman, & Tuck, 1981).

Aduen et al (2014) reviewed driving records of 3000+ adults from six major US cities. Participants were recruited through probability sampling for the Strategic Highway Research Program 2, and driving information was obtained in two ways:

• Participants self-reported their driving record for the year before the study began. This kind of data can be questionable, however, for several reasons:
  • Drivers in general are likely to overestimate their driving skills and attribute accidents or near-accidents to temporary driving conditions (such as poor roads, bad weather, or carelessness of other drivers on the road) and so may not be very reliable (Reimer et al., 2005).
  • In a study where participants are asked about their driving difficulties and accidents, they may not recall all accidents or near-accidents, and further may not report the accidents or near-accidents they do remember. This may be due to impression management and embarrassment, or because the accident was never reported.
• Participants’ cars were fitted with a special monitoring device for one to two years. As a result, this data is more reliable, as it was collected without relying on driver’s memories or judgments, and the devices can record more data (such as average driving speed, stopping distance, etc…) and record it more accurately.

The results were interesting:

• 8.5% of the sample was diagnosed with ADHD (compared to 5% of the general population estimated by the American Psychiatric Association). Drivers with ADHD, of note, were less likely to have high school diploma or college degree, more likely to be in the extreme income groups (making less than $29K a year or more than $150K a year), and less likely to be married. While these findings likely have nothing to do with their driving, all these findings were statistically significant and consistent with what has been found about adults with ADHD in other studies.
• Drivers with ADHD were at a 222% increased risk for multiple traffic violations and single/multiple accidents when compared to the general population; in other words, they showed 100% of the same risk other drivers showed, plus 222% more, meaning they had three times (322%) as many accidents as drivers without ADHD. After correcting for demographic factors (younger age, male sex, lower SES, unmarried, and number of miles driven per year), drivers with ADHD still were at a 127% increased risk; in other words, they showed 100% of the same risk other drivers showed, plus 127% more, meaning they had twice (227%) as many accidents as drivers without ADHD.
• When compared to drivers with depression, drivers with ADHD had almost twice as many instances of multiple violations (187% increased risk), and were twice as likely (112% increased risk) to report an accident was their own fault (there was no correlation between depression and report of being at fault).

Fischer et al. (2007) followed 147 drivers with ADHD and compared them to 71 drivers without ADHD. The ADHD drivers had been diagnosed between the ages of 4 and 12 years old at an outpatient clinic at the Milwaukee Children’s Hospital. The researchers followed them into their teen years (over 70% continued to meet all diagnostic criteria for ADHD into their adolescence). Most (92%) were not taking ADHD medication during the study. The average age of drivers in both groups was 21 years old:

• ADHD drivers reported having more tickets for reckless driving, having more suspensions or revocations of their licenses, and being two to three times more likely to have driven with a suspended or revoked license.
• They also showed less safe driving practices, as found in self-report questionnaires and interviews. They reported more auto crashes, especially hit-and-run accidents, as well as more expensive auto crashes. As noted above, sometimes self-report is less reliable, as people may not judge their own skills or recall accidents accurately. In this study, the researchers gathered two more kinds of data:
  • Family members of the drivers with ADHD were interviewed. They rated the driver’s ADHD symptoms as more severe and reported more unsafe driving practices than did the drivers themselves.
  • Trained driving instructors and actual driving tests were also used in this study. Driving instructors reported that drivers with ADHD showed more unsafe driving practices.

ADHD and Alcohol

Studies of teens drivers with ADHD show they experience the same risks as adult drivers with ADHD. However, two further points are worth noting:

• Barkley et al. (2004) note that adolescents with ADHD report consuming more alcohol per week than their peers, and becoming intoxicated more times in the prior three months than their peers. As a result, adolescents with ADHD are more likely to drive while under the influence of alcohol. Given that alcohol is a disinhibitor and impairs motor control, adolescents with ADHD are more likely to be impaired in their driving by alcohol.
• Teen drivers with ADHD are also at three times the risk for injury in accidents as other drivers. Of greater concern though is that teen drivers with ADHD are more likely to be killed in auto accidents.

In one study (Barkley et al., 2006), researchers interviewed over 100 drivers (just over half did not have ADHD, just under half did have ADHD) about their alcohol use. Consistent with past research, they found that drivers with ADHD reported more accidents and tickets for traffic violations. However, they also found that adults with ADHD reported using more alcohol than their peers without ADHD, and as their alcohol use increased, so did the number of accidents and tickets.

For the study, they divided drivers (average age 29-33 years) in three groups:

• The first group received a high dose serving of alcohol – they received 2 oz of pure grain alcohol in a 16oz juice drink, followed within 15 minutes by 1 oz of pure grain alcohol in an 8 oz juice drink (this condition was chosen as it raised the blood alcohol level of participants to the level required for legal intoxication in their state).
• The second group received a low dose serving of alcohol – they received 2 oz of pure grain alcohol in a 16oz juice drink (this raised their blood alcohol level to half that required for legal intoxication in their state).
• The third group received an alcohol-free 16oz juice drink, which had been lightly sprayed with a mist which included alcohol so that it smelled slightly of alcohol.

After consuming their drink, participants were given an attentional task, and then placed in a driving simulator. The researchers compared drivers with and without ADHD in all three conditions:

• They found that drivers with ADHD showed more attentional errors than drivers without ADHD, and even small doses of alcohol significantly increased the number of errors they made on the attentional task, as well as the variability in their reaction time.
• Participants who consumed the higher serving of alcohol showed faster and more variable driving speeds, slower reaction times, and more collisions. These findings held equally true for drivers with and without ADHD.
• However, drivers with ADHD showed slower reaction time when braking to avoid an accident. Further, both low and high doses of alcohol delayed reaction time for braking even more.

Weafer et al. (2008) conducted a similar study in two parts, with a smaller sample and with younger drivers (average age 22-23 years):

• In the first part, rather than examining reaction and braking time, they examined variability in lane position, driving speed, and steering rate (meaning the number and size of steering corrections – more variability means more “wandering” and swerving).
• Using these variables, they compared three groups:
  • sober drivers with ADHD
  • sober drivers without ADHD
  • intoxicated drivers without ADHD
• Sober drivers with ADHD showed significantly more variability in lane position and steering rate than drivers without ADHD (though no differences in driving speed). In fact, sober drivers with ADHD showed almost as much variability in lane position and steering rate as intoxicated drivers without ADHD. They note some concern about this finding, as their sample had more women with men. Recall from above that men are more likely than women to be involved in accidents, and so a sample with just as many men as women likely would have shown greater variability in driving.
• In the second part, they compared a sample of sober and intoxicated drivers with and without ADHD (half the participants were in part one, half were new to the study, and more participants were male than female, as is found in most samples of those with ADHD):
  • Intoxicated drivers with ADHD were more likely to underestimate their level of intoxication and overestimate their ability to drive before they entered the simulator, compared to intoxicated drivers without ADHD. This effect increased as the drivers with ADHD consumed more alcohol.
  • Intoxicated drivers with ADHD showed significantly more variability in driving speed, lane deviations, and steering rate; in fact, they showed more than twice as much variability as intoxicated drivers without ADHD.
  • Curiously, intoxicated drivers with ADHD showed greater impairment with smaller doses of alcohol. The authors were unsure as to why, but note that perhaps drivers with ADHD failed to realize their impairment after smaller doses of alcohol; however, they realized the impairment after larger doses, and made more efforts to monitor their driving performance once they started driving.

Weafer et al. (2008) note that in this study, drivers with ADHD did not take their stimulant medication. Some might argue that as a result the study does not reflect how drivers with ADHD function when actually driving. However, the researchers make two points:

• First, medication compliance is highly variable, and so many drivers with ADHD who are prescribed stimulant medication generally drive without taking it. Further, only 10-15% of adults diagnosed with ADHD take stimulant medication.
• Second, more accidents involving young adults happen over the weekends and at night, and more alcohol is consumed by young adults over the weekends and at night. These are the times when younger adults are least likely to have taken prescribed medication. Further, even when they took the morning dose of medication, the effects have likely worn off by night.

Thus, these findings likely do reflect actual driving conditions. And on the topic of medication and driving…

Medication and Driving

Medication for ADHD can also be of great benefit when driving.

Barkley et al. (2005) studied 53 adults (average age of 37 years) using a driving simulator. Participants were divided into two study groups:

• Everyone in the first group was given a driving test in the simulator to assess their basic driving skills. They were then tested a second time after being given medication. This group was divided into three medication sub-groups; the first sub-group was given a placebo (basically a candy designed to look and taste like real medication, but which has no effect), the second was given a low dose of stimulants, and the third was given a high dose of stimulants.
• The second group was just like the first, except that they were tested first with one of the medications (again, a placebo, a low dose, and a high dose), and then took the driving test to assess their driving skills.

Why would the researchers do this? It’s possible that there is a practice effect. People who used the driving simulator once might show better driving skills the second time they use it because the first time they drove served as practice.

• On the one hand, if everyone were tested without medication, then tested again with medication, everyone could show improvements the second time which we would think were the result of the medication, but which actually were the result of simple practice in the simulator.
• On the other hand, if everyone were tested with medication, then tested again without medication, everyone could show improvements the second time which were the result of simple practice in the simulator, and this would reduce the benefit of the medication and make us think it was not very help.

By having two groups (one tested without and then with medication, and the other tested with and then without medication) and combining their scores, the researchers hope the benefits of practice will “average out” and not affect the results of the study. When researchers do this, it is called counter-balancing the study.

The researchers did something else of note in this study. They made this a double-blind study, which means two things:

• First, the drivers did not know the purpose of the study, which reduces bias. Why is this important? Well, those who believe in stimulant medication might, intentionally or unintentionally, do a better job while driving with the medication. Those who do not believe in stimulant medication might, intentionally or unintentionally, do a better job while driving without the medication. By keeping the purpose of the study a secret until after the study is done, the participants are less likely to do something that would “throw” the study.
• Second, this kind of bias is, of course, also a risk for the research assistants. However, it is difficult to hide the purpose of the study from the research assistants helping conduct it. Instead, research assistants in a double-blind study do not know which participants are in which conditions. That means in this study, they did not know whether they were observing a driving test in which the driver was, or was not, given medication.

The driving test was given during the day, and a range of variables were monitored:

• When drivers with ADHD were given medication, steering control, driving at a consistent speed, and using a turn signal all increased as compared to their driving without medication (all these differences were statistically significant). Further, the number of (virtual) things they struck, driving time, and the number of accidents they had decreased as compared to their driving without medication (again, all differences were statistically significant).
• They found that the higher dose of stimulants did not always produce the best effects for each driver.
• Those given placebos also had small improvements in driving, but the authors attribute this to practice effects. Remember, the placebo is a fake medication, and so driving without fake medication and driving again with fake medication allows for practice effects. Thus, this finding shows the researchers were wise to counter-balance the study groups.

While many studies of drivers with ADHD use driving simulators, Barkley et al., 2005 note that Cox conducted a similar study using teen drivers (16 to 19 years old) on an actual driving task (16 miles of rural, highway, and city road driving approved by the Virginia DMV for assessing driving competence). These teens drove with observers in the car who watched their actual driving. Cox and colleagues found similar results, in that small doses of stimulants could produce significant increases in driving performance.

Conclusions

Over the past 30+ years, the research has consistently shown that both teen and adult drivers with ADHD are at two to four times the risk of committing traffic violations and experiencing auto accidents. Largely this is the result of poor judgments, poor adherence to the rules of the road (especially in drivers under the age of 40), and more variable control of the car while driving. However, teens are at a much higher risk of injury and even death as a result of an accident. While alcohol use causes impairment for all drivers, both small and large doses of alcohol cause significantly greater impairment for drivers with ADHD. This appears to be the case because these drivers are prone to underestimate the impairment of alcohol before they drive, as well as to show more variable control of the car once they start driving. Small doses of stimulant medication have been shown to significantly improve driving performance. However, most people with ADHD are not prescribed stimulants, while those are prescribed stimulants may not take them regularly. This is especially of concern with teen drivers, who are more likely to drive on weekends when they are more likely to have skipped taking the medication, or at night when the effects of the medication have worn off. There are some online resources at the end of the article for parents who are concerned about their teen drivers, including tips, examples of driving agreements and contracts, and ways to minimize distractions while driving.

References

Resources

• ADDitude Magazine article on driving
• report of a study on ADHD and manual transmissions
• article on ADHD and teen driving