There are longitudinal studies that show that untreated adult ADHD patients are less safe as drivers.
Here is an article:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4195639/
And I cite one sentence from the result: "Effective treatment of adults’ ADHD improves symptoms, emotional lability, and patient functioning, often leading to favorable outcomes (eg,
safer driving, reduced criminality)." (No, the article is not saying that ADHD = criminality.)
I had done hours of CME (continued medical education on ADHD), and the psychiatry professors (from medical schools) who lecture cite these longitudinal studies, which indicate that untreated ADHD patients have more auto accidents, more job turnover, less educational achievement, more divorces, etc.
I don't have these articles "on the tip of my tongue," but such results would be consistent with the condition. It has nothing to do with IQ, it only has to do with maintaining focus and attention. Even non-ADHD drivers are subject to lapses in concentration, and we must be careful and attentive when we drive. How much more so for the ADHD patient.
The point here is that ADHD is not "environment specific" -- it isn't just a problem functioning at school, but it affects other environments as well -- work (esp. in adults with ADHD), home life, driving, participation in sports (see more on this last item below).
Generally speaking, is he on his medications when he is driving? I have parents who take their kids off for the entire weekend, every single weekend. I tell them that is their choice, but usually point out that the child may benefit from being on the medication on the weekends.
Finally, I have a psychiatrist I worked for who is a consultant for major sports teams in Dallas. He started first with the Texas Rangers, and he is an expert at ADHD and makes sure that baseball players are 1) on the best medication for them (two classes of stimulants, and varieties within each class); and 2) the dose is optimized (not too low, and not too high). The answer is yes: baseball players with ADHD will make more "errors" (pun intended) in playing baseball.
So your post is in target, and what you are noticing is real. In addition to medication, yes, do use other strategies to aid your son in becoming a safe driver.
Edit: If you look at the article cited, in the back under
references, you will see that 75, 76, 78, 79 are all articles on the topic of ADHD and driving.